Any Interest in 6mm ? - Build Thread

[FuzzyGrub]

I have often wondered about a long prod airtube to use for the AR style stock vs the adapters.   Thinking out how to do all the adjustments was one of my concerns.  Looks like you spent some considerable time thing about the solution.   

[rsterne]

Well the 7/8" OD tube on a PRod is MUCH too small, and although the 1.25" OD of the MRod tube is large enough to machine down to 1.15" to fit inside an AR stock, if you did that the wall thickness on the tube would only be 0.045", which I think is marginal.... If you used steel for the tube, you might have enough strength not to kink if you fell on the gun, but with my tube being aluminum I thought it only a suitable solution for the long tube, which is 0.120" wall instead of 0.095".... That leaves me a 0.070" wall after machining....

I think part of the problem with fitting an AR stock to a PRod is that with the smaller tube ID (less than 3/4") and the short distance available for an insert to adapt that to an AR stock.... it makes the attachment a bit wobbly.... You could probably drill and tap the sides of the adapter and use a total of 4 screws to eliminate that issue.... Alternately, as you say, you could have a custom longer tube made, and then sleeve it up to 1.15" OD using aluminum or even plastic so that the weight wouldn't increase too much.... Then you still have to deal with the adjustment and preventing rotation, but the solution I used would work for that....

Bob

[FuzzyGrub]

Yep, I have a number of the adapters and they all seem to loosen over time.  You can't make a permanent fix without addressing the adjustment issue.   Was thinking a larger tube over the prod but never took those thoughts any further. 

[rsterne]

The stocks wobble around over the buffer tube, so there is always slack there anyways.... part of having an adjustable stock....

Bob

[FuzzyGrub]

The stocks wobble around over the buffer tube, so there is always slack there anyways.... part of having an adjustable stock....

I have used a plastic shim between buffer tube and stock to help with that wobble.   On the Prod, with the added adapter length, the "collapsible" aspect is severely limited, so locking it down doesn't cause me issues.

[rsterne]

I made the front plug and fill fitting for the long tube today....



It is a piece of 1" diameter 2024-T3 that is 1.4" long.... It is drilled through 1/8" for an air passage, and then drilled and tapped for four 10-32 x 1/4" long SHCSs with the head in shear to take the loads.... There is a single O-ring groove for a #117 O-ring.... The front is drilled and tapped 1/8"-27 NPT for a male Foster, and the back is drilled out a short distance to 5/8" just to gain back part of the volume lost to the plug.... I drilled the four 5/16" retaining holes in the tube at a 45 deg. angle to the vertical, so that the front plug can be removed without removing the barrel....



I'm quite pleased with this, the next job is to ponder how to make the reversed tank block for the short tube to hold my 500 cc Carbon Fibre bottle under the tube....

Bob

[rsterne]

Today I tackled the job I hate the most, but possibly the most satisfying if it turns out well.... I turned the stub and rear face of the tank block.... The piece I started with was a 1-1/4 x 3-1/2" piece of 6061-T6 rectangular bar stock.... I roughed out the stub material with my power hacksaw (finished with a hand saw) and then cut a piece 2-3/4" long, ending up with an "L" shaped piece of aluminum.... The small part of the "L" is for the round stub that inserts into the front of the tube, so I had to mount it in the lathe to make that round and the correct diameter, and cut the O-ring groove.... In the same operation, you face the rear surface of the block flat square to the stub.... The trick was mounting it in my lathe....

I have a 4-jaw chuck, with reversible jaws, but with them in the normal orientation one jaw was right out past the chuck face, because the block has to be offset so that the round stub is at one end.... I reversed that jaw, which solved that problem, but then the block has to sit on top of the first ledge on the jaw, and I didn't like the idea of trying to shim the block at the other end, there is incredible load when turning this piece, and it must be very rigidly mounted.... or it could jar loose and come flying out of the lathe, or break something (and maybe me).... I came up with the idea of reversing the opposite jaw as well, so that the block would sit on both of them, but NOT reverse the side jaws, so that they had more clamping area.... it worked perfectly, as you can see below.... Top and bottom jaws reversed, side jaws "normal"....



Before putting the block in the chuck, I laid out and drilled a center hole for the stub, using a 60* center drill.... The by using a 60* center in the tail stock, pressing the block against the two reversed chuck jaws, it was pretty simple to get it centered.... I have never turned such a large, offset block before, and the impact with the tool bit is considerable if you take too big a bite.... so I used the slowest feed rate (0.004") and could only remove 0.005" per pass off the flat surface beside the stub.... I had left the material 1/10" oversize, so that took 20 passes at over 3 min. each.... well over an hour just to machine that flat surface.... Then I had to turn the 1-1/4" square stub that was sticking out to a 1.05" circular one, drill the center air passage (1/8") and cut the O-ring groove.... Between the setup and machining, it took all afternoon, and some of the evening.... I must be the world's  ~ s l o w e s t ~  machinist.... 

It was pretty nerve-wracking, so I will have a good sleep and tackle the rest of the block tomorrow....

Bob

[MJP]

Nice setup Bob, if there is a will there is a way! 

Marko

[rsterne]

I think I cringed for over an hour straight, every time that piece of aluminum came around and smacked the tool bit.... and I was only running at 166 RPM....

