What's wrong with my threading technique?

[Yondering]

Jerry, I wasn't guessing; I gave an example that I have personal experience with. That's why I said "for example". It certainly could be a very minor difference in pitch, nobody knows. You're theory though is based on an assumption that both threads are of correct pitch diameter - but if you continue to feed the tool in further to cut a deeper thread, as the OP said he did, the wrong pitch will thread on farther and farther. That's what I experienced, and that's what the OP described. The actual values are not as important as the concept, and who knows if the OP actually meant a specific number of threads literally, or just as an example.

I said "once again" because you have a consistent habit of insisting your theories about what "should" be are more valid than someone else's actual experience. It detracts from the overall value of the forum.

David Powell, obviously we are trying to help the fellow, and what you suggested is pretty well covered under the previous suggestions that there is a pitch diameter mismatch. Any of those are possible, up to the OP to figure out what since there's no way for us to know for sure.

[Illinoyance]

OP mentioned buying a thread mike. Real Spendy! PeeDee wires are cheap.

[txfireguy2003]

Okay, to answer a few more questions.

The desired pitch is 5/16-24 UNF.

The tap is the correct tap.

The shaft major diameter was cut to 0.311.

Once threads were cut to desired depth, I touched the tops with a flat file and ran a triangle file through the threads several times.

I did use lube, a dark heavy oil.

My cuts were never more than .010" deep (on the radius, .020" on the diameter).

The male threads appear to match the tap as well as the thread gauge for 24tpi.

I'll take some pictures tomorrow afternoon when I get home.

The only thing I was thinking, was either a slight taper was created somehow, or maybe my threading tool isn't set exactly square to the part, maybe off by 1 or 2 degrees, but I don't think that's the case.

Thanks again for all the help guys.

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[garyhlucas]

When threading a cut of 0.010" near the end is HUGE! You could be deflecting the part like crazy at that size. The last cuts need to be much smaller, maybe 0.001"

[David Powell]

Okay, to answer a few more questions.

The desired pitch is 5/16-24 UNF.

The tap is the correct tap.

The shaft major diameter was cut to 0.311.

Once threads were cut to desired depth, I touched the tops with a flat file and ran a triangle file through the threads several times.

I did use lube, a dark heavy oil.

My cuts were never more than .010" deep (on the radius, .020" on the diameter).

The male threads appear to match the tap as well as the thread gauge for 24tpi.

I'll take some pictures tomorrow afternoon when I get home.

The only thing I was thinking, was either a slight taper was created somehow, or maybe my threading tool isn't set exactly square to the part, maybe off by 1 or 2 degrees, but I don't think that's the case.

Thanks again for all the help guys.

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Hello again. I am a pretty lazy machinist, for me the big thrill is not running the lathe or mill but getting steam up in the models I have built or repaired. Many of the threads I meet are fine threads including 40 32 and 26 tpi jobs. I want them to be straight, on size and generally " Correct" BUT I want them ' NOW" I have dedicated die holders for most sizes and I commonly single point thread using hss tools to about 75 % of thread depth and then , using the tailstock die holder, guided loosely( Ie with a reamed hole guided on a turned pin with about 5 thous total clearance) held in my fingers I take the final cut. Cheating in this way normally produces the threads I need. IF I pick up the wrong die holder, but the right size my fingers tell me within a couple of turns. I hope this is of interest and help to someone. Regards David Powell.

[txfireguy2003]

When threading a cut of 0.010" near the end is HUGE! You could be deflecting the part like crazy at that size. The last cuts need to be much smaller, maybe 0.001"

The last cuts were very small, 10 thou was the largest, first pass or two.

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[txfireguy2003]

Hello again. I am a pretty lazy machinist, for me the big thrill is not running the lathe or mill but getting steam up in the models I have built or repaired. Many of the threads I meet are fine threads including 40 32 and 26 tpi jobs. I want them to be straight, on size and generally " Correct" BUT I want them ' NOW" I have dedicated die holders for most sizes and I commonly single point thread using hss tools to about 75 % of thread depth and then , using the tailstock die holder, guided loosely( Ie with a reamed hole guided on a turned pin with about 5 thous total clearance) held in my fingers I take the final cut. Cheating in this way normally produces the threads I need. IF I pick up the wrong die holder, but the right size my fingers tell me within a couple of turns. I hope this is of interest and help to someone. Regards David Powell.

I'm having fun running the lathe, but my end goal is to support my precision rifle addiction, so whatever makes that easier. For this particular part (bolt handles), a die would be totally sufficient, but single point threading is the gold standard for other parts such as the barrel to action connection and muzzle threads. If I can't make this fit, chances are those won't work for me either.

