GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Show us your Custom Airgun Parts (TRICKS-N-TIPS) => Topic started by: Scotchmo on April 19, 2012, 06:36:28 PM
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I've got a big shop project in the works. I've been designing a spring gun that incorporates a lot of the features that I want. It will be a lightweight, short stroke, 12fpe, gas spring rifle. Intended primarily for field target. No wood. All aluminum (some steel for the barrel and other critical areas). I can usually make workable parts in the shop but this is going to be an ambitious undertaking for me as I'm a better designer than I am a machinist. When I was a design engineer, I could just take the drawing to the model shop, but this time, I'm making everything myself. I still have some parts and materials to order. I plan on starting in the shop in May.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE000-LAYOUT-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE001-RECIEVER-UPPER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE002-RECIEVER-LOWER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE003-PLUG-REAR-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE004-PLUG-FRONT-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE005-ADAPTER-GRIP-2.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE006-BUSHING-RECIEVER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE007-BUSHING-BARREL-FLOATING-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE008-BUSHING-BARREL-FIXED-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE009-PISTON-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE010-CHAMBER-AIR-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE011-PLUG-CHAMBER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE012-CAP-CHAMBER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE013-PLATE-LINK-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE014-BUTTON-INTERLOCK-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE015-TRIP-SAFETY-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE016-CATCH-CHAMBER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE017-LEVER-COCKING-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE018-LINK-LEVER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE019-BUTTON-LEVER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE020-CATCH-LEVER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE021-RING-SEAR-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE022-RAIL-LOWER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE023-RAIL-UPPER-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE024-PIN-INTERLOCK-1.gif)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE025-BARREL-1.gif)
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Beautiful drawings.... I'm kind of a back of the napkin, machine on the fly kinda guy.... Good Luck, and keep us posted with lots of pics....
Bob
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Beautiful drawings.... I'm kind of a back of the napkin, machine on the fly kinda guy.... Good Luck, and keep us posted with lots of pics....
Bob
Thanks Bob,
I usually do the back of the napkin design too. But when it gets complicated, I get on the computer.
Scott
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I don't have a good CAD program, and never learned to use one properly anyway....
Bob
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I don't have a good CAD program, and never learned to use one properly anyway....
Bob
When I started my first job, companies were just starting to look at mainframe and minicomputer CAD systems. They were expensive, huge and crude. I did my first projects on the drafting board as a junior engineer but within a few years got into CAD. I then saw that CAD on the PC was coming and got into that big time. I have not kept up on it in many years. I still use an old version of AutoCAD that I got when I was an application developer.
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companies were just starting to look at mainframe and minicomputer CAD systems. They were expensive, huge and crude.
Brings back fond memories of days past :)
My first job out of High School was for a electronics company that had just invested in a CADAM system. They put me though the training program but unfortunately they went belly up before the machine was delivered. It was a huge and crude machine as you described! A few years later when I worked for IBM, they had the mainframe cad systems that were leaps and bounds ahead of the standalone CADAM systems. Then came the IBM graphics PC/Terminals that off loaded all the screen drawing from the mainframe, then the PC/RT that ran CADCAM locally and about that time Autodesk released AutoCAd for the IBM PC and the world changed.
I use Inventor for all my mechanical design now.
BTW, your project looks amazing. I like the 12fpe Field Target aspect. Nothing quite like having a specific use or goal to drive a design.
Tom
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What's the swept volume?....
Bob
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What's the swept volume?....
Bob
* Side lever (right side)
* Side loading port (left side)
* .177 caliber
* 12fpe with 7.9gr pellets
* 1.8ci (29cc) swept volume
* Chamber and barrel axis on center
* Gas spring, 19mmx63mm, 132 bar
* Lube free, o-ring piston
* 14" barrel
* Gamo/Theoben style trigger
* Weaver/Picatinny scope rail with 30moa built in
* Weaver/Picatinny forend rail
* Add on rest w/Aschuntz style rail in front of trigger
* Somewhat modular
* M4 style pistol grip
* 1-3/16"-16 threaded female adapter for stock extensions
* Light weight, mostly aluminum
The 12fpe with the small chamber volume will require 6.7fpe/ci of swept volume. And I need a pellet/spring efficiency of about 37%. I have not gotten that yet in my current rifle so that area needs some further experimentation and development.
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Don't you normally use 5 FPE/CI as a reference?....
Bob
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I don't have a good CAD program, and never learned to use one properly anyway....
Bob
Bob,
progeCAD is a free downloadable program very much like autocad.
Lloyd
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Beautiful drawings.... I'm kind of a back of the napkin, machine on the fly kinda guy.... Good Luck, and keep us posted with lots of pics....
Bob
Thanks Bob,
I usually do the back of the napkin design too. But when it gets complicated, I get on the computer.
Scott
Scott, I am with you on that.
Having a good drawing makes the machining go so much faster and avoids a lot of restarts. Still, I do a lot of my stuff seat of the pants, too. And then go back and make the as-built drawings.
Lloyd
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Don't you normally use 5 FPE/CI as a reference?....
Bob
Yes. The 6.7 FPE/CI number is a goal. I have been using 5 because that is a fairly easy number to get with conventional airgun technology. I've pushed it up to 5.8FPE/CI using a Teflon ringed piston in my TF58. It has very low friction. It does not seal well at low speeds but does OK when fired. I'm not sure how long it will last. I've got about 1000 shots so far and it is still going good. I have a couple of other tricks that I have employed in this next design in order to try and get the FPE/CI up a little more.
The parachute seals that are commonly used are very forgiving as far as tolerance and contamination. When you drive them harder, they seal even tighter, but the friction also climbs fast at high pressures. And then the efficiency falls off. The only parachute seals that I have used are the Chinese polyurethane seals so my conclusions may not apply to better quality parachute seals.
A lot of people in Field Target that still shoot spring guns are switching over to Viton o-ring pistons with good results. In a well finished chamber, they seal good at all speeds, even with the soft tunes that they typically run in their ProSports and Tx200's.
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Scott PM sent. Craig
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A bit off topic, so I apologize in advance.... You mention "soft tunes", have you ever put a number to that term?.... EG would 4 - 5 FPE/CI be "normal" and under 4 FPE/CI "soft" while over 5 FPE/CI a "power" tune?.... Please give your opinion, using your own numbers if you can, I think it would be a valuable tool for people to assess their goals...
Bob
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I would call 4 FPE/CI a soft tune. A full stroke TX200 at 12fpe is at 3.9 FPE/CI. Some people are taking a combined approach and also shortening the stroke a little in their TX. I calculated that the short stroke TX's are running about 4.5 FPE/CI to get the 12fpe.
I guess that you can make arguments for the long, slow push vs the short, high pressure impulse. I don't know which is better yet.
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I wonder what the guns the Cardew's referred to as being in "pop-gun mode" (ie NO combustion involved) would work out to.... certainly would be a soft tune for sure....
Bob
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I wonder what the guns the Cardew's referred to as being in "pop-gun mode" (ie NO combustion involved) would work out to.... certainly would be a soft tune for sure....
Bob
Most soft tunes run in "pop-gun mode". My 5.8 FPE/CI rifle is also running in "pop-gun" mode.
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I made some refinements to the design and I started cutting metal yesterday. I'm giving my mill/drill a good workout and learning it's limitations. I decided to make two sets of parts since much of the time is the setup. I'm not a machinist so it's tough to hold some of the tolerances that I specified but I'm doing OK so far. Backlash is a b#%&*. No major mistakes. I did accidentally drill out a threaded hole that I thought was a pilot hole in a momentary lapse. I made a threaded plug, loc-tited it in and re-drilled and re-threaded to the original size. It's an internal part, so I won't even see it but I hate making stupid mistakes. It cost me an hour. I'm working on the lower receiver right now so it is mostly mill work. I look forward to working on the upper receiver as it has more lathe work.
I'm on a budget so I decided to try a 1-1/4" carbide core box router bit instead of an expensive ball end-mill. It actually works fairly well on aluminum.
When I did this design, I decided to stick with imperial units and inch threads. My first design project as an engineer was dictated to be metric as there was supposed to be a industry wide conversion taking place. When the design went to the tool and die department, they said that they were not equipped to work in metrics as all the machines were still in inches. I was asked to go back and dual dimension all of the drawings. Over thirty years later and we are still using the inch system. My lathe and mill/drill are are also graduated in inches. Doing two days of constant work makes me appreciate our backward system. I can work with metrics as it is very logical but there is something very intuitive about how we work in the inch system. 1/2's, 1/4's, 1/8's. All easy to remember and divide up in my head. I try to use preferred numbers in my designs and that makes it even easier.
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Scotchmo,
We should complain, LOL, almost everybody else in the world has to learn English, so we lucked out there. I have to say that a millmeter is a handy unit of measure sometimes, like when roughing out a hole and measuring the depth with a steel scale. And my lathe tailstock quill has 1/8's and mm's, so I always convert to mm's for the drilling.
Yup, too bad the metric conversion didn't happen when we were all kids.
Lloyd
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Scotchmo - very nicely detailed!
Funny you mention metric/inch conversions. I went to engineering school in Canada, so we learned both systems back then for everything. Try doing kinematics calculations using slugs... yeesh! Really makes you appreciate the metric system.
Over twenty five years later, I literally think in thousands of an inch, and can convert pretty much anything back and forth in my head. Lucky for me, our current CAD software (SolidWorks) can be set to automatically dual-dimension everything whether modeling or drafting.
I'm lucky in that I get to design, fabricate, and build equipment and prototypes (and g-job a few AG parts) where I work. Can't ask for any better job than that.
And any engineer worth his salt starts a design on the back of a napkin.
Looking forward to progress reports on this one...
JMJ
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I finished the most complicated mill part today. The front plugs for the lower receivers. My mill/drill has .023 and .014 inches of backlash in table dials. I lose track of it sometimes and am lucky to hold .02 tolerance at times. This would be an ideal part for CNC. But I would be happy with ball lead screws or a digital readout.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/lower-receiver.jpg)
This part houses the anti-beartrap and safety interlock mechanism. Not very exiting but necesary on a sliding breech spring gun. On my TF58, you need three hands in order to override the interlock and uncock the rifle. I compromised a little on this design. I did not try and make it idiot proof because I wanted to be able to safely uncock it if needed. The automatic safety does not go on until you disengage the anti-beartrap. So if you you want to uncock it, you pull the lever back to engage the anti-beartrap, release the safety and then pull the trigger. The anti-beartrap will catch the compression chamber and then you can hold the lever, release the anti-beartrap, and return the lever to the closed, uncocked positiion. At least that is the intent.
I made a 1-3/16-16 tap for the stock tube. I made it slightly undersized for a tight fit. I case hardened it. It was good practice for case hardening the sear ring that will go on the piston. The tap cut a decent thread in a test piece but it was too tight. I may just buy this one since it has to fit a standard AR15/M4 buffer tube. I may still make the tap for the compression chamber since I am also making the cap/plug so I can adjust the thread fit easily. The heating, case hardening and quenching seemed to take some of the edge off of the cutting edge on the homemade tap. It still cut OK. If you are only going to use it a couple of times in aluminum, you can probably forgo the case hardening.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/tap-00.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/tap-01.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/tap-02.jpg)
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man reading Scotchmo and Bob posts make me feel stupid :-[
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Well you qualify as the first person I have seen make a custom TAP.... Well done....
