GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Topic started by: nced on October 03, 2013, 11:57:17 PM
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In the past I've made custom springer pistons and even extended piston latch rods for TX200s, however I've always used a good adjustable die to cut the 10x1mm threads on the end of the o1 tool steel rods I use. After threading one end of the latch rods and machining the "catch" on the other end I harden and temper the "catch end". This has worked well for me however actually cutting a 10x1mm thread with a die takes careful heavy torque to turn the die.
Well.....I had the notion to practice cutting some threads on the end of a 10mm o1 tool steel rod so I viewed a bunch of YouTube "how to" vids, ground my lathe bit with the recommended 60 degree point and relief grinds, added the recommended change gears for a 1mm pitch, set the compound to 29 degrees, made sure the tool was on center of the 10mm stock that was mounted in my 4 jaw chuck and centered within .001 tir, slowed the rpm of my LatheMaster bench top lathe to min, touched the stock with the tool bit and set the cross slide scale to "0", advanced the compound .005, engaged the half nut when the "thread dial" hit "1" on the dial, then proceeded to cut a thread, the first pass was just fine.
After the first pass I disengaged the half nut, retracted the cross slide, moved the cutter past the right end of the stock, advanced the cross slide to "0", advanced the compound another .005, when the "thread dial" again rotated to "1" on the dial I engaged the half nut and took a deeper pass over the previous cut. Well....how about that.....the second pass actually followed the first pass cutting the thread deeper. This worked fine for several passes following the same procedure till (for some reason) the next pass didn't follow the previous cuts very closely messing up the thread!!! >:(
OK, I cut the messed up thread from my work piece, advanced the work and re-centered in my 4 jaw chuck and started all over again. This time I ended up with the full thread I was looking foe but I noticed that the thread was pretty ragged even though the last few passes were only .001 deep. ???
I'm wondering a couple things.........
1. How did the first threading attempt have a mis-aligned pass when I engaged the half nut at what I thought was "thread dial at 1"? I did notice that when the thread dial was at "1" the half nut didn't engage till the thread dial was a bit past "1". Is this normal?
2. Is the rough thread finish of the successful thread a product if the material (o1 tool steel) or poor grinding geometry (I thought I followed the YouTube instructions)?
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Ok it has been 45 years since trade school and cutting threads on a lathe. If I remember correctly the half nut has to engage exactly on the numbered line. As far as the roughness of the cut try some of that heavy cutting oil used for black iron pipe. Good luck
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Ed......the only thing I have run into that may have caused your first issue is the stock slipping in the jaws or possibly getting bumped between passes.
For problem #2......I would guess that maybe your sfm is on the slow side. Hard to say. It's normal to want to turn the spindle slow when threading due to the "rapid" disengage that is often necessary, but I get much better finishes running as fast as I can and being very quick on the disengage......lol.
If you are threading into a shoulder, you can cut a relief at the end of the thread to give you a bit of leeway on the disengage.
Mike
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Use the die to clean up the threads, after you get them close.
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I don't know about your lathe but some lathes you can not disengage the lead screw while cutting metric threads. What RPM selections do you have on your lathe?
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Ricky made an excellent point.
If that's the case on yours......you're going to have to stop the spindle at the end of the thread instead of disengaging the half nut.
Mike
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Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.
Threading makes a lot of tool pressure, especially on a small lathe. Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish. BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.
BobH.
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Use the die to clean up the threads, after you get them close.
That is a possibility since it would remove a lot of the pressure on the die thread cutters.
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Ok it has been 45 years since trade school and cutting threads on a lathe. If I remember correctly the half nut has to engage exactly on the numbered line. As far as the roughness of the cut try some of that heavy cutting oil used for black iron pipe. Good luck
Heavy cutting oil? I'm currently using Rigid thread cutting oil and I'll look for some "heavy stuff". I do have some chainsaw bar oil that's pretty thick stuff. ::)
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Ed......the only thing I have run into that may have caused your first issue is the stock slipping in the jaws or possibly getting bumped between passes.
