Bedding questions

T

toolmakr

Guest
After listening to your advice (bed receiver, barrel, or ?), I'm proceeding with traditional bedding the receiver. The Devcon plastic steel putty is on it's way and I've got a couple more questions.
1st There's a large pocket in the stock for a box magazine. Since this is a single shot Savage Model 12, there's no mag. Should I fill the 3" x 1.3" x over an inch deep pocket at the same time as the rest of the bedding? That way it would all be one solid mass. The Devcon supposedly shrinks very little.
2nd Is the reason for taping the sides, bottom, and front of the recoil lug to provide clearance for dis-assembly and re-assembly? Or is it to provide clearance for movement during firing recoil? The sides of the Savage lug are tapered a lot, so assembly clearance will be there even if it's not taped.
Thanks again for your advice.
Kevin
 
Second question--The goal is to bed it so there is no movement of the action within the bedding upon firing. Tight action screws often don't clamp things together well enough to prevent the forces generated by recoil from moving things around if the recoil lug is improperly bedded. Any movement risks re-settling in a different place. Even extremely small changes from shot to shot are detrimental to accuracy.
 
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The front and back sides of a lug should be parallel. Taping facilitates
removal. The rocking motion required to get a barrel action out of the bedding can and does ruin bedding. Do it gently. I don't always tape both sides of the lug. Lately, I have been taping the non torque side. that is
the left side viewed from the drivers seat. A round action will always
try to torque in the stock, and regardless of bedding material will compress
slightly more that flat bottom actions, so not taping the torque side
may help that. I don't think there is any benefit in filling the entire
magazine area with epoxy. A piece of wood seated in bedding material
will work fine for that, even aluminum. Leave it sufficiently below the surface
to allow covering with epoxy in your final bedding. No high spots !!!
It is Important and rarely mentioned. Any fine edges must be removed,
because they break off when disassembling and reassembling only
to find themselves under something they should not be.-- good luck
and wax everthing
 
toolmakr said:
After listening to your advice (bed receiver, barrel, or ?), I'm proceeding with traditional bedding the receiver. The Devcon plastic steel putty is on it's way and I've got a couple more questions.
1st There's a large pocket in the stock for a box magazine. Since this is a single shot Savage Model 12, there's no mag. Should I fill the 3" x 1.3" x over an inch deep pocket at the same time as the rest of the bedding? That way it would all be one solid mass. The Devcon supposedly shrinks very little.
2nd Is the reason for taping the sides, bottom, and front of the recoil lug to provide clearance for dis-assembly and re-assembly? Or is it to provide clearance for movement during firing recoil? The sides of the Savage lug are tapered a lot, so assembly clearance will be there even if it's not taped.
Thanks again for your advice.
Kevin
Click to expand...

Regarding shrinkage and resins. It's a percentage so its tied directly to the volume of material being used. A 2"X4" brick of Devcon is going to shrink more in linear distance than a shell .050" thick.

If it were me:

I'd see about making a block from material similar to the stock's parent material to fill your mag box. That's a pretty big cavity to fill with just resin.

taping the lug on the sides and front helps to get the dern action out of the stock without rocking it and beating up the bedding. It also helps avoid the lug galling/scraping the bedding during assembly. Last, it ensures the lug floats off the bedding on the bottom. A recoil lug that gets crap under it can prevent the action from registering in the bedding properly. (think kids story "Princess and the Pea.")

FWIW the best tape I've found for this (cosmetics afterwards) is the super thick 3M electical tape. Super sticky, easy to trim with a razor, and it gives a nice slick finish down in the pocket. Here's a stock I bedded that illustrates it pretty well.

DSC_0102.jpg


Good luck.

Chad
 
I just looked it up again on the Devcon tech data sheet. It shrinks 0.0006 in/in. "cured shrinkage". Doesn't sound like much, but I agree with your ideas about filling the large pocket with something. Thanks for the other comments and pics. Nice work, as usual, Chad.

