Does a barrel tenon need an undercut at the end of thread?

C

csdilligaf

New member
When I thread on my Prototrak 1630 lathe with conversational programing I run in full CNC mode and do not need an thread relief at the end of the thread. Can I just taper out the thread under the recoil lug and call it good. Or is there more of a stress riser issue this way and may be a fracture point there? I've been doing the thread relief but dont know if it is needed.

thanks
 
csdilligaf said:
When I thread on my Prototrak 1630 lathe with conversational programing I run in full CNC mode and do not need an thread relief at the end of the thread. Can I just taper out the thread under the recoil lug and call it good. Or is there more of a stress riser issue this way and may be a fracture point there? I've been doing the thread relief but dont know if it is needed.

thanks
Click to expand...

Not needed in your situation.
 
It's actually better not to use an undercut for a couple of reasons. I make the diameter behind the thread fit the hole on the recoil lug within .001 as a locator. Also if you ever want or need to set the chamber back, you don't have to worry where the groove is going to end up along the thread.
 
"Prototrak 1630 lathe with conversational programing I run in full CNC mode"

I'd give my left one to have such a machine & the knowledge to use it!

Many of us use machines that are primitive compared to yours but still get the job done with no relief cut. The area left at the shoulder is covered by the recoil lug, or the first couple of threads in the action are removed if the recoil lug is not used. Some actions come that way, and on Remingtons it's part of truing.

Regards,
Ron
 
The only time I use a relief cut is on an action that has the threads in the receiver right out to the face, Mauser as an example. Most of the Wichesters and Sako actions have a relief in the front of the action threads; the removeable recoil lug on the Remingtons and savages act like that relief.
 
csdilligaf said:
When I thread on my Prototrak 1630 lathe with conversational programing I run in full CNC mode and do not need an thread relief at the end of the thread. Can I just taper out the thread under the recoil lug and call it good. Or is there more of a stress riser issue this way and may be a fracture point there? I've been doing the thread relief but dont know if it is needed.

thanks
Click to expand...

Threads are definitely stress risers, particularly next to a shoulder. You can get a reduction in stress concentration by tapering out the threads, or cutting a relief down to the minor diameter. Adding a radius, the larger the better, between the tenon and the shoulder also reduces stress concentration. But the torque used to tighten barrels and the number of cycles are so low that the joint never gets close to fatigue failure in normal use. Still, adding these features contribute to the overall optimization of the design, and to the extent that it gives you more confidence in your rifle, they can't hurt.
 
Rflshootr said:
It's actually better not to use an undercut for a couple of reasons. I make the diameter behind the thread fit the hole on the recoil lug within .001 as a locator.
Click to expand...
Measurement taken when it's cold, right? And what is it when things heat up?

Also if you ever want or need to set the chamber back, you don't have to worry where the groove is going to end up along the thread.
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As long as the groove is the minor diameter of the thread, not an issue. It really only takes about 5-6 threads generally, probably less in this particular joint. With 16-tpi, that's 1/3 inch, if my math hasn't abandoned me again. Why not have them at the end of the barrel (chamber end) to aid in alignment.

Speaking of alignment -- Now 4Mesh, who use to make his own actions, favored an 8-tpi thread...Assume he's a smart guy (that's a safe assumption), and figure out why...
 
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I just traded for a farley black widow, and i really like the short .700 tenon. When it comes time for a set back i will just cut the hole dam tenon off and start over. Nothing much lost!!! I wish all my actions had this tenon. Lee
 
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