Home Forums Hunting & Shooting Gunsmithing Acraglas

So, does Brownell's manufacture their own epoxies or merely repackage someone else's? If someone else�s, who�s and what product?

As for JB weld, I've used it once for a temporary job on an action that was waiting for another mcmillan. Compared the specs with the various devcons and as I recall the compression strength was less though still substantial and shrinkage numbers the same or less.

I doubt very seriously that Brownells actually "makes" anything they sell. Like most businesses they contract for the manufacture of their name brand products using their specs.

It says allot about Brownells that they carry, in addition to their own Acra Brand, both Devcon and Marine-Tex.

Where did you find the compression strength of J-B Weld? Their web site lists the specs...but there is no spec for compression strength. Their specs also say it has "zero" percent shrinkage. That's actually a bad thing for rifle bedding because you are constraining your receiver in the stock.

Perhaps compressive strength?
Tensile strength: 3960 psi
Adhesive strength: 1800 psi
Flexural strength: 7320 psi
Compressive strength: 10733 psi
Shore D hardness: 85 (1 week)
Shrinkage: 0.0%
Tube size (each): 28.4g, 100mm x 20mm.

As to shrinkage I recall the others I looked into went further than one decimal so I am thinking if JB went went a couple more decimals there might be a number.

I was going by the info on their web site which does not list a compression strength that I could find anywhere.

http://jbweld.net/products/jbweld.php

Acra-glas comes with a little pouch of fiberglass flock. This is to thicken it up and give it more tensile strength. Mel told me that he found that using Kevlar or carbon fiber in lieu of the fiberglass make it that much better. I'll put a mix of Acra-glass and kevlar fibers up against your Devcon any day!

Mel showed me his method for using steel wool to bed and seemed to be the cats a$$. He said that the bedding held up very well and was easy to make look good- remember, he was only bedding walnut stock- not synthetic like we mostly do today. I'm not sure that uniformity has anything to do with a good bedding job. I have seen excellent bedding jobs that comprised three different materials. Nothing uniform about that!

Manufacturers give us what we want to pay for, not necessarily what is best. I suspect that if you added a pouch of kevlar fibers instead of fiberglass flock, you would price the bedding kit out of reach of many who use it.

Although I don't use that method anymore (too time consuming) I never had a problem with it. Mel was an old guy 10 years ago and maybe just didn't want to come into the 21st century!

All this has me wondering just how much compressive strength do you need? Walnut has always worked pretty well except for splitting (which is another issue) with a compressive strength on the order of 7,600 psi for American grown species (USDA Forrest Service). Now Devcon specs its 2 Ton Clear Epoxy at 11,000 psi and I'd suppose it's a good quality standard grade epoxy.

Seems to me that getting the mixing ratios as close to perfect as you can (and mixing thoroughly) to get the best mechanical properties is more important than what you stuff into the mix. (I see that Brownells says adding powdered steel to Acraglas Gel increases compressive strength a whopping 10 percent.) That does help to get a consistency that lets you apply it for best results. For product specifications I'd be more interested in shrinkage and lot-to-lot repeatability. Once I get a product to perform with a specific technique I don't want anything to change.

I've used Acraglas and it works fine, but I like using a 2:1 slow curing liquid epoxy... I mix it thoroughly, then add Cabosil as a thickener to turn it into a gel. I add whatever color I want and it's good to go.

I do use JG Weld for attaching scope bases/screws to the receiver. It's a relatively weak epoxy as epoxies go, and is perfect for this application.

The supposed attributes of different products have little meaning unless they were all tested using the same criteria...and that criteria is relevant to the intended use.

Say epoxy A's compression test used a 6 inch thick block and epoxy B's test used a 1/16 inch thick block and each of them withstood 10,000 psi before failure. Which one is stronger and more appropriate for rifle bedding?

Some serious gnat straining going on here...

When someone recommends brand X as being great I've found that it's often instructive to find out how they arrived at that conclusion so you can make your own decision as to how that relates to your particular needs.

If that's "straining gnats" then I guess I'm a first class "gnat strainer."

I imagine there is an ASTM standard for testing epoxy, there is for everything else. Of course there's no guarantee that stated figures were arrived at through the standard method, but a reputable manufacturer would be foolish to cook the numbers.

In any event compressive strength is compressive strength no matter what size if the material is homogeneous and the stress is evenly distributed and there are no unaccounted for external variables. (What you want in a testing standard.)

"...and the criteria is relevant to the intended use."

That's a good point, impact resistance and creep would be concerns in choosing a bedding compound. That's where experience with a material is probably the best measure as what you'd expect to be the testing standard would be quite different than the conditions in a bedded rifle. A good reason to stay with materials that are known to work well.

