Set Screw Gas Block Internal Deflection

[trackmagic]

I have heard people talk about a downside of set screw gas blocks being that the screw induces a dimple into the barrel that can affect accuracy. I have access to FEA software that I could quantify the dimple size. I thought this board may find the results interesting:

Setup:
0.750" OD
0.224" ID
Force applied to 0.170" Dimple on bottom (Not sure the actual diameter these are drilled, but it should not impact the results much)
Force: 580lbs (8-32 Set Screw at 19in-lbs of dry torque)
http://i1343.photobucket.com/albums/o795/trackmagic218/Setup_zps5eoyplvt.jpg (http://s1343.photobucket.com/user/trackmagic218/media/Setup_zps5eoyplvt.jpg.html)

Results:
Deflection inside bore = 0.00001"
http://i1343.photobucket.com/albums/o795/trackmagic218/Internal%20Deflection_zpsfgqsatto.jpg (http://s1343.photobucket.com/user/trackmagic218/media/Internal%20Deflection_zpsfgqsatto.jpg.html)

[SomeOtherGuy]

Very interesting, thank you for doing this and sharing.

[jerrysimons]

I have searched couple times and found nothing, what is the spec for gas block set-screw torque?

[azoutdoorsman]

Nice info.

[AM-15]

I have searched couple times and found nothing, what is the spec for gas block set-screw torque?

Steel threads is about 19 in.lbs. max
Aluminum threads is about 10 in.lbs. max

[jerrysimons]

Steel threads is about 19 in.lbs. max
Aluminum threads is about 10 in.lbs. max

Based on 8-32 thread size? I put 30in/lbs on the set screws with the teeth on last build

[AM-15]

Based on 8-32 thread size? I put 30in/lbs on the set screws with the teeth on last buildhttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjABahUKEwiw0cm-1KnHAhVDNIgKHfqCAzA&url=https%3A%2F%2Fwww.engineersedge.com%2Ftorque_table_sae.htm&ei=vnDOVbD6DMPooAT6hY6AAw&usg=AFQjCNEjPgugcwo2qLH0MtFduDAkFQmO1w

[BufordTJustice]

Based on 8-32 thread size? I put 30in/lbs on the set screws with the teeth on last build
The SLR adjustable gas blocks and Black River Tactical's MicroPin gas block use 10-32 set screws. I have torqued many sets of McMaster Carr knurled cup tip screws into these blocks at 20-25 inch pounds without issue.

OP, thank you for sharing this.

[BufordTJustice]

I have heard people talk about a downside of set screw gas blocks being that the screw induces a dimple into the barrel that can affect accuracy. I have access to FEA software that I could quantify the dimple size. I thought this board may find the results interesting:

Setup:
0.750" OD
0.224" ID
Force applied to 0.170" Dimple on bottom (Not sure the actual diameter these are drilled, but it should not impact the results much)
Force: 580lbs (8-32 Set Screw at 19in-lbs of dry torque)
http://i1343.photobucket.com/albums/o795/trackmagic218/Setup_zps5eoyplvt.jpg (http://s1343.photobucket.com/user/trackmagic218/media/Setup_zps5eoyplvt.jpg.html)

Results:
Deflection inside bore = 0.00001"
http://i1343.photobucket.com/albums/o795/trackmagic218/Internal%20Deflection_zpsfgqsatto.jpg (http://s1343.photobucket.com/user/trackmagic218/media/Internal%20Deflection_zpsfgqsatto.jpg.html)
I wonder what the bore deflection, if any, could come from taper pins in a std FSB?

[trackmagic]

I could run the same model with a taper pin. The only issue is I'm not sure how to quantify how much force the pin puts on the barrel. If I had the taper angle and a distance for how far the pin is pressed in I could use that. If anybody has those numbers I'll run the program.

[Gunfixr]

A taper pin that fits properly engages the whole taper. That would mean it is meeting metal all the way around from one end to the other.
Therefore, pressure would be more or less of an effort to swell the hole from one end to the other on the part of the pin.
Because part of the pin would be across the bottom part of the barrel, there would be some pressure, but it would be spread across the whole area it contacted.

A set screw is direct inward pressure, concentrated in the contacting area of the end of the screw.

I would think the taper pin would apply substantially less pressure, and may not deflect into the bore.
The pressure is not concentrated on the barrel, as it is also pressed into the gas block/sight base at either end of the hole at the same time.

Sent from my C6916 using Tapatalk

[Macht]

What FEA package is that? It doesn't look like Abaqus or Ansys. Nastran?

