Starting a .308 ELR build

[Scotchmo]

OK, I understand the design now (I think).... The green parts carrying the O-rings in the drawings in Reply #22 are metal, then?.... and designed with usual safety margin without the support of the cross block?.... I am nervous about printed parts delaminating, the layer-to-layer adhesion results in Z-direction tensile strength of only about 3000-6000 psi (X-Y about twice that), depending on material and layer thickness and width.... Check out my thread on Tensile Testing in the 3D printing section....

Bob

The green cylinders are horizontal and vertical transfer port tubes. They will be brass or steel. I might eliminate the upper o-ring from the vertical transfer port, as that o-ring is constrained by the plastic receiver. Though not holding air directly, the receiver sees force when the o-ring is under momentary pressure. I used o-rings constrained in a plastic cross block in an earlier build and it was not an issue.

I’m also considering eliminating the horizontal transfer port tube as my last two builds simply used a single o-ring sealing against the barrel face. I added the horizontal tubular port this time to theoretically get a more positive seal as the back o-ring provides an energized seal of the tube against the barrel, in a similar manner as the vertical transfer port with its lower o-ring pushing the transfer port tube up against the cross block. The barrel can be pushed in tighter against the cross block to get a good seal if I decide against the more complicated energized seal. That worked fine in the past though it required a careful setting of the barrel pushed against the cross block before securing the barrel position in the receiver.

[Scotchmo]

I received an important part today. The TJ barrel liner is 61.25” long x 0.498” diameter. It’s a .308/1:10 twist. So far TJ has been able to make any length barrel that I’ve asked for. They are in Kentucky, so maybe they make long liners for Kentucky long rifles. The liner will go into a 60” long steel DOM tube 1” OD x 0.5” ID.

Now I just need to get into the shop more often to make some progress.

[rsterne]

Scott, you inspired me to 3D print tank clamps for my two benchrest rifles to attach the rear of the tanks to the plenum tube....



Three 6-32 x 1.25" SHCSs at different heights tighten the clamp in place....



Looks a lot better than just using a hose clamp and a spacer block like the one in the center of these clamps....

Bob

[Scotchmo]

Very nice Bob. I decided to hold off on my dual bottle version for now. It complicates the plumbing so more to deal with. Single bottle for now. Got my 400cc 4130 plenum, tank, regulator, scope, and a few fittings. I made some design changes and had to reprint most parts. 3rd times a charm so this last one should work. I had switched filaments and didn’t realize I was now using Sunlu “EasyABS” instead of regular ABS. Temperature and other settings were way off. This stuff is in between PLA and ABS. Doesn’t warp like normal ABS and runs at about 230 C nozzle temp. No ABS smell that I could detect. Layer-to-layer adhesion is super good. And so far haven’t had any moisture absorption problems. It should work better than regular ABS with unenclosed printers. Not sure if I want to compromise on the temperature resistance in the future, but I’ll print my one other roll of it before deciding.

[rsterne]

The plumbing problem goes away, I would think, if you put the tank block and regulator at the front like I did.... The only complicated bit is the block itself.... The tanks feed the regulator in an inverted "V" arrangement, and the reg. feeds the plenum at the front....

 



Bob

[Scotchmo]

.... The only complicated bit is the block itself....

Bob

Exactly. That’s the part I decided to forego for now.

[Scotchmo]

I made the 400cc plenum today. It’s a 4130 seamless tube. 1.25” O.D., 0.120” wall thickness, 36” long.

I contemplated using socket head cap screws to hold the end cap and valve, but I prefer threaded plugs. I would have tried to single point the threads, but my lathe has only a 7/8” bore in the headstock spindle. In the past, I have single pointed internal threads in ~18” long 1.25” tubing by using the steady rest, but the 36” tube is too long to do that.

I found a 1-1/16-20 tap on EBay for $30.69. Then I had to come up with a way to tap a straight hole. I used blue tape on the tap to mark the 3/4” depth. I started with the tap in the three jaw chuck and the tube in the steady rest. I was able to start the tap fairly straight for about one turn, but had no good way to grip the tube to turn it. It takes a lot of torque to turn such a large tap. I flipped it around with the tap still engaged and locked the headstock. I used two open end wrenches to carefully turn the tap, and clear chips, checking often to make sure it was still going in straight. The threads came out good.

At 4000psi internal pressure, this size 4130 tube has a 3.5 to 1 factor of safety using tensile yield strength of 63ksi. I plan on running at about 3600psi, but might go a little higher.

[YEMX]

VERY interesting design!!!

[Scotchmo]

Years ago, when these taps were more expensive, I had made my own to thread my home built airgun receivers for AR buffer tubes/buttstocks. My year 2000 version of AutoCAD can’t easily model threads, so I used that tap to thread the ABS .308 receiver. I did a trial assembly of the major components today. I still need to make the valve, electronics, etc.