For problem #2......I would guess that maybe your sfm is on the slow side. Hard to say. It's normal to want to turn the spindle slow when threading due to the "rapid" disengage that is often necessary, but I get much better finishes running as fast as I can and being very quick on the disengage......lol.
If you are threading into a shoulder, you can cut a relief at the end of the thread to give you a bit of leeway on the disengage.
Mike
LOL......I do use an undercut and I was even considering the purchase of a reducer to cut my spindle speed in half! LOL
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I don't know about your lathe but some lathes you can not disengage the lead screw while cutting metric threads. What RPM selections do you have on your lathe?
Well....my lathe is a LatheMaster bench top lathe that I bought years ago and it's larger than the normal minilathes weighing in at 200 pounds......still real light as far as lathes go. Here is a pic of the LatheMaster behind the Harbor Freight minilathe that I used in the beginning.........
(http://img.photobucket.com/albums/v734/wved/Machine%20Tools/7x148x14lathe.jpg) (http://smg.photobucket.com/user/wved/media/Machine%20Tools/7x148x14lathe.jpg.html)
Anywhoo......the LatheMaster has an "inch thread" lead screw and the stepped pulleys give 125,210,450,620,1000,2000 rpm.
I think I'll turn the threaded end my 10mm stock to 3/8" and change the gears for 24 threads per inch (3/8"-24 is real close to 10x1mm) and give it a try again. Perhaps this will work better with the LatheMaster inch lead screw for using the "thread dial". I tried cutting the metric thread by turning off the lathe without disengaging the half nut then reversing the motor.......problem was judging the amount of "over run" as the chuck was coasting to a stop so I attempted using the "thread dial".
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Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.
Threading makes a lot of tool pressure, especially on a small lathe. Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish. BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.
BobH.
Would there be any benefit to adding some top rake to the bit? I do know that when the compound is set at 29.5 degrees only the sharp "leading edge" is doing the cutting when advancing the compound for increasing the thread depth but I'm wondering?
As far as turning the spindle by hand......I've considered fabricating a hand crank that can be inserted in the spindle bore (only 3/4" for the Lathe master") and is of a "split collet design" with a draw bar that would get tightened to the MT3 part of the spindle. That way I could have a REAL slow cutting speed and a lot of torque from a long crank.
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Try a little relief on the top of the tool bit the back right corner should be the low spot. The cutting oil you are using should be good for the job may be add some grease to thicken it up a bit if it runs off the part un less you are using a flood type application.
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You said you have a thread relief. If you have a bar for threading ID's use that. Thread from the back side starting in the relief and thread coming out in reverse. This will allow you to run the RPM you need and be able to stop the chuck to manually run the chuck forward to get the tool back to the thread relief.
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You said you have a thread relief. If you have a bar for threading ID's use that. Thread from the back side starting in the relief and thread coming out in reverse. This will allow you to run the RPM you need and be able to stop the chuck to manually run the chuck forward to get the tool back to the thread relief.
Humm....I use a quick change tool post so perhaps I could put the bit in the holder upside down, adjust the bit height, and run the lathe in reverse to accomplish the same thing.
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[/quote]
Humm....I use a quick change tool post so perhaps I could put the bit in the holder upside down, adjust the bit height, and run the lathe in reverse to accomplish the same thing.
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I've done that before. Especially running brass, it throws the chips down instead of up and all over you.
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Ricky made a very good point, if your lathe was not designed as a "metric" lathe, engaging and disengaging the half nut is a game if chance. I have used small engine lathes most of my life, and on small critical parts, I have turned the spindle my hand. RPM can and will eat your lunch, if you can't control the spindle.
Threading makes a lot of tool pressure, especially on a small lathe. Typically, my last 2 or 3 passes are made without advancing the compound rest, this relieves the tool pressure and improves the thread finish. BTW, there is no substitute for a good cutting fluid that keeps a buildup from forming on the cutting tool. Keep the tool overhang to a minimum and avoid allowing the work to flex.
BobH.
Finally some single point threading success!!!