Bob
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great job on the tap , guess you didn't want to try internal threading on the lathe?
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Wow! Seems a bit crazy but results are what count. I am curious why you would not attempt a single point internal thread first? How much muscle does it take to start that thread with the full profile lead? I bet you got a workout! When I need a custom tap I just get out my check book :)
Nice progress and nice work!
Tom
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great job on the tap , guess you didn't want to try internal threading on the lathe?
I have never cut an internal thread on a lathe but I did want to try it. I set the lathe up to do it to see if it was possible. The upper receiver tube that I will be threading is 24 inches long and 1-1/4 inch OD. My headstock will pass a little under an inch only. So I needed to hang the tube out and use a steady rest. My lathe bed is 4 feet total length. I had to remove the tail-stock and crank the carriage all the way to the end of the bed. If I went out far enough to get the tool into the tube, the thread dial would not engage since it hit the lead screw support. I would have to rig up an unconventional tool holder that extended to the right off of the carriage and curved back but it seemed easier to make a tap. I could do it on the short compression chamber but I don't want a thread relief on it, so it gets a tap too.
Wow! Seems a bit crazy but results are what count. I am curious why you would not attempt a single point internal thread first? How much muscle does it take to start that thread with the full profile lead? I bet you got a workout! When I need a custom tap I just get out my check book :)
Nice progress and nice work!
Tom
It took some muscle. But it cut smooth with no hangups. I'm poor (or cheap). Just for the two taps and large end mills that I would need would be about $250. So I made do. I'm going to give the receiver tap another go. And hopefully get the fit right this time.
I'm a little envious of your milling equipment and skills.
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instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
super simple to make a crank too... just get you some scrap roundstock (delrin alu steel doesnt matter) drill you a hole the length of it, then turn it to just under the size of your headstock bore, then cut the pipe at a angle in the middle.. Get you another peice of scrap round square whatever you got make you a crank arm and handle then take a long bolt and bolt the whole thing together with the tube angles mated together... slide the whole thing into the bore of the headstck tighten the long bolt down and the angled ends try to slide past each other and grip the inside of the bore(Kinda like an old gooseneck on a bike would stay down into the forks) .... you now have a removable crank
kinda like this except they have a collet shown attached at awell
http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html (http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html)
just remember to REMOVE the crank before ou power back up!!!!!
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instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
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That is a good idea. I'll have to try it. The other good thing about your method is that I don't have to incorporate any thread relief.
I won't need a crank. My lathe is 84 years old. It is a transitional model from when they were switching from line-shafts to self contained units. When I want to run it manually, I just keep tugging down on the drive belt.
This lathe is old but the ways are nearly perfect. I think it was retired before WW2 so it escaped the heavy use that the newer lathes got.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/lathe-00.jpg)
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It has been a while since I have made a thread on a manual lathe but I do remember the process quite clearly. I prefer not to have an undercut on an ID thread and would use the cross slide to back the tool out of the cut on the last couple of revolutions. I would uses a dial indicator and a mighty magnet on the carriage travel to determine when to start the cross slide back out. Once the tool is clear of the thread, disengage the half-nut, back up the carriage, run you cross slide back in to 0, make the depth adjustment on the compound slide and make another pass. I prefer to do the threads under power to avoid jerkiness in the peripheral speed of the cutting tool to get the best surface finish possible.
I can see where the crank method would be invaluable on a blind bore where a slight hesitation would be disastrous!
Tom
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instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
super simple to make a crank too... just get you some scrap roundstock (delrin alu steel doesnt matter) drill you a hole the length of it, then turn it to just under the size of your headstock bore, then cut the pipe at a angle in the middle.. Get you another peice of scrap round square whatever you got make you a crank arm and handle then take a long bolt and bolt the whole thing together with the tube angles mated together... slide the whole thing into the bore of the headstck tighten the long bolt down and the angled ends try to slide past each other and grip the inside of the bore(Kinda like an old gooseneck on a bike would stay down into the forks) .... you now have a removable crank
kinda like this except they have a collet shown attached at awell
http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html (http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html)
just remember to REMOVE the crank before ou power back up!!!!!
Well knock me over with a feather..I have heard it all now...
toss the crank handle away... peddles man peddles and a seat...
But all jokes aside it could be useful on some threads.
small threads in a blined hole or against a shoulder..
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instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
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That is a good idea. I'll have to try it. The other good thing about your method is that I don't have to incorporate any thread relief.
I won't need a crank. My lathe is 84 years old. It is a transitional model from when they were switching from line-shafts to self contained units. When I want to run it manually, I just keep tugging down on the drive belt.
This lathe is old but the ways are nearly perfect. I think it was retired before WW2 so it escaped the heavy use that the newer lathes got.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/lathe-00.jpg)
That sir is a darn pretty old lathe.... very nice looking *(&^ I would trade out my 9x20 for a lathe like that anyday :)
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Update on my progress:
The lower receivers are almost done. I specified stainless steel on many of the small parts but I don't have much SS material and did not want to order any for small items. In some instances I just used what was in the shop. Usually mild steel. For the safety interlock, I cut up a bicycle fender bracket:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/trip-00.jpg)
The next picture show the guts of the lower receivers of rifle A and rifle B (I'm building two):
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/abt-00.jpg)
The next picture shows the guts of one rifle with the anti-beartrap catch up:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/abt-01.jpg)
The next picture show the release button pushed and the catch retracted:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/abt-02.jpg)
Finally. The assembled lower receivers with the catch engaged on the left and released on the right. The picture is dark but you may be able to see the part inside the receiver that trips the safety when the button is pushed:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/abt-03.jpg)
I think I'm about ready to burn out if I keep it up. I'm taking a break and going to the Los Angeles area this weekend. I'll be shooting a field target match in Chino Hills with CASA. I'll be using my TF58. Maybe someday I'll use my Scotchmo Express in a match.
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Is it just me, or do those last two pictures, when viewed as a single picture, give a bizarre effect of a hand with a big crack in the wrist just laying on table. Whew!
That case hardening compound you were using. How does that work? Do you need an oxy-acetylene torch?
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"Thing" is my helper.
The Cherry Red case hardening compound requires that you heat the part up until it is cherry red. I use an oxy-acetylene torch with a big tip.
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I was back in the shop today after a few days away. I did some work on the grip adapter/frame. I am still getting frustrated dealing with the backlash in that mill/drill. In these parts there is an area that I could not avoid it - I had to change direction and deal with the transition from conventional to climb milling. Mostly just cosmetic errors resulted.
I used a Harbor Freight hole saw to cut the 1-1/8" holes. Half of the teeth broke off before I finished but it got done. Not the best tool for the job. I wondered what that crunching sound was as it rotated.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/grip-adp-00.jpg)
I have a lot of metal to hog away.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/grip-adp-01.jpg)
The next picture shows the rough milled parts. I don't really have the capability to cut the small fillets and radii. I broke an 1/8" ball nose end mill while trying. I switched to a 3/16" in a couple of places.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/grip-adp-02.jpg)
Time to get out the files and do a lot of finish work.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/grip-adp-03.jpg)
I'll be starting on the upper receiver in a couple of days.
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Looking good :) Boy, you work fast!
I'm sure you already know this so I'm hesitant to throw it out there. You can try to use your axis locks to add enough drag to overcome the the tendency for the cutting tool to pull the work piece to the head of the back lash when climb cutting is necessary.
Tom
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Looking good!!
Man I can't believe you did those holes with the hole saw. But it worked and sometimes thats what counts.
If ya got a 4 jaw chuck for your lathe you could have done it on the lathe or hunt ya a boring head for your mill. Or rough it out with an endmill and then come in with a fly cutter to clean it up.
But hey good job of making do with what ya got,, Keep it coming!!!!!
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Ditto with what Tom said. Climb cutting is my favorite way of milling but its not that great if you're trying to side mill lets say .250 or over , it will catch and feel like its gonna destroy the whole machine and easily ruin a part.
P.S. I see also that you're using a 4 flute endmill, while you can use it to mill aluminum it is way better to get a 2 flute cutter or just use lots of coolant with the 4 flute, it gets hotter and will melt the aluminum right onto the cutter and can ruin the cutter and the part. I hunt down those endmill lot deals on ebay or goto Harbor Freight
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...You can try to use your axis locks to add enough drag to overcome the the tendency for the cutting tool to pull the work piece to the head of the back lash when climb cutting is necessary.
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I don't know if the mill/drill has an axis lock. I tried tightening the gibbs but then it hangs up at different points in the travel so I have to readjust it each time. I just leave it at the best average setting.
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P.S. I see also that you're using a 4 flute endmill, while you can use it to mill aluminum it is way better to get a 2 flute cutter or just use lots of coolant with the 4 flute...
I figured that out when I clogged up some small four flute mills. I now use two flute for most of my aluminum cuts. I do use that large 4 flute to hog away large amounts. It seems sharper than my other mills.
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Yeah we all learn by mistakes. I had just recently messed up a 4 flute end mill and the part which was 1/2 aluminum pipe that I was making a slot in it, didn't take but a few seconds of milling to screw it all up.
I used to work at a mold shop with great big CNC machines that were drilling aluminum and I still being a lil green let one of the machines run out of coolant and it snapped a one inch drill bit right off with a horrible noise that went with it. And it was about a $100 drill bit and the boss was not happy.
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For an axis (crossfeed) lock on my lathe carriage I took out one of the setscrews and locknuts near the middle of the Gibb and substituted a hex head bolt that I can tighten with a wrench.... I leave it loose for most machining, but I can tighten it up to lock the crossfeed when I'm milling.... Simple and effective solution.... The milling attachment on my lathe already has a lock on the vertical travel.... and of course there is one on the longitudinal travel on the carriage.... so I have the ability to lock two axis and then feed with the third....
Bob
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I would like to thank MichaelM for originally suggesting that the lathe could be turned manually to cut threads. It worked great. Even if my lathe had the room, which it didn't, cutting threads under power can stress me out when in a tight space.
The 24" receiver tube mounted on the lathe with the tailstock removed:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/thread-00.jpg)
I had to remove the thread dial for more carriage travel. Since I was turning the lathe by hand, forward and reverse, I never disengaged the half nut. So I did not need the dial.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/thread-01.jpg)
The setup ready to cut threads:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/thread-02.jpg)
And the threaded receiver tubes:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/thread-03.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/thread-04.jpg)
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Looking good !!! Don't ya love it when you over come a challenge. I love the lathe but I have only threaded on lathe once and that was when a master machinist was teaching me how to do it.
Keep up the good work!!!!
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I love seeing how folks improvise and get good results. Nice job on the internal thread, hanging it out there in space with that steady rest.
I really enjoy single point threading but what really helped me do a better job was studying the thread form diagrams
http://en.wikipedia.org/wiki/File:ISO_and_UTS_Thread_Dimensions.svg (http://en.wikipedia.org/wiki/File:ISO_and_UTS_Thread_Dimensions.svg)
to see how the threads were actually supposed to look. I had always thought that the major dia of the external thread was the nominal dia at the pointed crests. I was wrong. The major diameter is actually truncated down to the nominal diameter. When I was making them pointed, I was making the thread cross section too thin. If that makes sense. :P Anyway, the picture shows what I am trying to say.
Lloyd
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Glad I could help out!! looks like you got a good thread cut there
I know alot of people that cut threads manualy like that just because it IS alot less stressful then cutting threads under power, no counting dials, no grabbing for the feed lever, and can start and stop your threads with much more precision.. not to mention cutting up to shoulders or threading in a blind hole where a single lapse in timing can crash the lathe and wreck a part...