The comment about torque got me to thinking. It would explain the large sideways distance between groups of different bullet weights. Since the sides of this lug are tapered 10 deg or so per side, I'm wondering if applying tape for clearance to just the front and bottom surfaces, and leaving the sides tight might help contain the receiver torque. Whaddya think?
Kevin
 
No way. If anything ever happens to change the relationship - say shrinkage in the stock wood or an overgenerous tightening of the screws - then you could jam the lug into the bedding & bugger up the perfect fit you were aiming for. Tape clearance on it!
 
Good point John. I wasn't thinking about dimensions changing, say from wood drying out. Of course that would affect the fit in other areas also. I do see your point though. Perhaps like an earlier mention of taping the non-torque side. Makes sense.
Regards, Kevin
 
Someone here smarter than me (which consequently doesn't take much) feel free to help me along.

I had some tremendous help years ago from a guy who makes a very nice living designing fasteners for the aerospace industry. The particular subject was in regards to thread shear, tensile/torsional loading of a fastener, and all sorts of other fancy engineer vocabulary words.

Long story short, a 1/4/28 fastener snugged up at 40lbs inch generates something like 1100 or 1200 lbs of tensile load.

Store that in your RAM for just a moment.

Now figure out the surface area of a typical 1.350 OD receiver that measures 7" in length. For sake of simplicity we will just say it's free of any/all trigger features and peripherals but just kinda keep that in your mind as the figure will be a little optimistic. Our receiver has roughly almost 15 square inches of contact surface. If its a 3 guard screw action and each fastener is producing the claimed 1100 lbs of tensile load then (if I'm doing this right) those 15 square inches are distributing about 3300lbs of tensile pressure to the stock. Now, I'm fairly certain that as we move away from the 6 O clock position (where the guard screws are) the amount of load decreases somewhat as the action isn't expanding like a balloon, it's being pulled on one direction only.

Suffice to say there's a pretty good snort of pressure holding that action in the stock when the guard screws are snugged up tight.

Now, here's where I need the real help because I just don't know this kind of stuff. Say we have a 308 Winchester shooting 180 grain bullets. Say that bullet is being rotated 1 time for every 10 inches of forward movement. Say the velocity is 2500fps at the muzzle.

Just how many inches/lbs/tons of force are being generated in torque to rotate that bullet? I don't care so much for the acceleration although I'm pretty sure that plays into it in the form of torsion.

Now factor in things like friction coefficients, stickton, and whatnot and if possible explain (layman terms please) if it's feasible for this bullet to actually have enough ass behind it to rotate an action in it's stock. I'd go so far to speculate that it's not the action rotating, but the action actually twisting (torsion) the stock during recoil. If that is the case then rather than focusing so much on the recoil lug I'd be looking to improve the torsional resistance of a rifle stock.

I'll reiterate that this is all speculative and guesswork for me as I don't have the education to sort this out.

Turkey day almost here!

cheers,

C
 
I REALLY like the way you think. I've been thinking about equally pressing issues, no pun intended. Yes, acceleration is involved. In fact, this whole discussion is all about acceleration and Newton's 3rd (I think) law about action and reaction. Pushing the bullet that way shoves the barrel and receiver, and eventually the stock this-a way. Since the mass of the stock is off-center, the whole kaboodle rotates up with a "moment" of torque as it is moving generally backwards. The mass of the scope offsets the mass of the stock, but since the scope is less, the whole assembly is still generally rotating "up"

Now the whole issue of right or left (clockwise or anti) torque as viewed down the barrel really messes with my mind. In spinning up the bullet clockwise as it is going down the barrel, it's the barrel that is doing that work, so I would think that the barrel is going to be getting torqued anti-clockwise.

As this is related to bedding, I will ask Who cares where the stock is pointed? Since the scope is attached to the receiver/barrel, and the stock is merely a handle. Some have said that the reason for bedding is to ensure that the action returns to the same place in the stock after recoil. Who cares where the stock is pointed?

No, the reason for bedding is to control, or at least make repeatable, how the receiver and barrel twist, contort, and generally move around in a gazillion different directions during the time after the primer ignites and before the bullet leaves the barrel. This complex motion that I have only begun to grasp is what makes you benchrest guys crazy.