Getting the ratio of resin to hardener as exact as possible (and thorough mixing) is important. Mechanical properties fall off quickly as you diverge from the proper ratio, more or less quickly depending on the system. I think that's where people get poor results with a known-good product.

+1 on that, Brad!

However...since we're already in the gaack, I just wanted to say a few things. I'm a mechanical engineer and work in the composite material industry and the company I work for has developed many different polymer materials for use in reinforced composites, i.e. carbon, glass, Kevlar, etc. fibers and I was in charge of the testing group for some time, so have experience in testing in compression, tension, and shear of all types of composite materials. What I want to relate to everyone is compressive strength is not really what you're after here. What you want is a material with a high compressive modulus. In simple terms, modulus is defined as the stress divided by the strain at any given point. Or in other words, it is the slope of the linear portion of a stress-strain curve that you would get when you perform a compressive test on any material. I don't want to bore you by going into further engineering details on this since if you're really interested you can find this info on the net very easily I'm sure. To illustrate why compressive strength is not what you want I would tell you that I can give you two different materials, each with the same (equal) compressive strength, but one material will compress (strain) under load significantly more than the other before it fails. For bedding materials, we want a very stiff (meaning strains or compresses very little) material so that upon firing the rifle, the bedding material will move (strain) as little as possible. If it moves alot, we might as well not have bedded it at all. Typical unreinforced epoxies usually have a compressive modulus of around 1-3 Mpsi. For comparison, 6061 Aluminum has a modulus of about 10Mpsi, Titanium alloys around 16Mpsi, and steel around 30Mpsi. So you can see that if you combine any of these metallics in particle form to an epoxy, you can increase the modulus quite abit and given the same volumetric quantity for each, you'll increase the modulus more by using the steel. However, you'll also increase the weight more by using steel than by using aluminum. Also, since we are dealing with a particle reinforced material, the strength of these "metallic composites" will still be dominated by the epoxy matrix, thus you won't see a very large change in the strength of the material.

We can also use non-metallic fiber particles in a similar manner to increase the modulus of the bedding material and also to keep the weight down. One must remember that the fibers we're talking about such as E-glass, carbon fiber, and kevlar, by themselves have pretty much zero compressive strength and stiffness...sort of like pushing a rope. Therefore, typical properties of just the fibers themselves are given in terms of tensile strength and tensile modulus, but if these fibers are reinforced with an epoxy matrix, we would expect the compressive properties to be similar to the tensile properties for continuous fibers. So, for comparison purposes, I'll note that the tensile modulus of glass fibers are around 10Mpsi, carbon fiber can run anywhere from 8Mpsi on up to 100Mpsi, with most typical carbon fibers used in industry around 50-60Mpsi. Kevlar modulus runs around 18Mpsi. Since it would be very difficult to use long, continuous fibers for bedding a rifle, most people use chopped or milled fibers (very short) to add to the epoxy for bedding, thus the mention by many of using chopped Kevlar or glass to add to their bedding epoxy. Since we can't use continuous fibers for bedding, and since we can't align the chopped fibers in the proper orientation to take advantage of their strength in their axial direction, the benefits in modulus that we get by using chopped fibers in a bedding epoxy are not quite what you would expect based on the modulus numbers given above. While we will see some increase in modulus by using these fibers, it probably won't be much better than if you add a mettalic, since a metallic is a homogenous material and has the same properties in all directions, unlike the fibers. This is why you don't hear of everyone using chopped carbon fibers in epoxy for bedding compounds even though carbon has a much higher modulus than the other materials mentioned.

As a side note, you can also see from the numbers above the reason why we all like the carbon fiber reinforced stocks since the use of carbon fibers in continuous form greatly "stiffens" the stock due to it's very high modulus. The stock makers can orient the fibers in the correct direction and thus take advantage of carbon's high modulus. Coupled with carbons light weight, it's the best material by far for this application. Since the amount of bedding compound you're adding to the stock doesn't amount to much in terms of weight, whether or not you use metallics or non-metallic particles makes little difference. For this reason, I went with stainless steel particle reinforced epoxy when bedding my .416 Rem.

Sorry for my long winded writing, but I hope some find it useful and that this doesn't stir up too much of a hornet's nest. Perhaps everyone is asleep by now!

No matter how much compression strength the epoxy or glass bedding has, it still amounts to a very thin layer of bedding compound on top of a piece of wood (assuming you are bedding a wooden stock), which has an even lower compressive strength.

