Cliff Grays Podcast with Aaron Davidson

[BBob]

Just to add info: When you tighten common action/barrel thread joints to the same torque spec but use different thread TPI's (16,18, 20 tpi) the amount of rotation of the barrel before full lockup at that torque are all noticeably different. Basically when fitting/timing fluted barrels you have to account for different amounts of rotation per the tpi used in that joint at a specified or desired torque. What is better? I don't have an opinion on that as they all work.

I'll add that anything north of 100ft lbs starts to rapidly get harder and harder to hold without slippage. Not impossible but that's about the point where I've found it to start to noticeably change and where you might need a good insert type barrel vise over something like a Viper style.

 

[nm.otter]

discussion going on right now over at snipershide in a thread started by a guy saying 100 fl/lb tq on prefits is stupid and shouldn't need more than 35.

After reading all (and understanding most) of that, I'm not going to worry about Tikka prefits.

Also downloaded the Vaughn book for mid-winter reading.

 

[Klooop]

............

• Owner of PVA recommends 70 for standard bolt face and 100 for magnums and provided some calculations to justify it (responding to my questions that mimicked some in this thread). However, he indicated the pre-load with the above torque values exceeded the force of the rounds being fired.

Barrel Torque Ludicrocity

lol. Really though. 100ftlbs for prefit barrel torque? ****, I know this isn’t necessary. AR15 armorer schools is min 35ftlbs unless old school barrel nut needs advanced to next cut out for gas tube clearance. Then it’s 35-65ftlbs range. I run 55ftlbs on my bolt gun Savage style barrel nut...

www.snipershide.com

Those are the basic calcs I would expect to see, bolt preload as a function of applied torque. Then comparing to axial thrust from firing a round. Good stuff!

One thing I didnt see discussed is substantial variability in k-factor, torque uncertainty/error, etc. It is possible to have ~25%-35% preload variability for similar hardware, torqued to similar conditions. Ref NASA-STD-5020 for some additional fun reading on spaceflight fastening systems.

It's possible a lot of issues are simply a result of low nominal margin to separation on the preloaded joint. We always size fasteners to work at both min and max preload for a given torque.

Oh, and these barrel threads are nowhere close to anything other than normal localized yeilding...faster systems are frequently sized to 70% or 80% (or more) of yield for high cycle fatigue applications, which still keeps the overall faster in the elastic range.

I wouldn't have any concern torquing these barrels to a couple hundred ft-lbs as long as the tooling and resulting headspace can accommodate it. Galling would be a concern, too - especially for stainless joints.

Good discussion here. I need to read more into the posted book.

 

[Aaron Davidson]

If we assume that the joint moves, then we only need to consider how to get it to "tighten" back to center.

Thread finish is a factor. Scope base screw holes penetrating and interfering with the thread are a problem. If separate, recoil lug touching the barrel is an issue. Any touching of the barrel to the stock (or bedding).

When the joint comes loose, everything needs to be allowed to move naturally back the "center". The 60 deg thread does most of this, but if something causes a hitch, then its an issue.

Sidebar, this is why we started threading barrels with a CNC lathe 15 years ago. Surface finish....

 

[BBob]

Sidebar, this is why we started threading barrels with a CNC lathe 15 years ago. Surface finish....

While it may not be able to fully rival CNC some of us have put effort and $$$ into machining the best threads we can make on manual machines. Good equipment with a careful and repeatable setup with good inserted tooling (tested and purposefully selected) goes a long ways in achieving this. I can say I can hit repeatable barrel timing pretty easily when everything is the same or near same which wouldn’t happen if your technique and setup wasn’t good or repeatable. For the common action thread pitch’s I have pretty repeatable numbers for barrel rotation at various torques and am able to hit a small window of rotation when torquing a barrel. I’m sure you (Gunwerks) do too.

 

[Aaron Davidson]

While it may not be able to fully rival CNC some of us have put effort and $$$ into machining the best threads we can make on manual machines. Good equipment with a careful and repeatable setup with good inserted tooling (tested and purposefully selected) goes a long ways in achieving this. I can say I can hit repeatable barrel timing pretty easily when everything is the same or near same which wouldn’t happen if your technique and setup wasn’t good or repeatable. For the common action thread pitch’s I have pretty repeatable numbers for barrel rotation at various torques and am able to hit a small window of rotation when torquing a barrel. I’m sure you (Gunwerks) do too.

For sure. You have to have the right insert. Most are using inserts designed for the 1000 rpms you need to get the correctly calc'd surface speed at the 60 rpms a human can manage manually threading. The micro tearing result on the thread surface creates friction that can "stick" a barrel in an unusual position.