strength of 6-48 compared to 8-40

[big5ifty]

How much stronger is 8-40 compared to 6-48 ?

Is it a function of the area of thread ?

Is there a reason not to use 5mm-0.8 for a rail ?

 

[gunor]

Shear strength is a function of the cross-sectional area of the cylindrical part of the screws. The 8-40 would be stronger than a 6-48.

Copied that.

And a nice thread…

SEMI-PRO SCOPE MOUNTING

http://stonetip.com/diagrams/alignment_bars.pdf - should convince the skeptical that pointed bars...

www.opticstalk.com

Why introduce metric screws? At least in the US, we use Imperial for our gun screws.

 

[big5ifty]

We use metric. Much easier to find metric screws here.

 

[Clark]

A lot of guys think the stress on scope base screws is in shear. The screws are clamping with tension. The shear forces are between the scope base and the receiver.
Some things to keep in mind:
1) a) The screws will fail in tension if torqed too hard if there is MORE than ~1.5x the root of thread engagement.
b) The female threads will strip out if torqed too hard if there is LESS ~1.5x the root of thread engagement.
2) Max torque ratings for screws may be for #5 grade, but that chart should be tempered with:
a) 100 % for dry threads
b) 75% for oiled or greased threads
c) 50% for waxed threads.
3) A scope may stay sighted in for years until oil gets under the scope base.
The coefficients of friction for scope base to to receiver connection:
Clean and dry: 0.76 - 0.80
Lubricated: 0.16
If there is 400 pounds of clamping force and 4 [6-48] screws, the 1600 pounds of clamping force can withstand 1200 pounds of recoil force on the scope with dry base to receiver connection. But if oil creeps under the base, only 250 pounds of recoil force on the scope mass will be resisted.

Using Quickload:
142 gr bullet maximum acceleration is from 500 fps to 2000 fps in 0.4 ms then an 8 pound rifle will try to accelerate from 1.27 fps to 5.07 fps in 0.4 ms. This is an acceleration of 9500 ft/sec squared.
If a 2 pound scope were part of that 8 pound rifle during that acceleration the force between the rifle and the scope would be
f = m a = 2 pounds 9500 f/ss = [9500 f/ss] [2 pounds / [Gc = 32 f/ss]]= 594 pounds recoil force on scope.

 

[jelenko]

a) The screws will fail in tension if torqed too hard if there is MORE than ~1.5x the root of thread engagement.
b) The female threads will strip out if torqed too hard if there is LESS ~1.5x the root of thread engagement.

What is " 1.5X the root of thread engagement" expressed in?

 

[Clark]

What is " 1.5X the root of thread engagement" expressed in?

expressed in a ratio.
Threads have a major and minor diameter.
axial engagement = 1.5 times the root = 1.5 times the minor diameter
1.5 = engagement in inches / minor diameter in inch
The inches unit cancels.

 

[jelenko]

Thanks!

 

[big5ifty]

Bigger is better ?



From 6-48 to 5mm 0.8.

 

[GenePoole]

8-40 plus pinning with oversized pins is the way to go for a R700:

 

[rr2030]

Topics and information such as in this thread, is why I enjoy this forum.

 

[3Sigma]

A lot of guys think the stress on scope base screws is in shear. The screws are clamping with tension. The shear forces are between the scope base and the receiver.
Some things to keep in mind:
1) a) The screws will fail in tension if torqed too hard if there is MORE than ~1.5x the root of thread engagement.
b) The female threads will strip out if torqed too hard if there is LESS ~1.5x the root of thread engagement.
2) Max torque ratings for screws may be for #5 grade, but that chart should be tempered with:
a) 100 % for dry threads
b) 75% for oiled or greased threads
c) 50% for waxed threads.
3) A scope may stay sighted in for years until oil gets under the scope base.
The coefficients of friction for scope base to to receiver connection:
Clean and dry: 0.76 - 0.80
Lubricated: 0.16
If there is 400 pounds of clamping force and 4 [6-48] screws, the 1600 pounds of clamping force can withstand 1200 pounds of recoil force on the scope with dry base to receiver connection. But if oil creeps under the base, only 250 pounds of recoil force on the scope mass will be resisted.

Using Quickload:
142 gr bullet maximum acceleration is from 500 fps to 2000 fps in 0.4 ms then an 8 pound rifle will try to accelerate from 1.27 fps to 5.07 fps in 0.4 ms. This is an acceleration of 9500 ft/sec squared.
If a 2 pound scope were part of that 8 pound rifle during that acceleration the force between the rifle and the scope would be
f = m a = 2 pounds 9500 f/ss = [9500 f/ss] [2 pounds / [Gc = 32 f/ss]]= 594 pounds recoil force on scope.

This might hold true for a rail with a recoil lug or pin, but not for those without. Once the friction from the normal clamping force is overcome, those screws WILL be in shear and bending and tension during a shot. Which load causes them to fail? All of them. You can't put a blanket statement on the levels as scope, scope rail, ring, action surface finish, rail bedding, rail surface finish, screw clamp load, gun weight, cartridge, gun CG all come into play.

Suffice to say, a recoil lug on a bedded rail with oiled screws torqued properly is pretty solid. A pin would be even better.

 

[Machinist John]

I use 8-40 on big bore hunting rifles, I have used 10-32 on the really big boomers.

And I do not hesitate to put in dowel pins after the rifle is range tested and sighted in.

Big Bore Ruger No. 1 single shots have large dowel pins in the rib to the barrel. Very tight, I have removed a few.