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Glad I could help out!! looks like you got a good thread cut there
I know alot of people that cut threads manualy like that just because it IS alot less stressful then cutting threads under power, no counting dials, no grabbing for the feed lever, and can start and stop your threads with much more precision.. not to mention cutting up to shoulders or threading in a blind hole where a single lapse in timing can crash the lathe and wreck a part...
Michael,
+ 1 on what you said. Cutting them under power does cause a serious pucker factor. I am going to use your crank idea, too. Have been using the big key in the chuck to do the turning and it is pretty unsatisfactory.
Lloyd
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Glad I could help out!! looks like you got a good thread cut there
I know alot of people that cut threads manualy like that just because it IS alot less stressful then cutting threads under power, no counting dials, no grabbing for the feed lever, and can start and stop your threads with much more precision.. not to mention cutting up to shoulders or threading in a blind hole where a single lapse in timing can crash the lathe and wreck a part...
Michael,
+ 1 on what you said. Cutting them under power does cause a serious pucker factor. I am going to use your crank idea, too. Have been using the big key in the chuck to do the turning and it is pretty unsatisfactory.
Lloyd
yea and making a crank is a super simple thing to do, seen everything from the angled end pipe(like old goosnecks) to old worn out collets and making a cone shaped nut and a peice of allthread so that when they tighten it the cone on the nut SPEADS the collet open to grip the bore of the head... most people use whatever scrap they have to build themselves their threading crank LOL
me I used a chunk of 2 inch delrin rod to make my gooseneck type of crank then a scrap of wood the make the crank handle and a round peice of wood to make the handle to hold onto
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I milled the features in one of the upper receiver tubes and started the bushings that go inside. The front bushing uses a taper lock to secure the barrel. A barrel change should take only about 60 seconds in the finished rifle. Unscrew the front nut (collet) and slide the barrel out:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/bush-00.jpg)
The funny marks are from the chuck. I threaded the collet after it was made it and there was not much to grab. Next time, I'll thread it before I cut it off of the hex stock.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/bush-01.jpg)
I'm only finishing one upper receiver right now. No real economies of scale in that process. Just a lot of counting, cranking, and tool changes between setups.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/slot-00.jpg)
I drilled and tapped all of the holes in the bushings so I could do a trial assembly and check out the ergonomics.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE-00.jpg)
I just ordered a scope for it yesterday. I need to take another break from the shop and get caught up on my non-hobby work. Then I have to update the prototype drawings as I have been marking them up as I work. So back on the CAD program. And I really need to straighten out my shop and clean up some piles of chips. I also need to order some more tooling. I really need a 12mm drill for the bushings (the barrel is 12mm diameter). I used a 31/64" bit so the bushings were a little looser than I would like.
It is progressing faster than I originally thought. The simple design of the rifle helps. The next major phase will be the power plant: piston, compression chamber (cylinder and head), cocking lever and linkage. I need to work with a little higher precision on these parts. I hope to start on that next week
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Looking awesome!!!
I couldn't help notice that you milled those slots dry and I don't know what it is about aluminum pipe but you always need to use some type of coolant when milling it. I can see the aluminum is all gummed up on your cutter.
Whats the part under the barrel at the front?
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I did use WD40 on that slot. The little chips stick to it. No gumming there but it is often a problem on the smaller bits.
The part near the front is a Magpul Fore Grip. I plan on using this rifle in Hunter Field Target which allows a bipod. I have found that a piston gun prefers an unattached bipod. And I like to rest it way out there to help steady my aim.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/foregrip-01.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/foregrip-00.jpg)
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Hey Scotchmo I am really digging this build and if ya don't mind I would like to build your gun too. I was looking around my shop and realized I have all the material to build it. The Scotchmo Springer!!! I like it!!!!
PS I meant to ask you, why is the front of the barrel like that? with the threaded nut thingie ?
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Your welcome to build one like it but your on your own for now. I'm still revising the drawings as I build and I still need to see if it is even going to work. I just made some design changes to the safety interlock. It was not working right. I need to remake a couple of small parts and get that working before I start the power plant. I don't know what my final plans will be for the drawing package. If it works out, maybe I'll do a short run production.
I don't like set screws, hence the threaded nut thingie. It secures the barrel. The barrel slips into the back side of the nut. When you tighten the nut, it clamps around the barrel.
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Aahh I see so in that picture the barrel is not attached?
And I had a bummer day, went to the surplus place in another city and this place sells metals and such and I could not find any of the dimensional aluminum used in your design so I guess I will try ebay
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The barrel floats in a close fitting bushing at the breach and is secured only at the muzzle. Here is how it attaches:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/bush-02.jpg)
I have ordered metal on eBay but here are two better sources:
http://www.stockcarsteel.com/ (http://www.stockcarsteel.com/)
http://www.speedymetals.com/default.aspx? (http://www.speedymetals.com/default.aspx?)
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I'm also posting the same info over at LD's Airgun Club on the Delphi Forum. LD designed the USFT air rifle. I got some of my inspiration from the USFT.
I made some good progress on my rifle over the last week even though I had a bad toothache. I ended up going in for a root canal a few days ago. That kept me out of the shop for a couple of days.
I finally started on the power plant. These parts are key to getting the efficiency that I want so I worked hard to hold a tighter tolerance. The following picture shows the cylinder, piston, sear ring, and gas spring:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/PWRPLANT-00.jpg)
The sear ring will be case hardened and it then threads onto the aluminum piston:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/PWRPLANT-01.jpg)
I have few small thing to do on the rifle. But only two major parts remaining to make, the lever linkage and the compression chamber head. I also have to prep the barrel.
Here is what it looks like currently (minus the power plant):
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE-01.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE-02.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE-03.jpg)
If all goes well, I'll be testing by next weekend.
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Lookin' good ...
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Very sharp, keep up the good work!!!!
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You are really working like a mad man on this project.... it is hard to keep up ! LOL
Looking very good, BTW.
I have a question about the arrangement of the aluminum piston and the gas spring. In the picture you have the body of the gas spring inside the piston. When the gun fires, it looks like the weight of the body of the gas spring will have a accelerate along with the piston. If you turned the gas spring around so that its rod fit into the aluminum piston, then only the rod of the gas spring would have to accelerate. The heavy part of the gas spring wouldn't have to move so the aluminum piston ought to accelerate faster.
Just a thought.
Thanks for sharing this with us and I have learned some tricks by following this thread.
Thanks,
Lloyd
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I'll take the opportunity to answer about the piston weight.... but I'm not a real springer guy so I'm sure Scotchmo will correct me if I get it wrong.... If the piston is too light, it will bounce off the air in the compression chamber before the pellet moves.... If it's too heavy, it will tend to crash into the end of the chamber.... Light pistons favour light pellets, and heavy pistons favour heavy pellets.... Since most pistons are steel, I would assume Scotchmo is putting the body of the ram inside the piston to bring the weight back up to a more normal value.... After all, the usual springer has half the weight of the spring acting as piston weight as well.... There is an optimum piston weight for any given design....
Bob
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I'll take the opportunity to answer about the piston weight.... but I'm not a real springer guy so I'm sure Scotchmo will correct me if I get it wrong.... If the piston is too light, it will bounce off the air in the compression chamber before the pellet moves.... If it's too heavy, it will tend to crash into the end of the chamber.... Light pistons favour light pellets, and heavy pistons favour heavy pellets.... Since most pistons are steel, I would assume Scotchmo is putting the body of the ram inside the piston to bring the weight back up to a more normal value.... After all, the usual springer has half the weight of the spring acting as piston weight as well.... There is an optimum piston weight for any given design....
Bob
Bob,
Good point about the extra light aluminum piston and the gas spring body becoming part of the mass of the moving piston.
A long time ago I tried drilling a bunch of holes in a B3 piston to get more power. Needles to say, it was a disaster.
Scotchmo,
I bet you have already done your homework on the piston weight.
Lloyd
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I have considered the piston weight issue carefully and also the direction to install the gas spring.
All things being considered, I'm going for the lowest possible piston mass. I want maximum piston acceleration and the shortest lock time possible. I'm going for an efficiency that is somewhat above the norm. That means higher pressure. A slow piston won't give me that. The lighter piston will mean that timing may be critical. Because of hysteresis, the gas spring is a little more resistant to bounce than the wire spring. That will help with the lighter piston. It will be geared toward shooting 7.9gr pellets. In general, a heavier piston mass should act as an equalizer, but at the expense of optimization for a particular pellet. So a production rifle that fires a variety of pellets OK would favor a heavier piston. My rifle is targeting a very specific window and I hope to be able to utilize the advantages of a light piston. I'll find out soon enough if my theories hold up.
The direction of the gas spring does not affect piston mass as much as one would think. I'm going by external measurements at this point. I would need to disassemble a gas spring in order to do a thorough analysis. The mass of the gas spring housing is very close to that of the rod and piston inside of the gas spring. Having the body forward is also a more stable arrangement. Also, a future iteration will use a separate piston head and skirt. The gas spring body will act as the piston for the rifle. The N-Forcer gas spring that I'm currently using does not have the body grooves that would facilitate that arrangement but most of the other brands do.
I'm getting excited at the prospect of finally putting it over the chrony!
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My project uses a N-Forcer as well. Because the piston shaft is much heaver (larger diameter) than a commercial ram, its mass is about 40% of the total so it really becomes a question of which direction facilitates the design best. I my case, the 6m charging port will be used to hold the ram to the front of the piston.
Using the ram with a piston head only is an interesting idea. One of the major differences between an industrial spring like the N-Forcer and a drawn body commercial spring is the the industrial springs have very solid shaft bushings in the design so they need little or/no additional support. I'm not sure I would trust the grooves in the spring body for sear engagement but one possibility would be to silver solder on a release ring of appropriate size and material. There may many ways to do this and possibly eliminate excess piston skirts that may add binding and drag during to the cocking and firing cycles. Again neat idea.
Light weight pistons. I like the minimum lock time and if the timing works out and bounce is not affected it can only be a good thing. This on one of those things that has been tried before but with other purposes in mind (power) and then reported as failure. since power is not your goal it will be interesting to see how this works out. I'm making my pistons out of 7068 AL (Lloyd stop drooling!) to keep them light as well, hoping that the weight coupled with the short stroke will give the coveted extremely short lock time. I plan to try both nickel plating and typeIII anodize with PTFE infusion to give the sear engagement the hardness required for minimum wear. I also plan to embed accelerometers in my test pistons to measure the bounce and synchronization.
Great project with lots of innovations and keep of the good work! So close it must be killing you :)
Tom
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Why not make the piston with a tight fit to the shaft and then just fasten. It looks Like there is a hole in the end. I picture what looks like an engine piston with rings and instead of a perpendicular hole for the connecting rod a parallel hole in the center that fits tight to the shaft with a hole for a screw, or has that already been said. Wow that opens up a lot of options for modifying existing platforms to O rings and gas springs if the shaft is strong enough, the piston might only need to be a couple inches long.
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... the piston might only need to be a couple inches long.
Sounds like a good idea and I looked at doing something similar. But when laid out, you quickly realize that the piston has to be as least a little longer than the stroke. Unless someone can come up with a unique way of cocking and latching the piston that circumvents that requirement.