Even if the epoxy or glass doesn't compress, the wood under it acts as a compressionable cushion, which amounts to about the same thing. The only advantage to glass and epoxy bedding that I can see is that you have an almost perfect and stress free molded fit between the wood and metal. If the wood warps, so will the thin layer of bedding compound that is bonded to it, with resulting uneven stress placed on the metal parts.

In my opinion, that is why piller bedding works so well. The action is resting on two metal pillers, and even aluminum has greater compressive strength than bedding material, and the action parts are held securely by steel screws.

No matter how much the wood and bedding material shrinks, expands, shifts, compresses, cracks, breaks or warps, the action is still resting securely on the pillers.

As I said earler, I'm not trying to promote Devcon, so it isn't "mine". And it isn't a question of the fiberglass flock that comes with the Acraglas kit vs. whatever filler somebody else wants to add, I simply stated my preference for compounds that come with the filler already mixed, i.e. putty. For a metal based epoxy I have come to prefer Brownell's Steel-Bed, because I like the texture and the fact that it is filled with stainless steel.

I've been fooling with this stuff since the 1960s, when original Acraglas was the first or certainly the most touted bedding material. Later, it fell out of favor as a PRIMARY bedding material, and most builders of competition rifles went to Devcon or a similar material. I'm not aware of any such rifle makers or stockers who currently use original Acraglas for primary bedding areas. Do you know of any smiths who build HP competition, or benchrest rifles, who use original Acraglas? Military armorers? If you are happy with it, that's fine. I still like and use original Acraglas with and without fillers, but not as a primary bedding material.

Paul

Of course but remember what you're coupling the force into. Would modulus make a meaningful difference so long as it was the same or greater than the stock behind the bedding? (Walnut on the order of - wood is so variable - 1.7Mpsi, your shoulder a bunch less and not a fixed point) And elasticity? Does it matter that the bedding compresses tiny a bit so long as it does so consistently and returns to its former dimensions?

Useful stuff I think. The better you understand the materials, the mechanical system you're building, and the better you define what that system should do, the better the results.

Brad, all of the good products seem to work fine. But I don't know everything, sooo far from it. I'm interested if anyone can make a case for something different.

123...,

I think you're getting to the meat of the nut, just what do we want the bedding to do. I want the recoil lug to transmit force to the stock evenly across its entire surface. A complete, even contact between the lug and the stock. I want consistency, the contact points between the action and the stock should be exactly the same from shot to shot. For pillars I want changes in stock dimension and action screw force to not strain the action. Other than that who cares what the bedding compound does.

These quotes above, I think, have hit on what is actually going on and what the bedding should do. The modulus of the bedding compound is probably not that important since the forces will be transmitted back to the wood or synthetic stock, but we do want the bedding to be at least as good and hopefully better than the stock material to minimize any movement and maximize the transfer of forces. It seems pretty obvious to me now that there are a wide array of materials and methods for bedding stocks and most people seem to be having good success with their respective method, so I suppose that illustrates just how non-critical the actual material used for bedding truly is. It's more critical how the rifle is bedded as opposed to what material is used to bed it.

I'm not trying to take away from the discussion, just opining that there's really little real-world difference between most epoxies for the very simple/non-demanding requirement of bedding a rifle.

I've got far more expereience with epoxy than I care to remember. Once upon a time I installed commercial grade epoxy floors for an enormous variety of uses. Everything from fully grounded epoxy flooring for McDonald Douglas aircraft assembly to the Anhueser Busch Brew House floor to Operating room floors, and everything in between.

Said all that to say, I'm seriously familiar with epoxy! Few realize, but there are only three makers of epoxy in the entire world. The stuff is sold and re-labelled/blended as other products. JB Weld, for instance, is really a trowel-on epoxy made to be mixed with sand for chemical resistant applications. I know that because I troweled hundreds of gallons of the stuff in slit trenches at a Dow Corning Chemical plant. Would add, the way we bought our epoxy was in 55 gal drums!

Bottom line, nearly ANY epoxy is rigid enough to give perfect results in a rifle... the application just isn't very demanding. Pillars in conjunction with nearly any decent epoxy will get the job done.

My .02

Sorry Brad, but do a google search for Manufacturers of Epoxy and you will find FAR more than three companies just in the USA...let alone the entire world. China has a ton of them.

I would also disagree that rifle bedding is a "low-demand" application considering the force it must withstand from the recoil of a high powered rifle cartridge being delivered in milliseconds.

Can't say the current state of affairs as to actual mfg's... a few years ago there was just three. There are a LOT of "blenders" that call themselves manufacturers but aren't.

As to a rifle's bedding being a high demand application, I think that's nuts...