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[J Tiers]

Were you actually running in reverse, with the tool held cutting edge up? That sounds like what you said.

The 5/16" is 0.312" 0.311 is a bit large for that, the 1/8 H flat is not accounted for in that. That can make the threads jam. But, once you cut down enough, it should have screwed-in completely, and have just been loose. That in itself should not have made it jam at some point in the thread, unless there was swarf or a burr on the internal threads, which a tap normally does not do, it usually forms the thread pretty well.

If the thread tool is off-angle, you would expect to have a tight thread, but probably not one that goes a certain distance and jams. It takes a good bit of off-angle to make a difference, due to clearances in the thread. Once you made the thread smaller by more cuts, it should then have fit and gone all the way through.

Let's look at the OTHER side of it.... What was the tap? There are plug, bottoming, and taper taps. The first two make a good thread if run solidly through, so that the end of the tap sticks through significantly. But the taper tap has a long nose for alignment. If it is not run WAY through, it may make a series of tapered threads, which are actually of the wrong shape, not fully formed. That would cause much the same behavior, but should be obvious enough to not be an issue. The previous poster who asked if the nut was the same either way was also driving at this point.

We assume the nut acts the same either way, unless the OP says differently.

It does sounds as if there is a taper in terms of the depth to which the threads were cut. Possibly the boring bar is doing what often happens with actual boring, and is changing depth as it goes along. BUT, threading in reverse, on the backside. that seems to be less likely to taper to be fatter at the chuck end of the threads. It is more likely that the bar may ride out the further it is into the cut (as often happens when boring), and "further into the cut" is at the end of the piece when running reverse and making a standard thread. That would not really do what the OP is saying.

I think we need pics. And also some measurements, at least of the OD. If the OD tapers, that could be the issue right there.

But we do not know which part is tapered. We don;t even know if EITHER part is tapered. All we have so far is that it ACTS as if it were tapered.

NEED MORE INFO. PICS. MEASUREMENTS.

[txfireguy2003]

It was a spiral flute tap (the type that pulls chips out of the hole) with a modified bottom profile. The "nut" is actually a knob, and the threads don't go all the way through, so I can't check it by flipping it around, but I do have a through hole so I can see inside and there is a lot of thread left inside the hole, so I don't think I'm hitting an internal taper. I'll take some pictures and try a few of the things mentioned here tomorrow evening after work.

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[Arcane]

Did you ever actually try threading a simple plain jane commercially available 5/16-24 bolt into your knob?

[JohnMartin]

Possible you got what is known as a "drunken" thread. That's one where the overall pitch is correct, but backlash in the leadscrew and halfnuts lets the carriage on occasion jump ahead, giving a pitch that varies over a short distance.

One way to avoid it is to feed using the compound set at the thread angle, which loads the tool on one side and thus loads the halfnuts as well. Or, some choose to keep a little hand pressure on the carriage feed wheel. Other ways as well.

[Richard P Wilson]

Did you ever actually try threading a simple plain jane commercially available 5/16-24 bolt into your knob?

Just what I was going to suggest. Run a commercial bolt into the knob, run a commercial nut up your cut thread. run a die up your cut thread, see if that makes any difference. This is the first time you've told us the thread doesn't go all the way through the nut/knob, isn't it?

[txfireguy2003]

Just what I was going to suggest. Run a commercial bolt into the knob, run a commercial nut up your cut thread. run a die up your cut thread, see if that makes any difference. This is the first time you've told us the thread doesn't go all the way through the nut/knob, isn't it?

Easier said than done, as 5/16x24 is not a common thread if I'm not mistaken, it's an oddball gunsmith thread. I don't have a die in this pitch yet, but looks like I'll be buying one.

So, here's what I did today after work:
Took this picture of the problem thread


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[txfireguy2003]

After that, I cut it off and tried again. See the pics below.

After those pics, I continued cutting, using the compound to feed into the material, several spring passes etc and cut until I got to my expected final depth of cut. Not even close to fitting, so I kept cutting deeper and deeper (60 thou deeper on the compound) before finally giving up when I crashed the tool into the end of my threads. Even still, it matches my thread gauge perfectly, matches three different taps perfectly, but won't even start to accept the female threads. Then I crashed and gave up. It's time for dinner anyway.

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[oxford]

5/16-24 is standard fine thread. Get yourself a commercial nut and bolt, and see what fits where. Thread pitch gauge should be engaged in the thread not sitting on top point to point, you can’t tell what’s going on when it is like that. You also sometimes need to hold it up to a light and really look sometimes.