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Update:
I completed all of the parts except the barrel and did a trial assembly yesterday. I spent some time trying to figure out why some things did not fit right. I found a couple of manufacturing errors (mine). And a couple of design errors (also mine). No deal breakers. So rather than make new parts, I just modified the offending parts on the mill and attacked two others with a file.
Today I did a complete assembly of the rifle. I originally specified stainless for the steel parts but ended up using mild steel. I blued a few of the upper receiver parts this morning and then laid everything out. Here are some pictures:
First is of the piston assembly. Originally i had a PTFE ring at the back but I did not see in the layout that it would hang up on the trigger mount. So I cut it back. I still want to do a piston with a separated head and skirt and with a PTFE guide ring. Maybe in the next version. You can also see the wrench that I made to tighten the hardened sear ring onto the threaded piston skirt.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-PISTON.jpg)
The next is the head and piston:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-HDSPC.jpg)
Next is the head, head gasket and breach seal. See the brazing. Somehow the .125 roll pin hole ended up .140 diameter. I had too many drill bits out and must have grabbed the wrong one.:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-HEAD.jpg)
Parts for the power plant:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-PWRPLNT.jpg)
Exploded view:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-EXPL.jpg)
Sorry. I did not get any pictures of the barrel prep. It was pretty straight forward.
TaDaaa! -
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/00-SE-DONE.jpg)
I put about 150 rounds through it this afternoon/evening. Stay tuned for the report.
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I have been very excited the last couple of days. I couldn't even sleep in as I have been too anxious to get to the shop and get the rifle to the point of shooting. And yesterday was the big day.
First the bad news: My goal for this short stroke rifle is 12fpe. I'm only getting about 7.5fpe right now. Some of my design theories may not have held up or something else could be wrong. When I redesigned the rifle to take the longer LW choked target barrel, I did some crude tests and determined that it will probably cost me about 1fpe. But these barrels are renowned for their accuracy and I really want accurate. Using a 14.5 inch length may also be costing me a little. For this small chamber volume rifle, I estimated that 8 inch would probably be the optimum length. And the unchoked scrap piece that I cut off of the LW blank is about 8 inches long. I will eventually make a barrel bushing that adapts the shorter barrel piece so that I can try it out in this rifle and see if I can get the some more FPE. I should be able to get at least 9.5fpe even with the longer choked barrel, so I suspect that there are some other problem areas.
Ergonomics seem good but I should increase the finger relief on the cocking lever. And I think that the lever could be another inch longer. Cocking is very smooth though a little higher effort than I like.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST00.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST03.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST02.jpg)
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST01.jpg)
The final unscoped weight is 5-1/2 lbs. I like the quick shot cycle. It feels good. It is somewhat tinny sounding. Maybe since it is all aluminum and the hollow metal tube in the stock does ring some in my ear. Or maybe it has some piston slam.
Adding the scope definitely makes it top heavy. I like to try and get the CG of a spring gun along the bore line, so I need to add some weight underneath the rifle in order to balance it out. Perfect job for a fixed "hamster".
I tried a variety of pellets from 7.0gr to 10.5gr. Soft and/or light pellets worked best. The tight fitting CPH had only around 5fpe. The 7.33gr AA had almost 8fpe and my preferred pellet, the 7.9gr JSB had 7.5fpe.
Accuracy with the 7.9gr pellets was good almost from the start. I'm an OK shot using a bucket and shooting sticks. My current field target base line is the ability to get half my shots inside a half inch circle at 25 yards. This rifle is already pretty close to that. I think it will improve once I add weight and balance it and remove some of the hold sensitivity.
24 shots from 25 yards, bipod near muzzle:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST04.jpg)
12 shots from 25 yards, bipod back a few inches:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/01-TEST05.jpg)
I did make the design easily scalable. A redesign for a longer stroke with a greater chamber volume is almost a trivial matter. But I would have to build another rifle as many of the parts would have to be longer. I'm going to shoot it like it is for awhile. I was shooting field target with my TF58 when it was only 8fpe and did OK. The long shots are tough though.
I did a little research today on other rifles that have a similar chamber volume. There are not many but the following three are pretty close:
Diana 25
Gamo Delta Fox
IZH60
On all of them, the FPE is less than you would expect for the chamber volume. They all have less FPE than I am getting from the Scotchmo Express. It may be that when the chamber volume gets down to a certain point, it is hard to get any amount of power out. My rifle is probably sprung significantly higher than the three mentioned above. And since my efficiency is not that good right now, that is where the extra power is coming from. A future change to a shorter, unchoked barrel may help that.
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Fantastic, Now the real work begins :)
The overall appearance, fit and finish look great! No mistaking that rifle for any thing else. I'm curious why you chose such a long barrel length? As you expect, a shorter barrel may bring up the efficiency. I also wonder about the timing of the pellet release and its effect on efficiency on springers of any design. This may be the hardest variable to control but may yield the most results in terms of efficiency and shot cycle. I'm thinking of replaceable bushings for the barrel lead, any thoughts?
Tom
Tom
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Fantastic, Now the real work begins :)
...I'm curious why you chose such a long barrel length?...
Simply because it is easier to shorten a too long barrel than it is to lengthen a too short barrel. And I twas thinking that I could be wrong in my estimates and that maybe there was a little usable FPE in those extra inches of barrel. So I erred on the side of too long. I was going to start out by attaching the full 23.5 inch long barrel blank. I decided that going to that extreme was probably a waste of time.
...I'm thinking of replaceable bushings for the barrel lead, any thoughts?...
The lead makes a difference. I started with almost no lead and then cut a small lead. I gained about 20fps. I like it tight so that the pressure gets high before the pellet "pops". You want it to "pop" just before the pistons would bounce. Your breech may lend itself to having an insert. Barrel bores are not always perfectly centered. So take that into account.
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I have rally enjoyed watching this very impressive project unfold. Nice work.
I totally understand your hardly being able to sleep when it gets to this final stage. A new day always brings a fresh perspective and a new idea or two!
Lloyd
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Congrats on bringing a really beautiful and well thought out design to completion.... I'm sure you'll overcome any shortfalls in the tuning process....
Bob
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I have put about 500 rounds total through the Scotchmo Express. It is starting to show some areas that will need to be addressed to make it a reliable rifle. I used a case hardened ring on the piston skirt for the sear engagement. The ring is made of DOM and hardened with "Cherry Red". The piston is free to rotate but in reality, it does not, which is fine. The surface of the ring looks OK but the core is soft and the edge where the sear engages is starting to bulge up and is digging into the aluminum receiver tube. It gets gritty feeling when cocking and aluminum shavings appear on the sliding chamber and start to jam it up. When I had it apart, I tapped the edge of the sear ring back down with a ball peen hammer but it is happening again in another spot. I need to find something that I can make a ring out of that is hardened throughout.
I had two barrels that I made from the LW blank. One was 14.5 inches long with a heavy choke at the muzzle. The other one was the cuttoff from the same barrel and turned backwards since it appeared to have a slight choke an inch from the breech. It is 8.7 inches long.
Long barrel:
10.5gr CPH - 5.6fpe
8.44gr JSB - 7.4fpe
7.9gr JSB - 7.6fpe
7.33gr AA - 7.9fps
Short barrel:
7.9gr JSB - 8.0fpe
So the long barrel is not the cause of the low power and the long barrel appears to be a little more accurate so it stays. I used a heavy steel barrel bushing and a weighted stock with the short barrel so it was not even a good comparison. I want this rifle to be light weight, so I may balance it a little, but decided that the ballast was just wrong when it conflicted with the design goal. I removed the weights and put the long barrel back on.
Something in the power plant has to be the issue. I'm running no lube so I thought maybe it was friction. I tried a couple of different lubes thinly applied. It started at 790fps, stayed around 720fps for a number of shots, but once any dieseling settled out, it really had very little improvement. So I removed all lubrication.
I did notice that the face of the piston head still looked too perfect. Other aluminum piston heads that I have used usually show some evidence of impacting the head of the chamber. The missing evidence could indicate that the piston is always bouncing and never making solid contact with the head. Maybe the piston is too light. The fact that heavy pellets do poorly also indicates this. I decided to dry fire it. I did it a lot. Dry firing feels about the same as shooting pellets, maybe even less jump. No crash, no clank, just a crack of supersonic air. It sounds like a pop gun. I bet that it would perform well with PBA pellets (I don't have any yet).
So I have two issues with the piston assembly. I need to think about the design and redo the piston and sear ring. The moving mass of the gas spring is about 3 oz. The current weight of the aluminum piston is 1.9 oz. A full diameter steel piston will be about 6 oz. Total current moving weight is about 5oz, and the new moving weight will be about 9oz. So if lack of piston weight is the problem, that should be enough to make a difference. I wonder if there is a close correlation between total piston mass and FPE.
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I am wondering if the taper at the front of the piston and the matching taper on the breech block is limiting the flow as they are coming together and mating up would be interesting to try a more traditional flat topped piston and breechface and leave everything else the same and see what happens,
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Good update. I feel very fortunate that you are working out these bugs and sharing with us. Everything you are going thru, I see as invaluable to my project and I'm learning a lot as you move forward. I kind of feel like a leach. :)
Deformation of the sear ring. How wide is you sear and area of you engagement patch? It seems like an unlikely spot for trouble but the DOM steel may have a poor compressive strength and the case hardening may have worked against you by annealing the underlying material. Did you use a carbon additive?
I don't think that piston mass and fpe necessarily go together. Generalizing, I expect the lighter piston to bounce more for a given swept volume. This may be the most important correlation ( weight/swept vol.), of course the spring rate, pellet mass and transfer port all play a role. With the dead blow piston being the holly grail, I suspect bounce to be the major killer of efficiency over all other factors.
The pba amo is a interesting idea. I have use the gold ones and found them to be very consistent in diameter and accurate in 22 cal.
I also wounder about an even bigger transfer port to get things moving sooner.
Tom
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I am wondering if the taper at the front of the piston and the matching taper on the breech block is limiting the flow as they are coming together and mating up would be interesting to try a more traditional flat topped piston and breechface and leave everything else the same and see what happens,
The idea for that cone and taper was to maintain flow while reducing head space. I don't see the problem but I could be missing something.
...How wide is your sear and area of you engagement patch?...
...and the case hardening may have worked against you by annealing the underlying material. Did you use a carbon additive?...
...With the dead blow piston being the holly grail, I suspect bounce to be the major killer of efficiency over all other factors...
The pba amo is a interesting idea. I have use the gold ones and found them to be very consistent in diameter and accurate in 22 cal.
I also wonder about an even bigger transfer port to get things moving sooner.
Tom
The sear engagement patch is narrow as it is a Chinese copy of a Gamo/Theoben style trigger group.
You are probably correct about the annealing because it is fairly soft in the middle and hard on the surface.
I want to get the lightest piston that will give me a dead blow with the 7.9 JSB pellets and I don't really care about the other weights. Maybe the 7.33gr if I fall short of my FPE goal.
I have found the PBA pellets to be inaccurate but I never shot them through a choked LW barrel. They should have a good shot cycle with this rifle since so far it gets it's best shot cycle with straight air. If I want to shoot the 7.9 better, I think I need a heavier piston to slow the shot cycle down and deliver the piston energy/momentum over a longer period of time.
My transfer port is already .161 diameter. If I go much bigger, a pellet could fall into the chamber.