Bob

[MJP]

Ever considered buying a mill with rotary indexing table?

Marko

[Finn]

Boring head is useful in these situations and it is easier to set up than rotary table.

[rsterne]

Too much money for my very limited retirement income, guys.... I don't even have a Mill, just the milling attachment for my lathe you can see in the background of that photo....

Bob

[rsterne]

I finished the tank block today.... My 300 bar, 500cc Carbon Fibre tank clears the main tube by just over 1/16" when installed.... Here is the side view, showing the regulator installed....



The notch on the top front is for a barrel band, there are two 6-32 holes drilled and tapped on the front face to attach it.... As you can see, there is a Picatinny rail on the bottom.... The hole there is for the plug for the vertical air passage.... The plug is a low-profile 10-32 SHCS with a # 008 O-ring under the head in the milled 5/16" recess.... There are four 10-32 screws to retain the stub of the tank block in the tube, again drilled at a 45 deg. angle so that the tank block can be removed without removing the barrel.... The stub seals into the tube with a single # 118 O-ring.... The 1/8"-27 NPT tapped hole in the side is for a gauge for the pressure downstream of the regulator.... Below is the rear view....



The method of mounting the regulator is different from common practice.... Instead of using the two (red) O-rings inboard of the threads in a bored hole, the hole in the block is simply drilled deep enough to clear the length of the regulator and tapped 1/2"-14 NPS.... At the top of the hole is a recess milled with a 7/8" end mill to a depth of 0.085" to accept a # 115 O-ring.... This is a snug fit over the regulator, in the groove between the threads and the shoulder on the bonnet, which is 0.900" OD.... When the regulator is tightened into place, that O-ring fits into the groove in the tank block, and seals the regulator outboard of the threads.... This trick, pioneered by Travis, is much easier to machine, and is used on the Flex and Cobra.... Qudos to him and Jim Gaska for coming up with the idea.... The three 5/8" wide grooves milled around the middle of the tank block and the back and sides are only to remove a bit of weight, and pretty it up a bit.... Even so, the block weighs a hefty 8 ozs.... 

Bob

[oldpro]

 I have no Idea how you pulled that off with just a lathe but Im giving you a standing ovation!!! Bravo. That must of had a pucker factor of 10 at least.....

[MJP]

Boring head is useful in these situations and it is easier to set up than rotary table.

Well I dont see anything difficult setting up my rotaries, horizontal or vertical.
Boring head would work also.

Marko

[MJP]

I have no Idea how you pulled that off with just a lathe but Im giving you a standing ovation!!! Bravo. That must of had a pucker factor of 10 at least.....

You are right Travis, imagine what Bob would build if he had a proper mill.

Marko

[rsterne]

Travis, setup takes longer than machining on a job like this.... Thanks for the praise, buddy.... Hope your back surgery goes well....

PS, I'm just about ready for the regulator and valve....   

Bob

[Rob M]

I have no Idea how you pulled that off with just a lathe but Im giving you a standing ovation!!! Bravo. That must of had a pucker factor of 10 at least.....

LMBO., until I did the reading , I thought bob simply permanently attached a cylinder to the block , and had to go back to turn the cylinder. The fact its all one piece is BONKERS, and the interrupted cut time must have been hours !

Bob, I just read that you sawed off some material beforehand, that's good to hear.

[rsterne]

Well I may be SLOW, but I'm not stupid.... *LOL*....

Bob

[rsterne]

A couple of days ago I got a start on one of the valves.... got it roughed out, made some parts for the inside, and then spent a day finishing off the design.... Today I finished the first valve.... Here is the inside roughed out.... The valve is 5/8" ID with a 5/16" throat and a 1/8" stem....



The spring retainer end cap is a vented wheel that fits into a recess in the front of the valve and is retained by an circlip.... The area for the wheel is 3/4" ID.... I made the 3/8" thick and machined out the inside, leaving a 1/8" vented web.... That was my solution to the difficulty of holding a 1/8" thick part for machining....



Here is a front view of the completed valve.... The two long 6-32 screws are the front mounting bolts for the receiver.... They penetrate the valve BEHIND the valve seat, straddling the throat, which allows me to use larger screws than an MRod valve and yet have a larger ID....



The O-ring is located behind the recess for the wheel, where the valve wall is full thickness.... The gauge is tapped directly into the left side of the valve, behind the O-ring, so no need to drag it across a large hole in the tube.... Here is the back view, showing the huge 0.257" port, which is on a 30 deg. angle towards the 5/16" throat for improved flow....



The four valve mounting bolts are 10-32 x 1/4" SHCSs set down with the heads in shear.... They are in staggered pairs, the top and bottom pair in a similar location to an MRod valve, but the ones on the sides are 1/4" further forward.... I had to do that to make sure there was enough tube material for proper strength on the RH screw, which sits ahead of the cocking slot for the hammer.... The slot ends exactly at the back edge of the valve body....

The reason for no photos of the inside is that this valve is my own home-made version of the ART/SS valve.... Since I was privy to the development of it, I was able to design and make my own version.... but out of respect for Travis, I am not divulging the details until he gets his patent in place.... My sincere thanks to Lloyd and Travis for their help while I was making the very intricate guts for these valves.... I made several versions before I managed to get any parts that were even usable, they are, to say the least, challenging.... and well worth their price, IMO....

Bob