(http://img.photobucket.com/albums/v734/wved/Airgun%20Mods/LatheThread.jpg) (http://smg.photobucket.com/user/wved/media/Airgun%20Mods/LatheThread.jpg.html)
I changed the "change gears" for 24 threads per inch, took a piece of 10mm O1 tool steel, mounted and centered it in my 4 jaw chuck, then successfully cut a 3/8"-24 fine thread on one end. I don't know if it helped but prior to cutting the thread I annealed the rod by heating to "blue temp" and let it cool slowly. Using the 3/8"-24 thread will work out great for some custom R9 pistons I have in mind which will feature a screw in latch rod similar to a TX200 latch rod. I think that this success has given me the confidence needed to attempt a second feature of the custom R9 piston..........threaded in piston end plug. To do so I'll need to do an internal and a mating external thread so this old 67 year old dog is still learnin' some new tricks.
The cut threads aren't as smooth as I would like however they are uniform and the 3/8"-24 nut screws onto the thread without wobble! ;D Definitely a closer fit than a normal nut to bolt fit! The reason I've been using O1 tool steel is the fact that I can easily harden the "latch end" with a propane torch and oil for quenching, however it's not the easiest allow to machine!
Anywhoo.....thanks for the help y'all!
When cutting in Imperial thread with my lathe having an inch lead screw I had no problem cutting the thread by disengaging the half nut after a pass, moving the carriage past the end of the work piece, then engaging the half nuts after returning the cross slide to "0" and advancing the compound a couple thou. I also took the advice to make a couple final passes without and extra tool bit advancing. As mentioned, the thread is a BIT rougher than I would like, however it will be a good base for the LocTite to bond when the latch rod is installed in the piston!
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That looks good. You have about as good of a finish as you're going to get on tool steel.
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I cut a lot of shop made taps from O1 and your main issue is is speed. For 3/8 O1 you should be up around 900-1000 rpm.
Give it a try on a piece of scrap. The difference in finish will be day and night. What's happening right now is the material is tearing off. Have a look at it under a strong magnifying glass and you'll see the tears.
I know the thread chase is whipping past like a Ferrari but one of the other posters gave you a good tip. Flip the tool upside down and run in reverse. Then your not threading to a stop ....which can be real intimidating at first.
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I cut a lot of shop made taps from O1 and your main issue is is speed. For 3/8 O1 you should be up around 900-1000 rpm.
Give it a try on a piece of scrap. The difference in finish will be day and night. What's happening right now is the material is tearing off. Have a look at it under a strong magnifying glass and you'll see the tears.
I know the thread chase is whipping past like a Ferrari but one of the other posters gave you a good tip. Flip the tool upside down and run in reverse. Then your not threading to a stop ....which can be real intimidating at first.
OK....I'll give it a try. LOL.....at least I won't have to wait as long for the "number lineup" if I miss the "number lineup" on the threading dial the first time around! :o
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That's another good point.
Count the TPI on your leadscrew. It will typically be 8TPI
Any thread that is a multiple of 8 (like 24tpi)you can drop the hammer at anytime....no need to wait for your number to come up
Any other even TPI (10, 12, 14, 18...etc) you can engage at any division line
You only need to watch your p&q's on odd and half TPI
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That's another good point.
Count the TPI on your leadscrew. It will typically be 8TPI
Any thread that is a multiple of 8 (like 24tpi)you can drop the hammer at anytime....no need to wait for your number to come up
Any other even TPI (10, 12, 14, 18...etc) you can engage at any division line
You only need to watch your p&q's on odd and half TPI
Thanks so much for your good input!
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The hand crank idea, it could slide into the gear end of the spindle. You could use the binding technique used on older bicycles, the sliding wedge.
To clean up rough single point threads, I have resorted to using stainless steel tooth brushed to burnish the offending threads. Sometimes I have hit the thread tops with a strip of emery cloth. Both of these (fixes) were done with the work spinning.
BobH.
Would there be any benefit to adding some top rake to the bit? I do know that when the compound is set at 29.5 degrees only the sharp "leading edge" is doing the cutting when advancing the compound for increasing the thread depth but I'm wondering?
