Threading fit issue, tapered?

[tobnpr]

In speaking to one of my other gunsmith buddies, he says one of the old timer national benchrest champs, and custom bullet maker Roy Hunter, told him that he picks the loosest fitting barrel of the bunch... This blew my mind...

This is one of the first concepts I learned. I had been hearing from some "pros" that they cut their threads gnat's a** tight and literally lapped them into place. Discussed with a well known in the industry toolmaker. Worst thing you can do...

Threads just bring the parts together. It's the precisely perpendicular shoulder/receiver rings that give you the perfect alignment needed for a precision stick. "Tight" threads will interfere with that...

 

[Harry Knutz]

I've started using my thread mics, verified my threading insert is properly installed and not at any angles, and started taking thinner cuts when I start getting close to measuring depths.

This has helped some on the muzzle end, but I need to test the breech/chamber end that attaches to the receivers - this is the one I had been having most difficulties with.

In speaking to one of my other gunsmith buddies, he says one of the old timer national benchrest champs, and custom bullet maker Roy Hunter, told him that he picks the loosest fitting barrel of the bunch... This blew my mind...

A little bit of a loose fit on the barrel to receiver threads is much better than a tight fit. This allows the barrel tenon shoulder to align the barrel to the receiver. It's more important to get everything straight and dialed in.

A thread that is too tight is asking for trouble not just with alignment, But many times it will gall when you are trying to unscrew it.

20 or 30 years ago Gunsmiths were of the school of thought that tight threads were better, It has changed now to a little bit looser threads are much better now.

 

[extropic]

I've started using my thread mics, verified my threading insert is properly installed and not at any angles, and started taking thinner cuts when I start getting close to measuring depths.

This has helped some on the muzzle end, but I need to test the breech/chamber end that attaches to the receivers - this is the one I had been having most difficulties with.

In speaking to one of my other gunsmith buddies, he says one of the old timer national benchrest champs, and custom bullet maker Roy Hunter, told him that he picks the loosest fitting barrel of the bunch... This blew my mind...

Very good!

 

[gard]

I am curious about thread mikes, do they tell you anything more than measuring over wires? More accurate or just easier to use?

How would you know if the pitch was slightly off? What would be the best way to measure pitch? I am picturing something like a lead screw worn in one area or perhaps 1/2 nuts that move a little at one end of the cut. Obviously if the male and female threads are slightly different pitches it will get tighter the further it is screwed together. I can see where Jake M's suggestion of a thin nut might help.

 

[GeneT45]

I am curious about thread mikes, do they tell you anything more than measuring over wires? More accurate or just easier to use?

How would you know if the pitch was slightly off? What would be the best way to measure pitch? I am picturing something like a lead screw worn in one area or perhaps 1/2 nuts that move a little at one end of the cut. Obviously if the male and female threads are slightly different pitches it will get tighter the further it is screwed together. I can see where Jake M's suggestion of a thin nut might help.

They're pretty much like measuring over wires. They're nominally measuring pitch diameter, so deviation from that (beyond tolerance) should find the specific problems you mentioned. You need to swap points / tips by thread pitch, which means that for a single measurement wires are often more convenient (to me, anyway), but if you're checking a long thread at many places it's probably worth setting up the thread mic. Of course, if you frequently do the same thread, then you can leave your mic set up and have that convenience.

If you want mics for general threading, you may have to buy more than one set as they usually have a pretty limited range.

That's a long-winded way of saying that I have two threading mic kits and although I thread frequently, I don't use the mics nearly as much as I thought I would.

GsT

 

[WobblyHand]

I am curious about thread mikes, do they tell you anything more than measuring over wires? More accurate or just easier to use?

How would you know if the pitch was slightly off? What would be the best way to measure pitch? I am picturing something like a lead screw worn in one area or perhaps 1/2 nuts that move a little at one end of the cut. Obviously if the male and female threads are slightly different pitches it will get tighter the further it is screwed together. I can see where Jake M's suggestion of a thin nut might help.

I don't know a direct way, but if the pitch diameter varies along the length it's an indicator of wear, and possibly pitch variation due to wear.

I'd think true pitch variation without wear some place in the system would be tough to generate on a regular geared lead screw.

 

[WobblyHand]

I'd bet that under some conditions an ELS could generate variable pitch threads. Wow, that's both interesting and terrifying. At least in my ELS system I haven't observed it, but I haven't gone looking for that either!

 

[gard]

So I guess the 3 wires will give a measure of pitch diameter that is relatively independent of a tiny change in the thread pitch. The thread mikes are sort of measuring a combination of both because if the thread pitch of the mike is slightly different from the cut thread it will not fit all the way to the bottom?
So 2 possible different causes for the OP problem, either the pitch diameter changes (like tapered threads) or the thread pitch is slightly different between the male and female threads. I wonder if blueing might show, it seems if the pitch was off you would see contact only on one side of each thread? No idea how to get a thin coating of blueing on the threads.

 

[WobblyHand]

Thread mics have different anvils corresponding to thread pitch, but they cover a certain range of pitch. At least for normal threads, they assume 60 degree threads. I'd guess they are sized not to bottom out within their pitch range.

Thread wires also need to be chosen according to expecting pitch, so that the correct geometry is present so the equations are valid.

Regarding the original question of the OP, are both male and female threads cut in the same direction? If not, and there's different wear on the leading and trailing edge of the lead screw thread, could this account for subtle difference in pitch? Or pitch variation ? In other words, have the lead screw and half nuts been inspected microscopically for asymmetric wear? Are the thread angles the same (mirror image) and square to the screw's axis?

Can both threads be cut in the same direction? If they are now, I'm running out of ideas. Life line anyone?

 

[Ischgl99]

I am curious about thread mikes, do they tell you anything more than measuring over wires? More accurate or just easier to use?

How would you know if the pitch was slightly off? What would be the best way to measure pitch? I am picturing something like a lead screw worn in one area or perhaps 1/2 nuts that move a little at one end of the cut. Obviously if the male and female threads are slightly different pitches it will get tighter the further it is screwed together. I can see where Jake M's suggestion of a thin nut might help.

For the case you mention, thread micrometers or wires would be the first step in identifying the problem. You wouldn’t know if different measurements were cutting deeper in one spot, or if the pitch changes for some reason without further investigation. Thread micrometers are a direct reading version of wires, so there really isn’t much difference between them. The micrometers can be faster, and you don’t worry about dropping a wire in the chip pan, or wishing you had a third hand to hold everything while measuring.