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I did an interesting test today with the Scotchmo Express rifle. I filed down some pellets to make them lighter. The 10.5gr test was from earlier and is included for comparison. Here are the results of the test:
10.5gr - 490fps - 5.6fpe - 18% efficient - 3.1fpe/ci
7.9gr - 650fps - 7.6fpe - 24% efficient - 4.2fpe/ci
6.5gr - 750fps - 8.1fpe - 25% efficient - 4.5fpe/ci
6.0gr - 800fps - 8.5fpe - 27% efficient - 4.7fpe/ci
5.0gr - 890fps - 8.8fpe - 28% efficient - 4.9fpe/ci
4.5gr - 960fps - 9.2fpe - 29% efficient - 5.1fpe/ci
I see a definite trend. I obviously undershot my piston weight by a bit. There is at least 9.2fpe available. I just ordered some 4.0gr Skenco pellets to test. I'll even make these lighter until I reach the point where power and efficiency finally stops rising. Then I will increase the piston weight until the maximum power and efficiency coincides with a 7.33gr or 7.9gr pellet.
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Looking good, I like your testing method!!!
It might be awhile before I start my scotchmo springer project, The two Items I don't have are the 1.25 diam pipe and the the square tube, I have plenty of aluminum and steel for the other parts.
Keep up the good work!!
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Sounds like a good testing method.... looking forward to seeing how it works out....
Bob
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I was thinking about your data set and I believe it shows that the pellet mass has a larger effect on efficiency than the common consensus. I would have expected to see much less performance increase as the pellet gets lighter than the nearly linear relationship plotted and expected the stickion of pellet release to start to show its self as diminishing returns well before the 4.5gr mark.
29% efficiency in "pop gun" mode is pretty darn good at this point in development. A number that some guns don't achieve with combustion.
Tom
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What a neat project to follow! Next you will be building your own PCP :-)
Curious on the trigger - have you considered a Marauder trigger unit? The way the sear operates should be compatible with your piston design - it's quite a nice trigger. I know it would add cost but might be worth it to really fine tune the gun. I don't know design-wise how much stress it would hold up to though since on the Mrod it's expected to hold back far less spring force.
-Casey
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What a neat project to follow! Next you will be building your own PCP :-)
Curious on the trigger - have you considered a Marauder trigger unit? The way the sear operates should be compatible with your piston design - it's quite a nice trigger. I know it would add cost but might be worth it to really fine tune the gun. I don't know design-wise how much stress it would hold up to though since on the Mrod it's expected to hold back far less spring force.
-Casey
Casey,
The trigger will be addressed at some point. I would not feel comfortable with a PCP specific trigger. The current trigger has the basics of a decent design. The execution is a little sloppy but it works for now. There are two-stage trigger blades (CDT's GRT) that fit this style trigger group. I have put one on my TF58 and it is a big improvement. But this rifle has a fairly vertical pistol grip and really needs a more vertical trigger blade. I may get a GRT or I may make a custom blade for it.
-Scott
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hey-Hey!!!,
Instead of mounting an accelerometer to the piston, consider measuring piston speed. Machine a tone wheel( or more appropriately a tone rail) into the piston skirt and then catch the bumps/holes as they go by. The measurement stuff would then be stationary and not adding to piston mass or complexity( I can't warm up to the idea of wires comming out of the reciever, or believe that the connections will be reliable if they're anything resembling inexpensive ).
cheers,
Douglas
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Douglas,
Can you elaborate a little about the tone rail set up and how that could be done? Do you need some sort of DAQ card, or interface to a laptop to go along with a special software application?
Thanks,
Lloyd
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Not familiar with the tone rail. The term does make my imagination run wild though :) I chose mems accelerometers because I have experience with them . Also I have several several designs that are be easily convertible to this purpose with some simple software changes. The mass is very small (less than the metal that will be removed for the pocket), only 3 very small (30ga) wires are needed and be and simply looped inside the piston. This setup is for analysis only and never needs to be practical. The ultimate would be some kind of non contact linear sensor that could be mounted on the outside of the compression tube but I'm not sure what that would be. Maybe a tone rail?
Tom
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Some sort of linear hall effect sensor with a magnet in the piston? a linear positioning sensor?
Maybe I will ask the guys at work in the environmental testing lab. The do all sorts of shock and vibe testing.
-
prehasp a mrod trigger ? its got a trigger with a very slight curve to it and its match grade.. feels like a grt3 that i had on my titan
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Having accelerometers on the pistons would be a great diagnostics tool. If we also had a pressure transducer in the chamber, we would be set. The equations for what should be happening get a little hairy. And there are some big losses involved and I really don't know how those losses are distributed. When everything is working right, there are a couple of points in the shot cycle where it should be easy to calculate the conditions in the chamber. The point at which the pellet "pops" and the point when the piston comes to rest. I think if we compare the expected data to the sensor data, we could figure out where the losses are actually occurring. That would be invaluable in the quest for greater efficiency.
QVTom has a dual piston rifle design so I'm guessing that he wants to check the timing of both pistons. But on a single piston rifle, even an accelerometer mounted on the rifle receiver could provide some useful data.
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Scotchmo,
If you are really serious about a pressure transducer, and are reasonably comfortable with sensor -type electronic circuits, PM me. It will have to be interfaced to a fairly fast micro processor. Can be done with a 10-32 tapped hole.
Lloyd-ss
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Lloyd-ss,
I know the basics but not really comfortable with it. I'll leave it to the ELs or ELtechs for now. When/if I want it bad enough, I'll attempt it.
-Scott
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I have been running some rough numbers on adiabatic compression of air. Interesting. What if we have a piston that adiabatically compresses one cubic inch of air? We want that air to contain a certain amount of energy. What pressure does it take to attain 5fpe/ci? What about 6fpe/ci? And 7fpe/ci?
In reality, a piston gun is transferring energy from spring, to piston, to air, to pellet in an overlapping scheme so it is not as bad as it looks. But it shows why it may be very hard to push the air efficiency of a spring gun much over 6fpe/ci. Timing of the overlap becomes critical. At 5fpe/ci, the overlap can be very sloppy and still stay within an acceptable pressure limit.
I'll verify the numbers and post them later. I won't do it right away, so I hope somebody else can beat me to it.
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I have a very hard time trying to explain my ideas but I'm gonna throw this out there and hope to start the wheels turning.
A chrony is a timing device to measure how long it takes a projectile to travel from point A to point B, then it has a program to convert that time and known distance into FPS. If you didn't use the Sky Screens, you could use any kind of switch (probably need to turn on a transistor) for a input to start the time and stop the time. I'm thinking Pro Chrony here and using something other than the screens. If the distance traveled is not the same then the FPS would not be calibrated, but could be calculated, or just used as a relative measurement.
If you wanted to measure speed of the piston in a spring gun then a magnetic switch with a magnetic disc on the piston and two sensors on the outside of the cylinder might work (tube might need to be stainless) . It could be a very small port to measure pressure as soon as the piston moved, it could be differential pressure, any combination or switches for start and stop, trigger switch and pressure switch at the end of the stroke. The amount of pressure needed to make the switch would need to be set, just increase the pressure until it will not work then lower it slowly till it does....
Just thinking........
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I have been running some rough numbers on adiabatic compression of air. Interesting. What if we have a piston that adiabatically compresses one cubic inch of air? We want that air to contain a certain amount of energy. What pressure does it take to attain 5fpe/ci? What about 6fpe/ci? And 7fpe/ci?
In reality, a piston gun is transferring energy from spring, to piston, to air, to pellet in an overlapping scheme so it is not as bad as it looks. But it shows why it may be very hard to push the air efficiency of a spring gun much over 6fpe/ci. Timing of the overlap becomes critical. At 5fpe/ci, the overlap can be very sloppy and still stay within an acceptable pressure limit.
I'll verify the numbers and post them later. I won't do it right away, so I hope somebody else can beat me to it.
Probably with a loss at every transfer (spring to piston, piston to air) which could explain why pcp has an advantage not as much loss, just air to pellet.... 8) ???
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Probably with a loss at every transfer (spring to piston, piston to air) which could explain why pcp has an advantage not as much loss, just air to pellet.... 8) ???
The losses are pretty large in a piston gun. Only about 1/3 of the spring energy makes it to the pellet. We are losing about 67% of the spring energy.
The energy efficiency is important but I have also been thinking a lot about the air efficiency lately.
I spent an couple of hours reading a book from a link I found to an online copy of what looks like the predecessor to Cardew's "Trigger to Target". This earlier book was called "Trigger to Muzzle". The test equipment looks to be late 60's, early 70's era. But it is good research. It gave me some more insight.
Their test rifle had 5.9fpe, 1.964ci chamber volume, and a 20.4 ft-lb spring. That rifle is getting 29% energy efficiency and 3fpe/ci. And the peak pressure that they get is 1366psi. Their final graph shows an energy balance of the rifle throughout the shot cycle. That is the exact thing that is needed to diagnose what is going on. Their graph shows that at 7ms, virtually all of the remaining system energy is in the air volume. That is 16fpe. At that instant, the air is acting as a "container" for all of the energy. I don't think that I can have that situation in my rifle if I want to get over 6fpe/ci. The pressure would have to be very high. I hope I explained it clearly enough to gets some comments. So I'm thinking that there needs to be a much greater overlap of the pellet motion prior to the pressure peak. That way, the pellet is absorbing some of the system energy before the energy in the air climbs too high. Timing will be critical. Everything will need to be well balanced against each other. The lead-in that allows the pellet release will be critical. Maybe trying to time this overlap will be problematic. Even if it can be achieved, shot to shot variation may be large. I'm starting to question the wisdom of trying to squeeze more than 5fpe/ci from a piston gun.
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Scotchmo, I was trying to find Trigger to Target, can you read this online and if you can could you post a link ?
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Something to consider that might be part of the limiting factor.... There is a finite amount of FPE contained in a given volume of air that depends on the pressure.... and it's not even close to linear....
(http://i378.photobucket.com/albums/oo221/rsterne/Important/MaximumPCPEfficiency.jpg)
This graph is for Isothermal expansion, but that might be applicable once the energy is IN the air, as you talked about above.... Note that the limit is 6 FPE/CI at a pressure of 2000 psi.... That graph is based on the following calculator.... http://www.tribology-abc.com/abc/thermodynamics.htm (http://www.tribology-abc.com/abc/thermodynamics.htm)
I converted psi to MPA, used the calculator to get KJ, converted that to FPE.... Using the pressure drop (expanding to 1 bar) I then calculated the FPE/CI and graphed it using Excel.... I think you may be right that by starting the pellet moving while you are still adding energy to the air you MAY be able to tweak out a bit more....
Bob
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rsterne,
PCP's have a large volume of air and you need to use more of it in order to increase the energy per shot. Piston guns have a fixed volume of air and you have to use a higher pressure in order to get more energy per shot (at least according to Cardew).
If I use Cardew's model, the numbers don't look good. The Cardew graphical model shows a very sequential piston-air-pellet energy transfer. I'm thinking that modern piston guns have some overlap rather than completely sequential. And i hope to extend that overlap so that I can get the fpe/ci that I want without needing astronomical pressures.
Definitely non-linear in the pressures that we are dealing with.
I'll start another thread about this subject in the airgun forum so this one can get back on the build track.