As far as turning the spindle by hand......I've considered fabricating a hand crank that can be inserted in the spindle bore (only 3/4" for the Lathe master") and is of a "split collet design" with a draw bar that would get tightened to the MT3 part of the spindle. That way I could have a REAL slow cutting speed and a lot of torque from a long crank.
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I'm a little late to this party. O1 is free cutting and should thread very cleanly, the two things that are working against you are your tool and the back gear. You may want to consider an insert style tool or a pre ground natl V form tool designed for threading. The full profile inserts are the best thing since sliced bread, they cut the top of the tread using a wiper that give a very clean thread. A standard V form tool always leaves a burr on the top of thread regardless of the compound rest angle and the best way is to top it with a fine file. Avoid the temptation to keep cutting deeper, top with a file after each finish pass and test with your gauge. The back gear. Small bench lathes with a back gear are capable of all kinds of crazy harmonics in the gear train that will destroy your thread finish. If at all possible reduce speed as far as possible using belts to avoid gear harmonics. Another obvious thing to check is that you half nut and lead screw are super clean, crud can prevent full engagement of the half nut a can cause tracking errors.
Tom
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I'm a little late to this party. O1 is free cutting and should thread very cleanly, the two things that are working against you are your tool and the back gear. You may want to consider an insert style tool or a pre ground natl V form tool designed for threading. The full profile inserts are the best thing since sliced bread, they cut the top of the tread using a wiper that give a very clean thread. A standard V form tool always leaves a burr on the top of thread regardless of the compound rest angle and the best way is to top it with a fine file. Avoid the temptation to keep cutting deeper, top with a file after each finish pass and test with your gauge. The back gear. Small bench lathes with a back gear are capable of all kinds of crazy harmonics in the gear train that will destroy your thread finish. If at all possible reduce speed as far as possible using belts to avoid gear harmonics. Another obvious thing to check is that you half nut and lead screw are super clean, crud can prevent full engagement of the half nut a can cause tracking errors.
Tom
Thanks for the added advice! Is this the type tool you're speaking..........
(http://img.photobucket.com/albums/v734/wved/Machine%20Tools/ThreadingTool.jpg) (http://smg.photobucket.com/user/wved/media/Machine%20Tools/ThreadingTool.jpg.html)
My lathe is belt driven, however the slowest speed I can get is 150rpm.
The "ragged threads" are indeed a puzzle because I often use O1 for other projects and it did cut clean EXCEPT when I single point threaded the stuff!
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Ed,
That would be the 60 deg. V from threading tool. One insert will do a range of threads. The advantage is you only need 1 or 2 inserts to do all possible threads and you get external and internals with the same tool holder. The disadvantage is that you will need to top threads to remove the burr raised by the V form type.
This style you can use full or V form profile inserts and is the industry standard. The numbered inserts (8-24) are full profile types, you can see that the point cuts the V and the flat tops the thread to the correct major diameter, each insert can cut only one pitch. The A, G and AG will cut a range of pitches, the difference is the root radius and depth of the V.
(http://i1147.photobucket.com/albums/o553/QVTOM/Junk/links001_zps099615fe.jpg) (http://s1147.photobucket.com/user/QVTOM/media/Junk/links001_zps099615fe.jpg.html)
Tom
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Speed can make threading on the lathe a real adventure. I had a Logan 10" for a while, and ended up making a crank for the outboard end, so I could shut the machine off before the end of the thread, then crank the rest of it by hand at my leisure. Finish may have suffered some, but threads got done, and one reason why clean up with a die or tap was needed. Threaded spindle doesn't allow for upside down/backwards/reverse set ups, unless you are a thrill seeker.
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Please listen to those who have advised that metric threads can not be cut straight off the threading dial! They can NOT
The gears that connect the lathe spindle to the lead screw do not realign for many many turns once registration is lost. Do the math!
The spindle MUST be reversed and the carriage backed away without loosing registry.
Heck, for many metric threads on a lathe with a metric lead screw, you still can't use a simple threading dial.
For short threads on a lathe where reversing is fast and positive, Just leave the half nuts closed for the entire sequence.
