Gunsmithing Effects of Coefficient of Thermal Expansion

[treebasher]

OK here is an engineering question that I have been tossing around in my head for a while now.

When you are bedding a rifle do you want to match the CTE (Coefficient of Thermal Expansion) between the bedding compound (epoxy) and the CTE of the action or the CTE of the stock?

Here are some CTE's of common materials used in a rifle:

Epoxy, castings resins & compounds, unfilled ~ 31 x 10-6 in/in oF
Steel ~ 7.3 x 10-6 in/in oF
Steel Stainless Austenitic (304) ~ 9.6 x 10-6 in/in oF
Steel Stainless Austenitic (310) ~ 8.0 x 10-6 in/in oF
Steel Stainless Austenitic (316) ~ 8.9 x 10-6 in/in oF
Steel Stainless Ferritic (410) ~ 5.5 x 10-6 in/in oF
Devcon Technical Data including CTE's

It makes sense to me to keep the bedding material and the stock material as close as possible in physical properties to ensure a monolithic block.

Also if the stock/bedding material has a lower compressive strength than the action (steel) it should not exert any forces on the action that are great enough to cause deflection.

Has anybody done any testing on this? I can't remember seeing anything in <span style="text-decoration: underline">Rifle Accuracy Facts</span> by Harold R. Vaughn about this. I think I am going to go dust that book off...

 

[BOLTRIPPER]

Re: Effects of Coefficient of Thermal Expansion

are you one of them guys that weighs primers ?

 

[wnroscoe]

Re: Effects of Coefficient of Thermal Expansion

The best you can hope for is minimal shrinkage in the bedding compound you use. After trying Brownells and Devcon products I've settled on Devcon 10110. I believe the cured shrinkage is .0006". The release agent used will also play a role in the final fit. I've found that two coats of 100% Carnauba Wax buffed to a shine works better than anything else I've tried. The smallest of machining details in the receiver can be seen in the bedding when it's all finished. Don’t over think this, it isn’t rocket science.

 

[wnroscoe]

Re: Effects of Coefficient of Thermal Expansion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body">are you one of them guys that weighs primers ? </div></div>

Primers can be weighed??

 

[BOLTRIPPER]

Re: Effects of Coefficient of Thermal Expansion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: wnroscoe</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body">are you one of them guys that weighs primers ? </div></div>

Primers can be weighed??</div></div>


.......DOAH !

 

[wnroscoe]

Re: Effects of Coefficient of Thermal Expansion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: wnroscoe</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body">are you one of them guys that weighs primers ? </div></div>

Primers can be weighed??</div></div>


.......DOAH ! </div></div>

Damn, I missed the boat......................again

 

[treebasher]

Re: Effects of Coefficient of Thermal Expansion

I also borrow BOLTRIPPER's Prometheus to throw my powder charges

 

[hrt4me]

Re: Effects of Coefficient of Thermal Expansion

Boltripper has never owned his own Prometheus...

 

[Clark]

Re: Effects of Coefficient of Thermal Expansion

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BOLTRIPPER</div><div class="ubbcode-body">are you one of them guys that weighs primers ? </div></div>

I don't bother to glass bed 223 bolt rifles.
But I do glass bed 257s, 6.5s, 270s, 7mmMags, 300WinMags etc.
I need to include the stock as part of the recoil reaction.
The 30BR competitors try to include their shoulder mass are part of the recoil reaction.
The purpose of the glass bedding is to make the action to stock connection lower compliance AND more consistent. 0.001" between the recoil lug and stock bedding should close up with recoil.

 

[cdmurphy]

Re: Effects of Coefficient of Thermal Expansion

Treebasher -

I haven't heard of any testing done on this subject, but I would assume it would be best to match the CTE of the bedding material and the stock. They are the ones that (ideally) are bonded as well as possible, and any CTE mismatch would tend to stress this joint. It's possible this could lead to failure of the bond, and ingress of solvents, or other matter that might alter the point of impact.

As to your second point, any stock or bedding compound will be much weaker than the metal of the action, but the stress of any dimensional changes in the bedding will still have an effect. (It may look strong, but I'm sure you could deflect your barrel by several thousandths with just your bare hands.) This is the primary reason for using pillar bedding, or aluminum bedding blocks. The CTE of aluminum vs steel isn't a great match, but it's much better than steel to epoxy.

- Cameron

 

[BLAWTON]

Re: Effects of Coefficient of Thermal Expansion

The first thing i do is set the pillars let them set up for a day then i go after it. What i did was take two screws and turn the heads off of them. I put them in the action tape the lug with electrical tape and spray down with the release agent. Modeling clay helps you prep the stock and keep the bedding compound out of unwanted places. Like WN i use the 10110 now cause its the best stuff i have found. Once ready the BA gets layed into the stock and i use sergical tubing to hold the BA in place and a small shim under the barrel in the front part of the stock to keep everything nice and level. Keeps the rear of the action from coming up out of where you want it. It seems to keep this problem to a minimum

 

[TCA4570]

Re: Effects of Coefficient of Thermal Expansion

Stop weighing primers and draw it up!
Create assembly and run FEA.
All hotter=All softer
Sqeeze tighter.

http://en.wikipedia.org/wiki/Sigmund_Freud

 

[Gruff]

Re: Effects of Coefficient of Thermal Expansion

Don't sweat the small shit. It ain't rocket science. The putty will shrink more than any thermal coefficient will influence it.