Edit:
I can't let this one go yet. That Cardew graphical model that I was looking at has got to be wrong in at least one respect. I attached it here for discussion (hopefully not a copyright violation). I added a green dot where the pellet starts to move and that coincides with the blue dot that indicates the high pressure point. I just did a test by using a pound scale to initiate movement in a pellet that was seated in a barrel breech. The lowest pellet was a 7.33 JSB at 2 lbs. and the highest was a CPH at 8 lbs. The area of a .177 pellet is .0246 square inches. So even the heavy, tight CPH takes only 8/.0246=325psi to initiate motion. That pellet has got to be moving along at a good speed well before the Cardew's peak pressure is reached. I'm not going to start a new thread on this yet. Too many unknowns.
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The Skenco pellets came in today. I did a little bit of testing for FPE only. No accuracy tests. They are 4.0gr. and the lightest that I could find in production. They are very tight in the breech of the Scotchmo Express (SE). I have to use a flat bladed screwdriver to seat them. I included the prior tests for comparison.
10.5gr - 490fps - 5.6fpe - 18% efficient - 3.1fpe/ci
7.9gr - 650fps - 7.6fpe - 24% efficient - 4.2fpe/ci
6.5gr - 750fps - 8.1fpe - 25% efficient - 4.5fpe/ci
6.0gr - 800fps - 8.5fpe - 27% efficient - 4.7fpe/ci
5.0gr - 890fps - 8.8fpe - 28% efficient - 4.9fpe/ci
4.5gr - 960fps - 9.2fpe - 29% efficient - 5.1fpe/ci
4.0gr - 1030fps - 9.5fpe - 30% efficient - 5.3fpe/ci
The 4.0gr Skenco pellets definitely have a better shot cycle in the SE. They shoot smooth. I also tested the Skenco pellets in my TF58 for comparison. TF58 also has a red label N-Forcer gas spring. The TF58 has a 75mm stroke vs 59mm in the SE. Same PTFE piston ring. But the TF58 piston is about twice the weight of the SE piston. The Skenco pellets fit the TF58 breech with just light resistance.
7.9gr - 860fps - 13.0fpe - 32% efficient - 5.7fpe/ci
4.0gr - 1150fps - 11.9fpe - 29% efficient - 5.2fpe/ci
I doubt that these pellets are very accurate. I may check them for accuracy at some point. The data shows about what I expected. The TF58 with the heavier piston shoots a mid weight pellet with the highest efficiency. It also shoots the 10.5gr and 4.0gr almost as well. The SE with the very light piston shoots very light pellets with the best efficiency. I'm down to 4.0gr and still have not crested the peak.
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instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
super simple to make a crank too... just get you some scrap roundstock (delrin alu steel doesnt matter) drill you a hole the length of it, then turn it to just under the size of your headstock bore, then cut the pipe at a angle in the middle.. Get you another peice of scrap round square whatever you got make you a crank arm and handle then take a long bolt and bolt the whole thing together with the tube angles mated together... slide the whole thing into the bore of the headstck tighten the long bolt down and the angled ends try to slide past each other and grip the inside of the bore(Kinda like an old gooseneck on a bike would stay down into the forks) .... you now have a removable crank
kinda like this except they have a collet shown attached at awell
http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html (http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html)
just remember to REMOVE the crank before ou power back up!!!!!
Michael M,
I tried it last night for the same application as scothmo. Worked like a dream!!!
No more sweating about crashes or ruined parts.
The extra nuts on the right are just spacers for the long carriage bolt I had in the bolt-box.
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P1010006.jpg)
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P10100052.jpg)
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P10100012.jpg)
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Scotchmo,
Are you going to break down and machine a few Delrin pellets? Or is there anyway that we could stop you from doing it? You have identified a nice trend.
Lloyd
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Scotchmo,
Are you going to break down and machine a few Delrin pellets? Or is there anyway that we could stop you from doing it? You have identified a nice trend.
Lloyd
I thought about it. I don't need to now. If I want to try lighter than 4.0gr, those Skenco pellets would be easy to modify. I may shoot a couple of modified 3.0gr pellets tomorrow. The Skenco pellets are interesting. They say 40% metal so I expected a metal core. Not so. They appear to be a metal/plastic composite mixture. Probably zinc powder and maybe PVC or ABS. They are fairly accurate at 10yards but do 6 inch groups at 25 yards. BC is .0037 so they lose velocity fast.
I also tried some lead-free H&N FTT Green 5.56gr pellets. BC=.012. They were the best lead-free pellets that I have ever tried. Not as good as lead pellets but they are the first ones that I did not want to throw away after target shooting. I could hit the black most of the time.
The tight fit of some pellets in the breech of the Scotchmo Express got me thinking. I already have the materials for a couple different versions of the the heavier piston on order. And that should slow down the piston and give me more piston/pellet overlap. I originally thought that the tight pellets would be a good thing as that would let the pressure rise higher before the pellet is released. But if it is released too late, the light piston will rebound some and I lose FPE. So I spent some time on the barrel lead-in this morning. The idea was that if I open up the lead-in some, the pellet will release sooner, giving me more overlap.
The lead-in was originally on the tight side. I opened up so it is now on the loose side. It made a small but consistent difference:
Tight lead-in:
10.5gr - 490fps - 5.6fpe - 18% efficient - 3.1fpe/ci
7.9gr - 650fps - 7.6fpe - 24% efficient - 4.2fpe/ci
4.0gr - 1030fps - 9.5fpe - 30% efficient - 5.3fpe/ci
Loose lead-in
10.5gr - 537fps - 6.7fpe - 21% efficient - 3.7fpe/ci
7.9gr - 682fps - 8.2fpe - 26% efficient - 4.6fpe/ci
4.0gr - 1030fps - 9.5fpe - 30% efficient - 5.3fpe/ci
The loose lead-in gave me a 1.1fpe gain on the heavy pellets, a 0.6fpe gain on the mid-weight pellets, and no gain on the light pellets. The light pellets may be near maxed out and extending the overlap more may actually hurt them. When I go to the heavier piston, I expect to continue with bigger gains in the heavier pellets, some gains in the mid-weight, and maybe none with the light pellets. My goal is still to optimize it for the 7.33gr or 7.9gr pellets.
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Wow, very interesting results. You are collecting enough data for your own book. I imagine what your results show is that with the tight lead-in, the piston was bouncing back too early, and with the loose lead-in you have kinda given the heavy pellets a running start before the piston gets to the end of its travel, thereby being able to extract more of the spring energy. Nice.
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Wow, very interesting results. You are collecting enough data for your own book
Yes, quite impressive and coherent. The Cardew's wrote 2 books on airgun operation with lots of great practical experiments and analysis but Scothmo's data is far more telling and requires much less interpretation; the advantage of having a singular design goal
Tom
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This is for my fellow data crunchers.
I was determined to find the limit of how low I could go on pellet weight before FPE started falling. I ground down the heads on some Skenco pellets.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/skenco-00.jpg)
4gr Skenco pellets
1027fps
1021
1033
9.4fpe
3gr pellets
1132fps
1107
1144
8.5fpe
2gr pellets (loud crack = supersonic)
1280fps
1288
1287
7.3fpe
So the limit is around 4gr. This is the logic that I will use to increase the piston weight. 4gr pellets have the most energy and they can accelerate twice as fast as a 7.9gr pellet when the same force is applied. So I'm going to cut the acceleration of the piston by about half in order to more closely match the 7.9gr pellets. That means a doubling of the total moving mass of the piston. That will mean the relative overlap of the moving piston and moving pellet will now be optimized for 7.9gr pellets. That should get the power peak closer to 7.9gr pellets. I'm hoping it will also give me a little more power, but I have not figured out where it would be coming from, so a little bit of wishful thinking is involved.
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It will be VERY interesting to see if your "twice as heavy" piston indeed results in putting the peak of the power curve at, or very close to, 7.9 gr.... I'll be looking forward to those tests....
Bob
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!!! thats the beauty of hand turning your threads! and its such a great "junk drawer" mod.
Glad that something I picked up has been of help to two people I admire for their smarts and machining know how!!! you can send my complimentery disco double tube to.... LOL just kidding :)
instead of running the threads under power on the lathe you can make you a removable crank that attaches to the rear of the headstock so that when you want to thread you can just attach the crank handle set your compound and select the right gears for your thread.... then you just set your halfnut andstart cranking when you get to the end of your thread you back the compound off a tad reverse crank the handle till your carriadge is back to the beginning run the coupound back in and start cranking again...... basicaly everything you would do under power but your doing it manual and you NEVER disengage your half nuts so you never loose your index with the thread your cutting..... and no need for the threading dial..
super simple to make a crank too... just get you some scrap roundstock (delrin alu steel doesnt matter) drill you a hole the length of it, then turn it to just under the size of your headstock bore, then cut the pipe at a angle in the middle.. Get you another peice of scrap round square whatever you got make you a crank arm and handle then take a long bolt and bolt the whole thing together with the tube angles mated together... slide the whole thing into the bore of the headstck tighten the long bolt down and the angled ends try to slide past each other and grip the inside of the bore(Kinda like an old gooseneck on a bike would stay down into the forks) .... you now have a removable crank
kinda like this except they have a collet shown attached at awell
http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html (http://homepage.mac.com/bhagenbuch/machine/pages/spindlecrank.html)
just remember to REMOVE the crank before ou power back up!!!!!
Michael M,
I tried it last night for the same application as scothmo. Worked like a dream!!!
No more sweating about crashes or ruined parts.
The extra nuts on the right are just spacers for the long carriage bolt I had in the bolt-box.
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P1010006.jpg)
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P10100052.jpg)
(http://i226.photobucket.com/albums/dd79/loyd500/Disco%20Double%20Airgun%20Lab/Router%20fixture/P10100012.jpg)
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I made the steel piston yesterday.
Mass:
1.8 oz. - AL piston
5.3 oz - STL piston
That seems a lot heavier but it is really the total moving mass that counts. The spring contributes a significant amount. The gas spring that I use is about 5.2 oz. total weight. I use half of that number to estimate the moving mass of the spring. Part of it is stationary, and part is moving. So it contributes about 2.6 oz.
Total moving mass:
4.4 oz. - AL piston and gas spring
7.9 oz. - STL piston and gas spring
That is about an 80% increase in mass. I was going to go for about 100% but this is close enough for the next round of testing and it did not require any design changes, just material changes. The new steel piston has the groove for the skirt ring. It is a PTFE ring to guide the rear of the piston without needing lube. I had to cut it off of the aluminum piston early on because of a design error. I did end up using some lube in the gun. It still needs it in areas outside the chamber. The compression chamber slides in an aluminum receiver. Steel sliding on aluminum can start galling and then it just snowballs. A little common chassis grease there seemed to smooth things up a lot. Actually lithium based EP grease which is common these days. I also used a little 60% moly on some high load contact areas in the cocking and trigger mechanisms.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-04.jpg)
After a number of dry fires and shooting a lot of ultra light pellets, there is finally some evidence of piston bottoming (slamming?). Not an unusual amount though. The aluminum piston looks high tech but the blued steel piston looks ready for business.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-05.jpg)
This piston is made of CRS with a case hardened skirt. I ordered some tool steel and may make a tool steel piston later but I want to work the bugs out first. The tool steel is a little pricey and I don't want to waste it. I may also make up a brazed piston assembly with a tool steel sear ring. It will be based on a DOM tube. The current one piece piston takes too much drilling/machining when making out of a solid rod.
I will do some testing later today. Results will follow shortly.
