Drilling depth for small screw?

[holzwurm]

1. There is a tiny flat head screw that holds the nose cap and stock wood to the barrell a rifle I'm working on.  I'm pretty sure the screw size is a 5-40 flat head. I know how to jig every thing and corectly size the clearance hole through the brass and wood and countersink the screw head. What I do not recall is how deep to drill into the barrell to tap in three or four thread's for the screw.

2. Somewhere in the dim distance past I had a machinist tell me that the minimum number of threads in a blind hole to considered a screw properly seated was three. More are better of course, if you are working with soft metal and really shallow threads, but at some poiint you reach overkill and (in the case of a barrell) punch through.  He also repeated a formula which had something to do with Pi. 

3. Can anyone recall the formula?

[Stophel]

I've done one screwed-on nosecap, and made a dovetailed post to go on the bottom of the barrel and threaded it so as to avoid drilling holes in the barrel.  Plus, I was copying the original gun.   

[bgf]

If you want 3 threads engagement at 40 tpi, you need a hole 3x(1/40) = 0.075" deep, plus whatever you need for the bottom tap to clear (1 thread or 0.025"?).  So, a hole 0.1" deep, assuming that doesn't go too deep into the barrel wall.  The pressure is less that at the breech, but you still want some metal there.

Don't take this answer as the truth, my hope is only to goad someone who knows the right answer to chip in.  You could also work backwards from a safe wall thickness and see how far you can drill in to get maximum thread engagement, if it works out better.

[Blacksmoke]

Holtzwurm:    I do not use this method to attach my muzzle caps to the barrel because it does not allow for any forestock shrinkage or expansion.   I have seen many older longrifles where the forestocks were intirely separated apart because there was no provision made for shrinkage.   Wood is always moving and some provision should be made for it's movement.  I also slot my barrel tenons for that reason.  I attach my muzzle caps to the forestock with means of a rivet only and let the wood and cap move as they will.   Hugh Toenjes

[holzwurm]

The reason I'm asking is the original ((C. Wilheim Freund, ca 1750) was finished like this. The gun was last years pattern for the gun making cass in Kentucky and I saw a couple of other classmates doing this. 

[Jim Kibler]

holzwurm,

If you leave at least maybe .080" or so wall thickness, there shouldn't be any issues.  You could likely get away with much less, but I suspect leaving this amount will provide plenty of depth for your screw.  If you can get enough thread engagement with less, all the better.  As with blind holes in lock plates, it helps to make a flat bottomed drill and use a series of taps to create threads as close to the bottom of the hole as possible.  Hope this helps.

Jim

[bgf]

Regarding the movement problem, if the nosecap moves with the barrel and there is a little slop in the hole through the wood, wouldn't it be OK?  The gap b/t wood and nosecap might open up, but that is cosmetic at most and a tiny fraction of an inch at worst, only while the barrel is hot.

[Blacksmoke]

BGF:   Believe it or not -  I've seen forestocks on long rifles contract and expand as much as 1/8"just due to the change in humidity from one climate to another.  This especially true if you do not seal the barrel channel well.     Hugh Toenjes

[David Rase]

The rule of thumb is that a screw reaches its maximum strength when the threads are engaged the same depth as the diameter.  In the world I play in we require 1 1/2 the diameter of thread engagement.  I am sure you do not need full strength and 3 or so threads of engagement will be plenty.
DMR

[Jim Kibler]

I believe the project this pertains to is a short barreled, straight grained, English walnut stocked gun.  This all adds up to a decreased liklihood of expansion and contraction issues.

[Ron Scott]

Jim is correct in his recollection. The barrel length is 24.5 inches and the  stock is straight grain English Walnut.  The barrel thickness at the muzzle is pretty  thick as well.  I think Jim's estimate on depth is correct.  While the original rifle is no longer here for consultation, I think the diameter of the threaded screw was closer to a # 8.

[Acer Saccharum]

The screwed on nosecap is seen frequently on jaegers, which are short barreled. On some photos of originals, the wood has shrunken, and pulled out of the cap 1/16. The wood can slide in and out of the cap, by all appearances.