[Rob M]

looks incredible Scott , would be cool if the next one was dressed as a kentucky long rifle 

[Scotchmo]

I finished the electronics today. The 8.xx volt readout on the left is the battery voltage. The other readout is the capacitor charge voltage. The capacitor voltage can be adjusted up to about 60v to 62v. That is one of two hammer strike adjustments. The other is a plunger travel adjustment screw. When the trigger is pulled, an SCR is latched on and dumps the capacitor into the solenoid. A plus for using an electronic hammer is that the trigger mechanism can be a simple switch that makes for an easy to design/build trigger group with a good feel.

I ordered some 4140 steel for the valve and end cap. 1.25” round bar stock. Also an 8 ounce bottle of NU-BLUE to try on the steel parts. Up to now, I’ve always used Super-Blue.

[Scotchmo]

The 4140 steel bar is scheduled to arrive tomorrow. I plan to start on the valve build later this week.

I cast a bunch of the NOE 154gr (actually drops at 152gr) BBT and they came out OK. A little more wrinkling than I get with smaller 7mm cast slugs, but mostly acceptable. I really want to get the 172gr BBT if either NOE or Arsenal decides to make the mold. The 172gr might be more suitable for this build with its 1:10 twist barrel. I ran some performance numbers again and these are the predictions that I got:

Regulated to 3200psi, with .0032 second dwell, 152gr@1100fps, 411fpe, >30 shots from a 4500psi fill.

Regulated to 3600psi, with .0032 second dwell, 172gr@1100fps, 462fpe, >20 shots from a 4500psi fill

The longest/heaviest slug that will fit in the breech would be about 210gr. If I go for near max, I predict this:

Regulated to 4000psi, with .005 second dwell, 210gr@1225fps, 700fpe, 6 shots from a 4500psi fill.

All seems like a lot to expect, but I’ll be happy with 400fpe.

Here are some pics showing the rifle assembled with most of the finished/blued steel parts. Shown next to my 7mm build.

[rsterne]

What a BEAST!!!!

Bob

[Rob M]

looks awesome , I like the box tube breech

[YEMX]

That is TOO COOL!!

[Scotchmo]

I’ve been making some good progress on the valve body. But slow going machining chromoly steel. Being careful not to make mistakes that would cause me to start over. Very careful when tapping a couple of deep 6-32 holes. I’m an amateur machinist at best.

The numbers say that it needed to be a balanced valve, so that’s what it will be from the start. I using my same electronic controller design. Some of the valve specs:

0.079” (2mm) stem
0.332” throat
0.313” ports
0.09” lift
0.281” balance piston
About 0.013” valve seat overlap

I want it easy to open, but also want a good seal. The balance piston will keep the load on the seat low enough, so I’m going to start with an acetal valve head.

Cross section of model shown below, minus the two return springs (one for valve, one for solenoid). And a picture of just the main valve parts exploded.

[rsterne]

Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....

Bob

[Scotchmo]

Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....

Bob

Too late. I finished machining the valve body today.

I also started the internal valve parts today. Transfer port, balance chamber, balance piston, and grate, will be brass for this first try. Factor of safety is not great, but a failure is not catastrophic. Turning brass parts is so much easier.

[rkr]

[/quote]

Good design, glad to see you have the O-ring on the balance piston loaded the right way to avoid stiction.... Think about a radius on the front inner corner of the valve port, that is more important than the one on the outside of the corner.... If you can, angle the vertical part of the port about 20-30 deg. towards the seat, making the corner much more gradual.... You might have to move the seat forward a bit (or the transfer port back), if it's not too late for that....

Bob

Too late. I finished machining the valve body today.

I also started the internal valve parts today. Transfer port, balance chamber, balance piston, and grate, will be brass for this first try. Factor of safety is not great, but a failure is not catastrophic. Turning brass parts is so much easier.

Making that inner corner of the port rounded netted me 40 fps in my Evanix ...

[Scotchmo]

I got a little more work done. I still need to drill the vent holes for the pressure assisted valve (I don’t really like the term “balanced valve” in this case). I’m not currently setup to hold and drill tiny #60 holes in any precise way. I have some fixturing to do. I have a dozen #60 bits coming and a chuck that will let me hold and drill in my press and lathe with bits down to #83 (0.012”).

I’m leaving for Oregon in couple of days. I’m going to a three day airgun match that will have a long range event, out to 300yds. I’ll be shooting my 7mm in that event. The small drill parts should have arrived at home by the time I get back from Oregon, so work will resume then. There is a chance that I might have the .308 sorted and ready for RMAC (less than two months to go).