Here are some other pictures for your reference. One shows three N-Forcer gas springs. The top one is the 63.5mm spring that I use in my 59mm stroke SE. The middle spring is the 80mm spring that I use in my 75mm stroke TF58. The bottom spring is a 125mm spring that will go in a TF87 that I have in the closet.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-00.jpg)
The N-Forcer springs are industrial grade die springs. Die springs are available from a number of different manufacturers. High quality, high price. Typically rated for 500,000 cycles when used in stamping dies.
Consumer grade gas springs are also available. I found one that may go in the spare TF58 that I have. These consumer grade springs are inexpensive and are probably what is used in the production "Nitro piston" rifles like the Crosmans. They cannot be recharged, disassembled, or rebuilt. The industrial grade springs can be had for about $40-$70. I got this consumer grade spring for about $12.
Industrial grade N-forcer on top, consumer grade Suspa on bottom:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-01.jpg)
N-Forcer on left, Suspa on right:
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-03.jpg)
Edit: I recently got around to trying that Suspa in the TF58. The specs did not say that it was damped but it is. So it is not going to work in an airgun.
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I just put 70 shots over the chrony using 7 different types of pellets.
First impressions. The shot cycle now feels a lot more like a typical spring gun. It is harsher. Still brief but not like before with the light piston. With the light piston, after a shot, it was like nothing had moved.
No extra power appeared out of thin air. Darn. Well a little with one type of pellet. I'll get to that later. The FPE was now much less dependent on pellet weight. As expected, the light pellets lost some power though not by as much as I would expect. The heavy pellets gained a lot. The middle weight pellets gained or lost a little depending on the pellet.
Light piston (taken from earlier results):
10.5gr - 537fps - 6.7fpe - 21% efficient - 3.7fpe/ci
7.9gr - 682fps - 8.2fpe - 26% efficient - 4.6fpe/ci
4.0gr - 1030fps - 9.5fpe - 30% efficient - 5.3fpe/ci
Heavy piston:
10.5gr - 603fps - 8.4fpe - 26% efficient - 4.7fpe/ci
7.9gr - 692fps - 8.4fpe - 26% efficient - 4.7fpe/ci
4.0gr - 973fps - 8.4fpe - 26% efficient - 4.7fpe/ci
FPE seemed to be affected more by the pellet fit. Here are some results from some other pellets that were not included in the original tests:
8.44gr - 652fps - 8.0fpe - 25% efficient - 4.4fpe/ci (a little loose)
7.33gr - 667fps - 7.2fpe - 23% efficient - 4.0fpe/ci (loose fit)
5.56gr - 889fps - 9.7fpe - 30% efficient - 5.4fpe/ci (tight fit)
The tight H&N Tin pellets gave the most power 9.7fpe. And the loosest, the AA 7.33gr were only getting 7.2fpe. The 7.33gr shot a lot better with the light piston. That makes sense. With a loose pellet and a slow/heavy piston, the pressure never rises very high. With a fast/light piston, the pressure rise could be more dependent on the inertia of the pellet to provide the back pressure.
If the piston is heavy enough and slow enough, the pressure will never rise above the pellet's break away pressure and that is not enough to produce much power. I can imagine a very heavy, very slow, perfectly sealed piston. And a loose pellet that still seals well. As the piston moves slowly forward, the pressure is never going to rise very high and the pellet will eventually just drop out the end of the barrel at a very low velocity.
My lesson learned for today:
Light piston - pellet weight is a big factor.
Heavy piston - pellet fit is a big factor.
I would bet that even though loosening up the lead-in helped the FPE for the light piston, it probably hurt the FPE with the heavy piston.
Except for some other types of loose and tight pellets, the curve is now flat. That may be a good thing since I can shoot a variety of pellets without sacrificing power. But I prefer the old shot cycle of the light piston with the light pellets. At this point I would choose the light piston. I may try an intermediate weight piston and hopefully get a good compromise that gives me close to 10fpe with the 7.9gr pellets. That is about the most that I'm expecting at this point. I'm going to shave an ounce at a time off of the steel piston. I am out of other tricks to try and squeeze 12fpe from a 28cc, non-combusting piston gun. I'm thinking that the next prototype will be a slight redesign that uses a 72-74mm stroke. That will give me 36-37cc displacement.
I may end up turning this first prototype into a pistol/carbine. I shot a field target match last weekend with my TF58. I also competed in the pistol match by removing the butt stock from the SE. I knocked down a few targets shooting open class. Not a great score but all of the other competitors were shooting PCP pistols.
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Am I correct that with the heavy piston, at 8.4 FPE, you are virtually right on 4.9 FPE/CI swept volume (28cc = 1.7 CI, right?)....
Bob
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Am I correct that with the heavy piston, at 8.4 FPE, you are virtually right on 4.9 FPE/CI swept volume (28cc = 1.7 CI, right?)....
Bob
Pretty close. I had some rounding errors in my math. Probably closer to 30cc displacement or 1.8 CI. Also, the velocity data sets are pretty small and the gun was rehoned and has new seals again. Things could change with more extensive testing but I don't see it changing much.
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Watching this thread closely. Fantastic work, Scotchmo.
Also interested in the gas-ramming of the TF58, especially with a consumer-grade gas-spring versus the commercial grade unit.
JMJ
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Scotchmo,
All your observations about pellet fit have me wondering about something I have been thinking about for a long time.
In a springer, the compression is controlled mainly by the pellet fit, transfer port size, and the weight of the pellet. I wonder if anyone has ever incorporated a little pressure relief poppet that dumps the compressed air into the breech at a consistent pressure? Or what about a mechanical method to open a valve at the end of the piston stroke? I have no idea if either of those thoughts have any merit, but it seems like the resulting gun might be almost like a little pcp. Or maybe not.
Lloyd
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Scotchmo,
All your observations about pellet fit have me wondering about something I have been thinking about for a long time.
In a springer, the compression is controlled mainly by the pellet fit, transfer port size, and the weight of the pellet. I wonder if anyone has ever incorporated a little pressure relief poppet that dumps the compressed air into the breech at a consistent pressure? Or what about a mechanical method to open a valve at the end of the piston stroke? I have no idea if either of those thoughts have any merit, but it seems like the resulting gun might be almost like a little pcp. Or maybe not.
Lloyd
I thought about that too. If it is a mechanically opened valve, then we end up with PCP that is charged by a spring powered pump. So I define a spring/piston gun as one which has a direct transfer port from the compressing air to the pellet. As soon as we put a valve in it and incorporate any kind of delay, it is a PCP. The pellet release timing is the closest thing to a valve we should have. Delay it long enough and the gun settles out before the pellet even starts to move. That would be cheating. Better just get a PCP.
A mechanical valve would introduce some inefficiency anyway. What about a pellet with a built in burst disc? On second thought - forget it.
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I shaved a little over an ounce off of the steel piston. The last tests were done with 7.8oz. of moving mass (5.2 oz. piston and 2.6 oz. from spring). The total moving mass is now 6.6oz. (4oz. piston). That still 50% more than the original 4.4oz moving mass with the aluminum piston (1.8 oz. piston).
It feels a little better now but not as good as the aluminum piston shooting 4.0 grain pellets. I guess if I wanted the best shot cycle possible, I could use a 1oz. piston and shoot Styrofoam pellets at 12fpe. Not very useful so I make compromises in the shot cycle/recoil in order to move a heavier piston and heavier pellet. I think I'm at a good compromise now at 6.6oz moving mass. I guess I could shave one more ounce off but I doubt that would help much.
This gun is not going to be a 12fpe rifle. I'm going to call it good at 9fpe. Based on what I had done with slightly larger chamber volumes, I thought that I could get at least 10.5fpe and maybe even 12fpe. However, I think that when dealing with such a small volume of air, things like head space and timing become much harder to deal with. It might still be possible or maybe there is a limit of about 33% efficiency from non-combuting .177 piston guns. I took an IC engine class in college and we studied Otto cycle (internal combustion piston engine) efficiency. Maybe it's a similar limitation. Increasing the compression ratio in the Otto cycle increases the maximum theoretical efficiency. Increasing the compression ration in a spring/piston gun increases air efficiency. But both IC engines and spring/piston guns have material limitations that prevent us from ever achieving much higher efficiency.
I could put in a higher pressure gas spring and probably get a higher FPE. But it's not free. Increased cocking effort, maybe need heavier piston meaning heavier recoil, higher trigger loads. I'm not going to go there.
Results of today's tests with 6.6 oz. total moving mass:
10.5gr - 583fps - 7.9fpe - 25% efficient - 4.4fpe/ci
7.9gr - 708fps - 8.9fpe - 28% efficient - 4.9fpe/ci (good groups)
4.0gr - 970fps - 8.4fpe - 26% efficient - 4.7fpe/ci (only shot 2)
The H&N FTT Green are still shooting good though down a little from when tested with the heavier piston:
5.56gr - 867fps - 9.3fpe - 29% efficient - 5.2fpe/ci (decent groups for a lead-free pellet)
I also tried a variety of other pellets. This gun does not shoot one of my favorite pellets very well. The 7.33 AA pellets. But I was pleased with these two:
Crosman Pointed - BC tested at .020
7.4gr - 736fps - 8.9fpe - 28% efficient - 4.9fpe/ci (good groups)
RWS Hobby Wadcutters - BC tested at .011
7.0gr - 760fps - 9.0fpe - 28% efficient - 5.0fpe/ci (really good groups at 25 yards)
So now I have a few different pellets in the 5.56gr to 8.44gr range that shoot well at about 9fpe. I plan on shooting it in Hunter Class at the CASA match this coming weekend. I will use either JSB 7.9 pellets or the Crossman Pointed. Right now, it is put together using anti-seize on all of the threads to make it easy to keep taking apart. But it starts to shake apart after about 50 shots. Tomorrow i will disassemble it completely, clean it, and reassemble with purple thread locker.
I also decided that I need to avoid the feature creep that was taking the rifle away from it's intended design. I took off the big scope and put on a Bug Buster. I also removed any extra trim like the extra Weaver rail, fore-grip, and knee rest. It is now 6.5lbs scoped and ready to shoot. And it will lose a couple more ounces when I change the design back to a 10 inch barrel.
If I relax on the power goal, the design meets my expectations in most areas and even exceeded it in some. And I learned a lot from this exercise. I plan on doing some more builds from what I learned.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/SE-04.jpg)
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Congratulations on a fine project and learning experience.... You should be very proud of the results....
Bob
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Congratulations on a fine project and learning experience.... You should be very proud of the results....
Bob
Bob,
I have appreciated your feedback, comments, and insights along the way. I have noticed you are good at graphs and must have some program handy. Can you do one graph to show three overlaid curves of different colors with the following data?:
4.4 oz moving mass
10.5gr - 6.7fpe
7.9gr - 8.2fpe
4.0gr - 9.5fpe
6.6oz moving mass
10.5gr - 7.9fpe
7.9gr - 8.9fpe
4.0gr - 8.4fpe
7.9 moving mass
10.5gr - 8.4fpe (use 8.35fpe to get a curve)
7.9gr - 8.4fpe (use 8.44fpe)
4.0gr - 8.4fpe (use 8.35fpe)
Pellet weight on X-axis, FPE on Y-axis.
I think it will visually show how piston mass affects optimum pellet weights. Unfortunately, that 7.9oz data set looks flat, but it really is a slight curve when I throw in some other pellets. The resolution of the data is just too low. Assume rounding error so adjust slightly as shown.
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Here you go....
