Why to bed over the tops of the pillars

[gunsandgunsmithing]

According to Devcon's literature, the shrinkage rate for their 10110 putty is .0006/ubic inch. If the bedding is say .100 thick, it's shrinkage is only a tenth of that, so a very indiscerneable amount either way. I'm not sure over what period of time they use to measure it in it's "cured" state..or how old it is, iow. That said, if so, bedding shrinkage is a non-factor BUT...stocks shrink for a long time and wood both expands and contracts. Actually both do to a degree. Point being, fiberglass resins shrink a lot more than devcon's stated amounts and can still be curing for a long time. You can see it on some painted stocks over time, where it's slick as glass when new but a few years down the road, you may see the fiberglass pattern through the paint. The reason I think it's the stock more so than the paint shrinking is that you don't see it imperfections over metal, like sanding scratches for example, suffer the same thing.

Bottom line, I suspect most(not 100%) of the shrinkage in Al's pic and in general is from stock shrinkage more than the epoxy itself. IMHO, it may take literally years for some resins to fully cure to the point of no shrinkage at all. Again, jmo but if devcon's numbers are accurate, you simply can't see 60 millionths(@.100 deep) with the naked eye, which brings us to...how did "Al's bedding" shrink visibly. I'd say the substrate moved a lot more than the bedding is all. Also, what about the core material of the stock? I bet it really shrinks compared to devcon.

 

[AlNyhus]

Bottom line, I suspect most(not 100%) of the shrinkage in Al's pic and in general is from stock shrinkage more than the epoxy itself. IMHO, it may take literally years for some resins to fully cure to the point of no shrinkage at all. Again, jmo but if devcon's numbers are accurate, you simply can't see 60 millionths(@.100 deep) with the naked eye, which brings us to...how did "Al's bedding" shrink visibly. I'd say the substrate moved a lot more than the bedding is all. Also, what about the core material of the stock? I bet it really shrinks compared to devcon.

Just to clarify what I'd written in my first post......this is not a bedding job that I did.

It came to me in this condition. -Al

 

[LVLAaron]

Sure it did, Al. (just kiddin)

 

[Kracken McGraw]

Well if the bedding is shrinking, wouldn’t that area shrink too?

Maybe gravity doesn’t affect it behind the lug. lol.

 

[gunsandgunsmithing]

Just to clarify what I'd written in my first post......this is not a bedding job that I did.

It came to me in this condition. -Al

Sorry Al. I knew that and is why I put it in quotation marks.

 

[AlNyhus]

I knew you did, Mike. Just wanted to make sure everyone did. -Al

 

[DShortt]

If the epoxy shrinks, how do glue-in jobs ever stay tight?

Serious question. Not trying to be an Evinrude in the septic tank.

 

[V35]

Not trying to be an Evinrude in the septic tank.

That made me laugh, actually spit out a little beer.

 

[Alex Wheeler]

If the epoxy shrinks, how do glue-in jobs ever stay tight?

Serious question. Not trying to be an Evinrude in the septic tank.

Most of the shrinkage happens during curing. So if you bed the action first and let it cure, then you glue it in you wont have a problem.

 

[DShortt]

Most of the shrinkage happens during curing. So if you bed the action first and let it cure, then you glue it in you wont have a problem.

So the glue in is actually a 2 step process? First the action is bedded then it's glued in?

Same epoxy or two different types?

I really appreciate the benefit of your experience. Just wanted to say that beforehand.

 

[DShortt]

That made me laugh, actually spit out a little beer.

Well it's unfortunate that such a disclaimer might be necessary but so it goes.

 

[Alex Wheeler]

So the glue in is actually a 2 step process? First the action is bedded then it's glued in?

Same epoxy or two different types?

I really appreciate the benefit of your experience. Just wanted to say that beforehand.

I dont know how others do it, but I do 2 steps and use jb for the glue. I do not like the idea of a one shot glue in, dont know if guys do it or not.

 

[DShortt]

I dont know how others do it, but I do 2 steps and use jb for the glue. I do not the the idea of a one shot glue in, dont know if guys do it or not.

I appreciate your time. Thanks!

 

[LVLAaron]

@AlNyhus - How old is that stock... and what's it filled with?

 

[mikecockcroft]

So the trick is to buy composite stocks that are 10 years old.

 

[AlNyhus]

@AlNyhus - How old is that stock... and what's it filled with?

It's an older McMillan. Heavy 'glass fill, no graphite, no foam. -Al

 

[TaperPin]

Out of curiosity I googled polyester resin shrinkage, and the quote below is off an industry trade publication. I did not know about the additives that expand during the heat of the cure to offset some shrinkage. Maybe that pillar was bedded in a cold shop.

“The problem of resin shrinkage is that after curing, its space volume becomes larger, so we can reduce the shrinkage by adding some fillers.

The common inorganic fillers are calcium carbonate, talcum powder, aluminum hydroxide, etc. these fillers generally do not participate in chemical reactions and belong to inert materials. The shrinkage can be reduced by reducing the resin content per unit volume and occupying pits.

In addition, the shrinkage can be controlled by adding low shrinkage agent (LPA), which is mainly composed of some thermoplastic resins, elastomers or composite polymers. In addition to pit occupation, in the process of resin exothermic, thermoplastic resin will have a certain expansion, which can offset the volume shrinkage caused by the curing process of thermosetting resin, so as to reduce the shrinkage rate more effectively. “

 

[AlNyhus]

I had some fab stuff to do at the race shop today so took the stock with me. When the pillar was originally done, it was a 1" pillar that stopped .250 short of the bottom of the stock. Then a 5/8" opening was made in the stock. The circumference of that opening was glassed and fairly well finished.

My plan was to open that hole to .750 down to the level of the original pillar and then bore the original pillar out and make a stepped pillar with a .750 lower section to fit nicely in the stock. I went down in with the .750 end mill and as soon as I touched to bottom of the original pillar....it felt like mush. The original pillar simply pushed out with hardly any resistance. Looking at the pillar, you can see why....the epoxy grooves are narrow and shallow and very few of the grooves show any evidence of holding any epoxy at any time. There was no epoxy present or adhering around the periphery of the pillar hole in the stock.

 

[LVLAaron]

Is it possible the bedding didn't shrink... and the pillars worked their way up?

 

[AlNyhus]

Could be. But the rear 1/2" pillar was also high and it was stuck in there "...tighter than Dick's hat band." (Rooster Cogburn in 'True Grit')

Both action screws had been contacting the rear of the pillar holes. Even with a tight fitting lug, they undoubtedly took a battering over the seasons of competition..

No matter....all will be good in short order. .-Al