The motion happens on time and physical scales so diminutive that measuring it is impossible without very specialized equipment, so it becomes a sort of voodoo. And, as most of you know, it's only a small part of what makes a bullet hole where you want it.;)

Have a good Turkey Day!
Kevin
 
Given a rt hand twist barrel, the stock would be felt to rotate CCW from
the shooters position. The bullet initialy would like to go straight but in very short order is made to rotate. The opposite reaction to this is torque felt
in the stock. Much more noticeable in round bottom stocks than Flat.
Faster twists transfer more torque as do higher velocity and bullet weight.
That force begins in the barrel and is transfered thru the action. Given a flat
bottom action, it is transferred from matted flat to matted flat. The round
bottom action transfers thru the surface also, but is at the mercy of
friction and guard screws. These screws are probably capable of resisting
the torque, on there own, however screws are not perfectly alligned
from threads to head. Some being bad enough that the action can be seen to
rotate when screws are threaded in. Remington rolled threads are bad, none
are perfect. Harold Vaughn was able to demonstrate that barrel tenons
move under stress of firing. I think its fair to say that actions move in stocks
with the best bedding particularly round ones. by taping the non torque side
you allow removal and reassembly but allow the action to rotate no farther
than its location when bedded.
 
Many years ago, a successful but atypical (because he shot Remington actions) California highpower shooter, told me that, with conventional epoxy bedding, in a .308 match rifle, that their bedding tended to destabilize ,due to torque, when shooters switched from 168 to 180 grain bullets. To combat this several adaptations were employed. A half inch thick recoil lug was sometimes used, with one side bedded. Another approach was to tack weld (TIG) a square half sleeve to the bottom of the front receiver ring.

One of the advantages of pillar bedding is that higher action screw torque may be employed without distorting bedding. Round factory actions, with magazines are a mite short on bedding area. Increasing the unit loading on the bedding acts like a proportional increase in bedding area.

In my opinion, given what has been learned in the last few years, anything but pillar or glue in bedding, in a stock of compressible material, is second rate, evidence that someone was not willing to spend the money and/or time to do it right.
 
IMO, aluminum pillars which are flat topped and not radiused to match the action can also be compressed and are therefore no better than regular
bedding. I think caliber is important here
 
Some very good pillar bedding jobs have been done with the pillars and adjacent bedding of Devcon aluminum putty.

Several years ago, George Kelbly told me that back in the day when pillar bedding was new, that they soon learned not do it with metal to metal contact, that there should be at least a thin layer of bedding material between the tops of the pillars and the action.

I have done a job, for myself, with aluminum pillars that were much wider at the top and glued them into the stock so that they were 1/4" below the action, a Remington. I did the balance of the bedding with Devcon aluminum. It worked very well. As to flat topped pillars being no better than conventional bedding, I disagree completely. BTW, you know this how?
 
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Boyd,

Mike Allen, a former VP of manufacturing for Dakota/Nesika (Mike was my boss when I 1st got there) told me the very same thing when I started there. That pillars shouldn't contact the receiver. This was 100% opposite of what I'd been taught when building Olympic smallbore guns in Colorado.

Mike is a metallurgist. He worked in a non destructive analysis laboratory for the Dept of the Navy prior to Nesika. His explanation was that with moisture present, two dissimilar metals generate a charge when in contact with one another. In essence, it creates a battery and will lead to corrosion.

This is why you see the "ears" whittled in the pillar locations in my photograph. A better photo that illustrates this is shown on my website under the products section. (No, I'm not trying to sell stuff) I have modified the process a little over the years and I had begun leaving the two contact pads in place. They are machined to the same radius as the action and prior to bedding I "lap" them in promote maximum contact. I just strap a piece of emery to the OD of the receiver and stroke em back and forth a few times in a little jig I made. I mark the pillar pads with a Sharpie. Once the marker ink is gone I quit.