(http://i378.photobucket.com/albums/oo221/rsterne/Springers/ScotchmoExpress.jpg)
Bob
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Thank you Bob,
Can you make some adjustments for me.
Start the pellet weight at 0 and FPE at 0.
Add this data point to each curve:
0.0gr - 0.0fpe
And these points for the 4.4oz:
3gr - 8.5fpe
2gr - 7.3fpe
And go left to right for 0.0gr to 10.5gr
Scott
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Here are two graphs, with and without smoothing....
(http://i378.photobucket.com/albums/oo221/rsterne/Springers/ScotchmoExpress2.jpg)
Bob
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It will be VERY interesting to see if your "twice as heavy" piston indeed results in putting the peak of the power curve at, or very close to, 7.9 gr.... I'll be looking forward to those tests....
Bob
The graphs help to visualize what might be happening. It is obviously not a direct relationship for optimum pellet weight/piston weight.
I originally went with the following assumtion:
If the optimum pellet weight (PEL0) is found for the light piston (PSTN0), I could I could find the optimum piston weight(PSTN1) for a heavier pellet (PEL1) by using:
PSTN0/PEL0=PSTN1/PEL1
That does not appear to be right. I now think it might be more like:
(PSTN0 x PSTN0)/PEL0=(PSTN1 x PSTN1)/PEL1
If the pellet has an abnormal fit or the gun has a transfer port restriction, it may skew the actual number. But I think I'm on the right track now.
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I made the steel piston yesterday.
Mass:
1.8 oz. - AL piston
5.3 oz - STL piston
That seems a lot heavier but it is really the total moving mass that counts. The spring contributes a significant amount. The gas spring that I use is about 5.2 oz. total weight. I use half of that number to estimate the moving mass of the spring. Part of it is stationary, and part is moving. So it contributes about 2.6 oz.
Total moving mass:
4.4 oz. - AL piston and gas spring
7.9 oz. - STL piston and gas spring
That is about an 80% increase in mass. I was going to go for about 100% but this is close enough for the next round of testing and it did not require any design changes, just material changes. The new steel piston has the groove for the skirt ring. It is a PTFE ring to guide the rear of the piston without needing lube. I had to cut it off of the aluminum piston early on because of a design error. I did end up using some lube in the gun. It still needs it in areas outside the chamber. The compression chamber slides in an aluminum receiver. Steel sliding on aluminum can start galling and then it just snowballs. A little common chassis grease there seemed to smooth things up a lot. Actually lithium based EP grease which is common these days. I also used a little 60% moly on some high load contact areas in the cocking and trigger mechanisms.
(http://i1246.photobucket.com/albums/gg607/Scotchmo1957/piston-04.jpg)
Scothmo, The PTFE ring on the skirt, is this something you made or off the shelf? What is the profile? How much clearance are you running on your piston skirt to compression chamber wall? Any thoughts if the PTFE ring reduces drag and probable lifespan?
Sorry, lots of questions but It looks like an interesting idea and may help reduce the noise signature of a spring gun. Considering for my project.
Tom
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Scothmo, The PTFE ring on the skirt, is this something you made or off the shelf? What is the profile? How much clearance are you running on your piston skirt to compression chamber wall? Any thoughts if the PTFE ring reduces drag and probable lifespan?
Sorry, lots of questions but It looks like an interesting idea and may help reduce the noise signature of a spring gun. Considering for my project.
Tom
It is a standard hard Teflon "o-ring". Available from McMaster. I have run them in place of Viton o-rings for the piston seal also. But you need to use a floating pneumatic gland design since they do not like to have any squeeze on the cross section. When used as a piston seal, you rely on peripheral "squeeze" and a polished cylinder wall. They are lower friction than Viton, but don't seal as well. Sometimes they work better, sometimes they don't. Probably best to stick with Viton for the seal but Teflon for the guide.
For a guide on the skirt, I run it in a groove that holds the outside diameter of the ring out about .01-.02 larger diameter than the skirt. And then a couple or a few thou clearance to the receiver. That way there is no metal to metal contact.
My rifles use a separate compression chamber that rides in the receiver so the only thing that rides in the chamber is the piston head. The piston skirt is a larger diameter and rides in the receiver tube. The skirt diameter is about the same as the outside diameter of the sliding compression chamber.
The Teflon seal in my TF58 has at least 2000 shots through it and still going strong. As a skirt guide, Teflon should last a very long time.
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Wow - Very interesting project and data, guys!
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I competed in the Western States Field Target Championships in Grants Pass, Oregon this last weekend. I shot my TF58 in the rifle match and did OK. I had a lot of trouble the first day with all of the uphill shots on the lake course but I was competitive on the second day in the woods course.
In it's present form, the SE does not really have the power for the rifle competition. So I removed the butt stock from the SE and shot it in the open pistol competition with a 3-9x Bugbuster. I actually surprised myself and shot a 26/38 score. I was the only spring/piston pistol. I beat a couple of the other pistol competitors that had PCP pistols and 12x scopes.
It was all great fun and it gave me some ideas of what version 2 of the Scotchmo Express should be like.
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sounds great !! Glad to hear ya took it to a contest. Keep up the good work
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Scotchmo,
Congrats on the pistol competition with the SE! Sounds like you carried the flag well for personally engineered and built springers. Man, I am way too chicken to try anything like that, LOL.
Good job!
Lloyd
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I love to do the design and drawings, but darn if they ever get made... I have plans for so many muzzle breaks and sidewheels and rifle stocks... I don't do machining. And when I try to get someone talented with what I cant do, I have little luck finding them.
How do you go about finding a willing machinist ??
Richard
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I love to do the design and drawings, but darn if they ever get made... I have plans for so many muzzle breaks and sidewheels and rifle stocks... I don't do machining. And when I try to get someone talented with what I cant do, I have little luck finding them.
How do you go about finding a willing machinist ??
Richard
Richard,
Don't I still owe you something from a long time ago?
Shoot me a PM if that is the case.
Lloyd
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How do you go about finding a willing machinist ??
It has been my experience that there are many willing machinists, but very, very few inexpensive ones !
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How do you go about finding a willing machinist ??
It has been my experience that there are many willing machinists, but very, very few inexpensive ones !
Well part of the problem is the set up time for making stuff. Just as something simple as drilling holes on some pieces can't take time to get it set up. Then there is the cost of tooling and supplies, freaking expensive.
But if you hunt around there are some who will do it for half what others will do it for.
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Richard,
Don't I still owe you something from a long time ago?
Shoot me a PM if that is the case.
Lloyd
Ha! Lloyd... yup. I was just looking at that gun a day or two ago...Still setting up my desktop that has the plan! I'd be glad to get that piece to complete the gun!
Richard
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It has been my experience that there are many willing machinists, but very, very few inexpensive ones !
I wasn't necessarily looking for an inexpensive one...
Richard
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It has been my experience that there are many willing machinists, but very, very few inexpensive ones !
I wasn't necessarily looking for an inexpensive one...
Richard
Well, the phone book is full of "if you've got the money, we've got the time" machine shops, but watch out for "pirates" in those waters !
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Well, the phone book is full of "if you've got the money, we've got the time" machine shops, but watch out for "pirates" in those waters !
Pirates? After you invest a few hundred thousand dollars on equipment, a couple thousand yearly on liability insurance and more thousands on tooling, electricity, EPA number, worker comp, local property taxes, etc. One might be better off to invest in a ship and become a real pirate :) Providing goods and services for an agreed upon monetary exchange is hardly pirating. Let's get real. :)
Tom
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Agree with QVTom. The up front costs in machining are fairly steep. Lucky for me, I have access to a fully stocked machine shop at work and a friendly master instrument maker for guidance and advice. I mind my Ps & Qs, take it easy on the machines, and clean up after myself and I'm pretty much left alone when I'm in the shop. Since I design a lot of fixtures & prototypes, I'm allowed to fab a lot of stuff myself versus putting in a job request. No better feeling than making chips and ending up with stuff you made yourself, design to completion.
JMJ
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I've been asked if I do machine work for hire and I always say no. I have machined a few things for friends, no charge. I don't like to machine on someones existing parts because I do make mistakes.
I'm slow and I would be lucky to get minimum wage if I was making parts for others. When they are for me, it does not matter, because I enjoy making them and seeing a my projects come together.
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I agree with Scotchmo 100%.... that's why I only make stuff for myself....
Bob
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I too agree with scotchmo and rsterne. A lot of work goes into machining, easy to mess up your own stuff and hard to mess up someone stuff that your being paid to make.
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Piston gun power/recoil factor - my new term for another aspect of spring piston gun performance.
Recoil factor is used in powder burners to quantify the recoil in a given rifle. The closest equivalent in a spring piston gun would be piston-recoil/rifle-weight. But you can always add more rifle weight to get the number down. That makes the rifle less handy. I think a better measure of "shootability" would be FPE/piston-recoil. Increasing the FPE of a piston gun is good when the increase in FPE is larger than the increase in recoil. Piston gun power/recoil factor = PRF.
PRF = FPE/(piston mass x stroke)
If you can maximize the PRF and energy efficiency of a spring gun, you should have a smooth shooter. I've come to the conclusion that even though you can push the air efficiency of a spring gun over 5fpe/ci, it may be a losing proposition. Especially in a gun with a small swept volume. My next iteration of the SE rifle will have a 25% greater swept volume than this first version. I will not try to push the air efficiency over 5fpe/ci, but I will try and get 33% energy efficiency.
Currently I'm using the steel 6.6oz piston+spring mass in the first SE rifle. The aluminum piston gave 4.4oz piston+spring mass. The 6.6oz gives me about 9fpe vs 8fpe for the 4.4oz.
4.4oz piston & 2.3in stroke:
recoil energy = 4.4oz/16 x 2.3in/12 = .053fpe ft-lb (energy or fpe may not be the right term)
PRF=8fpe/.053fpe ft-lb=151
note: recoil is the shift in mass of the gun over a certain distance. Example: a 1 lb. piston with a 5 inch stroke will cause a 5 lb. rifle to recoil 1 inch.
6.6oz piston & 2.3in stroke
recoil energy = 6.6oz/16 x 2.3in/12 = .080fpe ft-lb (energy or fpe may not be the right term)
PRF=9fpe/.080fpe ft-lb=112
So which do you think is the better configuration? Another thing to consider; I can always increase the spring energy and the FPE and PRF will go up. But the energy efficiency will probably go down. Recoil will not increase but there will be additional dissipated energy in the rifle. That can translate into piston slam, reverberation, etc. and other undesirable characteristics. Spring guns have a lot of tradeoffs.
Some preliminary specs for the SE prototype 2:
Stroke = 75mm
Swept volume = 38cc (2.3ci)
Spring energy = 34fpe
Energy efficiency (goal) = 33%
FPE (goal) = 11.3fpe
PRF (goal) = 180
Weight, unscoped (goal) = 5lbs.
The specs are not as ambitious as the first SE prototype. I was optimistic on what I could do as far as pushing the performance envelope. That exercise humbled me some. The object is to make a very light weight, medium power, accurate, easy to shoot spring gun. The first prototype was partly successful. The second prototype will look pretty much the same as the first, but will have some refinements. I plan on starting the build for the SE P2 next month. I'm going to take my time on that one.
I added some afterthoughts in red.
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I like the idea of your PRF.... and I especially like the idea that the number is bigger when the factor is "better".... It just seems to make more sense to me that way....
Bob