You CAN disengage the half nuts at the end of the thread when using a threading dial. At the end of the cut, withdraw the tool and opening the half nuts at the same time. Then stop and reverse the spindle rotation catching the same number on the thread dial to back out as you used going in by closing the half nuts. When clear of the work to the right, stop the spindle, put on your cut, then start up again going forward. repeat as needed. Simple!
When the thread starts really looking like a thread, run the die over the piece to complete the job , If steel, the work should be dripping with sulphur oil.
If alu. then kerosene or mineral spirits
For small threads, I'm in the habit of cutting three heavy passes single point and then going right to the die. Saves time and trouble
Or just cut imperial threads the way you are doing.
To help with where to stop, fit a travel indicator on a magnetic base to show the carriage movement.You will be able to stop withing .005 with a little practice (practice cutting air)
SKIP CARBIDE on that little machine. You have neither the speed nor rigidity to support carbide tooling.
HSS is your friend.
cheers
Cal
a ps for clarity. The KEY aspect of using the threading dial in lue of keeping the half nuts closed during the entire operation is that the dial is picked up on the same revolution going out as was used going in. That is, it takes many turns of the spindle before the number "1" comes around a second (or third etc) time on the threading dial. The lathe spindle must be stopped and then reversed to catch the spindle and feed screw timing on the VERY SAME orientation.
So after releasing the half nuts and withdrawing the tool, don't just let the spindle keep turning for such a period of time that the threading dial pointer might pass the reference again! Confusion and failure would result. Close the half nuts on the VERY SAME line of the threading dial in every respect.
I've never been good at explanations, here is a second version
http://conradhoffman.com/metricthreading.htm (http://conradhoffman.com/metricthreading.htm)
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Metric Threads are a pain on an Imperial feedscrew, plain and simple.
I have had the best luck flipping the threading tool upside down and threading "away" from the work on projects which require a defined stopping point (shoulders/snaps/steps) when turning metric threads. Even with a quick change gearbox and carefully backing the lathe into position for the next cut, it is a tedious and often error-prone process if you are used to single point threading towards the work using the thread dial. To help prevent issues, I type up a Method of Technique to follow as a script: a good habit to get into anyway for a multi-step part. Especially for tasks which are ordinarily rote but actually aren't, for reasons such as these. Better to change everything and follow a script instread of just relying on talent, experience, training (though those really help) and Memory (Memory is an awfully fickle "helper" when machining).
Example:
Material: 316 Stainless Steel.
1. Place material in chuck with minimum of 1 3/4" protrusion not to exceed 2"
2. Face material square and zero feed indicator.
3: Turn Major Diameter to 0.875" +0.001/0.003" for a minimum of 1-1/2"
4: Turn Middle Diameter to 0.8125" +/- 0.003" to 1.000" +/-0.005" feed distance.
5: Turn Minor Diameter to 0.745" +0.000/-0.003" for 0.500" +/-0.005" feed distance.
etc...
Mark off and notate results on each completed step. It makes it much simpler to return to the work, especially if interrupted or a problem arises. Many times, you can spot a procedural problem or find steps which be simplified while writing the MoT: saves you time, tooling and material. Just like machining "air" when setting-up the machine (another good practice), spelling out making the part on paper can save a lot of headaches before the chips start flying.
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You can also do like I have done before I had a machine with a thread dial and that is to never disengage the half nut during the cuts. Just make sure you go beyond the end of the workpiece when you start again to take up the backlash in the leadscrew/nut. Basically just back out the cross slide to take the tool out of the cut and reverse the spindle to get back to where you were. Then you never have to worry about missing the alignment again. It takes a little longer and threading to a stop is more difficult but it works. My newer lathe has a nice thread dial so I do it the normal way now. I will add some things that have helped me a lot. Got these ideas from Dave Matticks who is a super cool guy and a very experienced machinist. First thing is that I use a mighty midget dial indicator holder when I setup the threading operation. I put it on the ways and setup the pointer on the carriage apron. Then I set the tool on center with the compound at 29.5 degrees. I bring the cutter to the right edge of the workpiece and set zero on the cross slide and the compound. Then I set zero on the mighty midget dial indicator. Then I back off the carriage apron until the dial indicator reads .100 and reset zero on the dial. This puts the cutter 100 thousandths off the right edge of the workpiece. I then dial in say .005" of cutter engagement on the compound and start the spindle. Make a pass, disengage the half nut, back off the cross slide, move the apron carriage back to zero on the dial indicator and then setup for the next pass. The dial indicator makes sure you always start at the same point and you can see your progress on the other dials more understandably. This helps you to not loose your place and kinda sets up a process that makes the silly mistakes harder to do. This has worked for many a thread without issue as long as I pay good attention.