[Joe Mamma]

I have heard people talk about a downside of set screw gas blocks being that the screw induces a dimple into the barrel that can affect accuracy. I have access to FEA software that I could quantify the dimple size. I thought this board may find the results interesting:

Setup:
0.750" OD
0.224" ID
Force applied to 0.170" Dimple on bottom (Not sure the actual diameter these are drilled, but it should not impact the results much)
Force: 580lbs (8-32 Set Screw at 19in-lbs of dry torque)


Thank you for doing this trackmagic. Could you run the same thing, but for a 0.625" outside diameter barrel (typical light weight diameter) and 33 inch-lbs of torque (on a 8-32 set screw)?

I know this isn't an exact science. But I am curious.

Thanks.

Joe Mamma

[trackmagic]

What FEA package is that? It doesn't look like Abaqus or Ansys. Nastran?

Its the FEA Ad-in for Solidworks called Simulation.

[trackmagic]

Thank you for doing this trackmagic. Could you run the same thing, but for a 0.625" outside diameter barrel (typical light weight diameter) and 33 inch-lbs of torque (on a 8-32 set screw)?

I know this isn't an exact science. But I am curious.

Thanks.

Joe Mamma

No problem, I'll do that on Monday and post the results.

[BigLarge]

What barrel material did you specify when running the program, or can you? I'm assuming 4150 steel, SS410, etc would have different deflections?

[trackmagic]

What barrel material did you specify when running the program, or can you? I'm assuming 4150 steel, SS410, etc would have different deflections?

I specified steel. In general steel all has the same modulus of elasticity regardless of alloy (29,700,000 psi). The modulus is a measure of how stiff the material is. Here is a data sheet for 4150:
4150 (http://www.matweb.com/search/DataSheet.aspx?MatGUID=f1ab27b2b649452ca522dbd0df967b8e&ckck=1)

410 Stainless has a modulus of 29,000,000. That is 98% as stiff as steel so they are both going to be pretty close.

[LRRPF52]

Excellent thread. I use no torque with dimples when I can, and secure the block with Red Loc-tite on precision builds. That also seals the gas block to the journal.

The set screw only serves as an index/counter-ratation/counter longitudinal travel device in a worst-case scenario.

[BigLarge]

I specified steel. In general steel all has the same modulus of elasticity regardless of alloy (29,700,000 psi). The modulus is a measure of how stiff the material is. Here is a data sheet for 4150:
4150 (http://www.matweb.com/search/DataSheet.aspx?MatGUID=f1ab27b2b649452ca522dbd0df967b8e&ckck=1)

410 Stainless has a modulus of 29,000,000. That is 98% as stiff as steel so they are both going to be pretty close.

Ok. Good work, and thanks for sharing.

[trackmagic]

Thank you for doing this trackmagic. Could you run the same thing, but for a 0.625" outside diameter barrel (typical light weight diameter) and 33 inch-lbs of torque (on a 8-32 set screw)?

I know this isn't an exact science. But I am curious.

Thanks.

Joe Mamma

Sorry this is later than I said:
Barrel OD: 5/8"
Clamp Force: 1100lbs (8-32 @ 36inlbs...closest I could find on a chart was 36inlbs)

http://i1343.photobucket.com/albums/o795/trackmagic218/58%20Block_zpsyrpr7ebb.jpg (http://s1343.photobucket.com/user/trackmagic218/media/58%20Block_zpsyrpr7ebb.jpg.html)

Looks like 0.00003" deflection on the ID.

[Ned Christiansen]

Very interesting and I have wondered about this, especially at the theoretical level, since actual observation has been: no effect.

.00001 deflection, I would say, is less effect than the dif between having corn flakes vs/ biscuits & gravy for breakfast before going shooting :) .

Which is not to say I think set-screwed gas block are OK. Far as I'm concerned the setscrews are for holding it in place while I drill for a cross pin.

I agree about tapered pins-- if an 8-32 gives .00001 deflection, a tapered pin will give much less, like less-to-immeasurable. Probably part of the reason they ere selected for the task.

Fascinating study, good job!

[jerrysimons]

Thanks op! How bout 10-32 set screw at 50in/lbs in the extream case on .625 gas block journal?

[Joe Mamma]

Sorry this is later than I said:
Barrel OD: 5/8"
Clamp Force: 1100lbs (8-32 @ 36inlbs...closest I could find on a chart was 36inlbs)

Looks like 0.00003" deflection on the ID.

Thanks. I appreciate you running these numbers.

Joe Mamma

[H Wyman]

Nice work OP.

Does a modeled ten millionth of inch translate to anything physically measurable?

You could probably sway a measurement that small by breathing on the gas block. How would heat from firing, peak and residual, affect deflection? How about heat treating or chrome lining?

[Eurodriver]

This is excellent info. Thanks for sharing!

[KUSA]

How about the effects of a clamp on block?

[cst]

Definitely curiots about a clamping block...but with all the forces distributed around the outside..I doubt you'd seen any internal deflection