I've not seen any improvement/reduction in performance with either method. My purpose was simply that when whittling those two little ears away sometimes the bedding would chip/flake around the contour and it made a cosmetic blem.

Interesting. ..

Chad
 
I think that George Kelbly was looking at it from an accuracy point of view, based on experience, not corrosion. As to small bore, I think that the forces are much less, and the barrel time a lot more, creating a different reality as far as the finer points of rifle tuning is concerned. My evidence is that the best of both types are markedly different in many respects. It is not that I think that contoured pillars are bad, just that one will get better contact, on a micro scale, with a cast interface, and the work can be done so that chip-outs are not a problem, by spacing the action far enough above the pillars to create a more durable filled epoxy layer. Also, I know that there is more than one way to skin this cat, and that if a thing works....it doesn’t matter if it fits into someone else's theory of operation.

One little anecdote about metal to metal contact of pillars on round actions... A former staff writer for Precision Shooting Merrill Martin, was an inveterate experimenter. Among his long list of experiments, was one that involved contoured aluminum pillars in the stock of a Savage .308 in which he shot 190 gr. bullets. After shooting the rifle he noticed fret marks on the action and tops of the pillars that he hypothesized were caused by torque induced movement of the action on the pillars. In his typical, out of the box manner, he sprinkled a few particles of dry silicon carbide grit on the tops of the pillars, reassembled the rifle, and retested the result. There was a definite change in the size and shapes of the groups, an improvement. It is from this that I learned the importance of traction, at the pillar action interface, when high firing torque is a factor, when using round actions. It would seem to me that this becomes progressively less important as the capacity of the cartridge, and especially the weight of the bullet is reduced, and as bedding area is increased.
 
Oky doky then.

I'll start mixing silicon carbide dust in my resins.

My tooling is going to looooove that!:D

Have a great t day everyone.
 
In order to do some calculations of torque, one needs to know the rotational inertia of the different masses involved, i.e. the bullet on one hand, and the rifle assembly on the other hand. Then throw in some general assumptions about the acceleration being linear, which it isn't, and all of the components acting as rigid bodies, which they don't. Then try to compensate and correct for all of these different "strength of materials" and elastic deformation values. Using finite element analysis programs, somebody a whole lot smarter than me could describe these motions pretty accurately. In the end you would have a lot of numbers on a sheet of paper that wouldn't help a whole lot in the longer term goal of putting a hole in the right place.

I would just like to visualize this motion of the various rifle components under recoil. I've been looking around, but virtually all the stuff I have found on the net is clips of idiots shooting shoulder cannons. I'm still looking for some serious high-speed videos of the sort that Savage said they used during the design and development of their accu-stock. If anyone knows of some good ones, let us know.

Happy Thanksgiving everyone.
Kevin
 
The numbers and equations are all fine, I just accept that the barrel does
torque in responce to bullet travel in a barrel. That it is transferred to
the stock in some manner or another and then to the shooter. To control
the shift that can occur its important to consider that different action shapes
may also transfer this in a different way. Some more repeatable than others.
Not all actions should be bedded the same way. A weatherby with the front
guard screw threaded into the bottom of the lug requires full contact at
the bottom of the lug. A Remington or savage ,no.thats just an example.
Now, I had not previously considered the galvanomic reaction of aluminum
pillars in contact with steel actions. IMO a well machined radius matching
the bottom of the action cannot be improved on. Flat topped pillars
are best left beneath the bedding surface allowing minimal compression
of the stock and allowing the the entire bedded area to recieve equal
loading from tension of the screws. Lapping pillars with emery around
the action with provide a larger radius , much the same as lapping scope rings
with a 1 inch bar. This can lead to problems in orientation when first
installed. We chamber within tenths, pillars can be machined to a great
fit also...BTW- the steel farrels installed as pillars in a savage are often
a bit high making a serious bedding fault. This allows the sales department
the claim that they are pillar bedded. In many cases those farrels can be
radiused down allowing the action to sit on the wood. Laminated stocks
being more resin than wood, are somewhat impervious to change. Fiberglass
can change as a result of post cure so they are not imune to change.
 
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