The thread you made I agree looks like it was tearing some. You can use a good thread file to clean up the threads if they get that way but make sure you stop tighter than looser because it will remove material more than you might think. A proper feed and speed is critical to threading and there are many calculators you can use to find it. A GOOD SHARP cutter properly ground is imperative. I have found that many of the commercial threading insert cutters kinda not so good on smaller lathes. A good cobalt or ground HSS cutter is better but you need to keep it sharp. Another thing Dave taught me was to get yourself an eye loupe or magnifying glass and visually inspect the cutting edge close up. What may appear to be sharp is often pretty crappy. Buy a set of hand help diamond hones and carefully hone the edges of your cutting tools before you use them. Even brand new inserts can benefit from a quick hone if you are careful about how you do it. Also on steel I find it is also important to not get too aggressive with cut depth, don't be afraid to advance .0025 instead of .005 on finish passes. The other side of that coin is if it is not cutting it is rubbing and that creates unbelievable heat at the cutter edge which will kill your cutter in short order, again SHARP tools....
Finally the idea of using a shoulder cut is a real good one that allows you to move faster safer. Relief cuts like this work well for that as does the upside down and backwards cuts as suggested earlier. I have done all of them and they are good suggestions. Good luck man. Peace
Pete
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You can also do like I have done before I had a machine with a thread dial and that is to never disengage the half nut during the cuts. Just make sure you go beyond the end of the workpiece when you start again to take up the backlash in the leadscrew/nut. Basically just back out the cross slide to take the tool out of the cut and reverse the spindle to get back to where you were. Then you never have to worry about missing the alignment again. It takes a little longer and threading to a stop is more difficult but it works. My newer lathe has a nice thread dial so I do it the normal way now. I will add some things that have helped me a lot. Got these ideas from Dave Matticks who is a super cool guy and a very experienced machinist. First thing is that I use a mighty midget dial indicator holder when I setup the threading operation. I put it on the ways and setup the pointer on the carriage apron. Then I set the tool on center with the compound at 29.5 degrees. I bring the cutter to the right edge of the workpiece and set zero on the cross slide and the compound. Then I set zero on the mighty midget dial indicator. Then I back off the carriage apron until the dial indicator reads .100 and reset zero on the dial. This puts the cutter 100 thousandths off the right edge of the workpiece. I then dial in say .005" of cutter engagement on the compound and start the spindle. Make a pass, disengage the half nut, back off the cross slide, move the apron carriage back to zero on the dial indicator and then setup for the next pass. The dial indicator makes sure you always start at the same point and you can see your progress on the other dials more understandably. This helps you to not loose your place and kinda sets up a process that makes the silly mistakes harder to do. This has worked for many a thread without issue as long as I pay good attention.
The thread you made I agree looks like it was tearing some. You can use a good thread file to clean up the threads if they get that way but make sure you stop tighter than looser because it will remove material more than you might think. A proper feed and speed is critical to threading and there are many calculators you can use to find it. A GOOD SHARP cutter properly ground is imperative. I have found that many of the commercial threading insert cutters kinda not so good on smaller lathes. A good cobalt or ground HSS cutter is better but you need to keep it sharp. Another thing Dave taught me was to get yourself an eye loupe or magnifying glass and visually inspect the cutting edge close up. What may appear to be sharp is often pretty crappy. Buy a set of hand help diamond hones and carefully hone the edges of your cutting tools before you use them. Even brand new inserts can benefit from a quick hone if you are careful about how you do it. Also on steel I find it is also important to not get too aggressive with cut depth, don't be afraid to advance .0025 instead of .005 on finish passes. The other side of that coin is if it is not cutting it is rubbing and that creates unbelievable heat at the cutter edge which will kill your cutter in short order, again SHARP tools....
