Single Point Thread Repair

[David2011]

Too many Dave's on here ! Yes , I understand what you're saying and I've done all of the above , and I've done many a BIG Barrel DoD guns . My point was from a newbie's perspective . Once you get the 29.5 procedure down , you can move on to whatever it takes . I can't tell someone that doesn't know his butt from a hole in the ground to do whatever works for him as he just doesn't know . I know wrmiller knows what he's doing , but unfortunately can't do the 40 tpi on his newer/older lathe .

Just to add . The barrel was threaded to size but not concentric . If the existing thread was picked up and threaded , it would be way undersized . Wouldn't trust that on any gun . May have to build it up and re-chase the threads for a decent repair .

No worries: I didn’t think your previous response was for me. I’ve never answered to “Dave” except to my Dad and one aunt, both of who have passed. For a few years I squadded at pistol matches with 4 other guys named Dave or David. We all quickly learned to ignore the derivative of the name that didn’t apply to us. Fortunately, we didn’t have to do any machine work to the barrels. It’s a battleship restoration so the breeches needed to operate properly and smoothly but they’ll never be fired again. They elevate and train like new now. We just finished all 6 of the 5”/51 caliber guns. They had to be finished before the ship is moved out of the dry dock; too big and heavy to put them back on the ship using a barge and crane. The dry dock has solidly mounted big cranes to handle that delicate job. Now it’s time to restore the remaining 3” guns and 7 more 40mm quad Bofors.

 

[mmcmdl]

Yep , I've been following your posts for awhile as it interests me . As far as too many Daves , we had 3 mechanics at Unilever with the same initials . DHM . No sense in engraving tools as the company paid for them .

 

[wrmiller]

Just to add . The barrel was threaded to size but not concentric . If the existing thread was picked up and threaded , it would be way undersized . Wouldn't trust that on any gun . May have to build it up and re-chase the threads for a decent repair .

Last time this happened, IIRC, I simply took the 'high spot' off the non-concentric thread, which got the comp a little closer to concentric with the bore. I also cut the thread just a bit looser for a little bit of clearance between the barrel's threads and the comp's threads.

I didn't try to completely repair the thread, because as you said, it would end up WAY undersized. This also added a bit more room for the red loctite. Since I added a bit of 'slop' to the thread interface, I turned a spud that was the barrel bore size on one end and the comp's bore on the other end. This kept everything in alignment as the red loctite set.

The pistol (a 10mm) has been running great for years now. My friend keeps telling me that he's going to replace the barrel and comp some day, but it hasn't happened yet.

I have a fair number of different sized pulleys for this lathe, and I'm sure some of you more numerically proficient types could figure out a setup for 40 tpi, but I'm not sure I can. I can do some of the more abstract math, but counting/timing gear/pulley teeth seems beyond me. It's probably a genetic deficiency....

Again, thanks guys.

 

[Jake M]

I can do some of the more abstract math, but counting/timing gear/pulley teeth seems beyond me. It's probably a genetic deficiency....

Probably not a genetic deficiency. Probably just a lack of practice.

If you can "math" at all gears are easy. The problem with gears is that it's SO easy to over complicate them. ESPECIALLY of you've got any higher math under your belt before you ever start to play with gears. It's (literally) third grade math, fifth grade science, but it's applied to a subject that you probably first saw somewhere in mid high school, and by then it's pretty tough to "unsee all of the other math you learned in between. Just like most things, it takes a general familiarity that comes with repeated dealings with the subject to make it "easy".

Not saying you "should" go and "math" all of your gears if you don't wanna.... Just that you could if it suited your needs in the future. And you're right, good call on the subject at hand- If there's ANY doubt on that subject... Well, there you go.

 

[wrmiller]

My 'gear train' is a bunch of different sized pulleys, driven by trapazoidal belts. And I think some of the belts have different pitches/tooth count.



The feed rate is currently very slow (slowest?) and is much finer than 40 tpi. I'll have to figure out the correlation between feed rate and threading tpi. Could be interesting.

The pulley setup is simpler for threading than it is for power feed.



Something to do.

 

[Jake M]

You only instead in pickup the thread with compound. Trying some other way is a big pain and will take time in most case.

It's really not a pain in any way. It's the exact same motions, on the exact same crank handles, using the exact same visuial/tactile method, since the dimensions are lost. Simpler in fact sometimes, especially if your universal threading tool has a tip radius maybe just a little different than the original thread. Rolled threads are the worst for this (I know the ones in question here are not that....). The 30 degree (or a whisker less) angle allows you to "find" the side of the thread instead of trying to estimate the center of the valley, at which point you just feed in the compound

=========================

The 29.5° was first used on very light lathes for cutting corse threads.

This is a very real thing that a lot of us here have to or choose to live with...

 

[wrmiller]

Feed rate on this lathe is approx. .003 to .005. No where near .025/40 tpi.. Time to make a custom 1.5" die holder for my little MT2 tailstock.