Finally the idea of using a shoulder cut is a real good one that allows you to move faster safer. Relief cuts like this work well for that as does the upside down and backwards cuts as suggested earlier. I have done all of them and they are good suggestions. Good luck man. Peace
Pete
Thank you Pete! It's good getting feedback from you again! I had no idea that tool sharpness was so critical. I guess I was afraid that too sharp of an edge would fracture easily. Do you have any advice concerning back relief angles for a thread cutter?
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I gotta be honest the toolbit I use most is one that Mr. Dave Matticks the master hand ground for me and sent to me many years ago. I have tried my best to recreate it several times using my green silicon carbide wheel and the fixtures I made. I have come close but never equalled the cutting smoothness of the one he ground. Just goes to show you that sometimes it is the hands and not the tool. Basically it is a very simple tool and I just did a google search for something similar and found this link...
http://img.photobucket.com/albums/v30/jglass/toolbitb.jpg (http://img.photobucket.com/albums/v30/jglass/toolbitb.jpg)
That is basically the tool grind he did for me and it works very well on a smaller manual lathe. I have since bought a 12x36 lathe and use indexable cutters occasionally reverting back to the single point ground bits he and I did when all else fails. For a long time I was afraid to even use it because it was so nice LOL and I did not want to screw it up. I have used it more as a guideline for what works and tried to match it with my internal and external threading grinds. I think if you grind one like that one and keep it honed sharp you will find it lasts quite awhile and cuts smooth. The one he sent me was so sharp the edge felt like a fine knife blade or better. It was ground from a 10% Cobalt blank as I recall and it was only like a 5/16 blank. He did that because I used to own a shoptask 3 in1 machine and the lathe was much smaller than my current lathe. Just a great kind and knowledgeable fellow. Peace
Pete
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Oh yeah I should add that the very tip of this tool is NOT a point. It actually has a small radius on it that helps keep the tip from breaking off too easily. Peace
Pete
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Thanks Pete!
I printed out the bit drawing and it appears to be a "double ended bit"........pretty slick. I do notice that all of the relief grinds are 10 degrees and the "top face" of the "pointy end" also has a relief. Perhaps the reason I got a "raggedy thread" is that I ground my bit with no top grind at all, only clearance angle on the sides.
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Yeah man....the top grind is rather important. Another tip Dave gave me is to buy one of these things...
http://www.globalindustrial.com/p/tools/test-measurement/charging-scales/60-degree-american-national-and-u-s-std-center-gage?infoParam.campaignId=T9A&gclid=CInxyo7tm7sCFUsV7Aodzy8Aog (http://www.globalindustrial.com/p/tools/test-measurement/charging-scales/60-degree-american-national-and-u-s-std-center-gage?infoParam.campaignId=T9A&gclid=CInxyo7tm7sCFUsV7Aodzy8Aog)
Then when you are grinding your toolbit you can use this to ensure you are actually at 60 degrees. Not just a quick eyeball either take it and hold it over your head in a bright light and look for light coming thru the sides. This will ensure your grind is accurate. I still have this tool and it is useful for more than just that. You can also use it to setup your tool tip height on the lathe for parts that have diameters of around an inch and a half or less. Just put it on the circumference of the workpiece at the open end and then setup your toolbit on the point level to the bed. The angle of the toolbit can also be adjusted with it. Basically I put the lathe tool bit in the holder, bring it close to the edge of your aligned workpiece and using this tool pointing along the Z axis of the lathe insert the toolbit into one of the smaller notches on the sides and bring the other side of the measuring tool to bear against the edge of the workpiece. Then shine a flashlight underneath it and adjust the toolbit to be square to the notch angle. Easier done than described. It really is a very handy tool for so cheap. Good luck dude! Peace
Pete