I have been struggling with this for years. I can stand it no more.....
When threading on the lathe my male threads are not consistent along the length. The front threads are loose and the back is very tight. What am I doing wrong? Is there an adjustment for this?
Sergeant,

I've run into this on some lathes. My cure is multiple spring passes. I think in my case it is from cutter deflection. Also running more rpm's helps.

Picture of your setup would help.

Are you threading close to the head stock*
or far away from the head stock, turning
between centers such as for a long barrel**?

In the first case one of many candidates for
'tapered' thread is that the tool is not set
correctly. Are you using the compound to move the
tool in or pushing straight in?

There are tables for the number of passes and depth for each of the
thread TPIs.

And often forgotten check with the lathe is ensuring
that the tail stock is actually in line with the head stock.
Otherwise you will definitely have a measurable taper. ***

Learned the hard way from researching on YouTube
that the last 3 passes be made just with the cross slide
pushing in to clean up.

Please let us know or show your particular set up.


Best.



* e.g., << https://www.youtube.com/watch?v=9y0MmvscBzg >>

** << https://www.youtube.com/watch?v=4bBg0XMIRHg >>

*** << https://www.youtube.com/watch?v=q5AslPQLQpw >>
Tubalcain << https://www.youtube.com/watch?v=tqjO4yPvhQs >>

Turning close to the headstock.
I have a muzzle brake threaded on an mandrel (a barrel cut off). I measured for concentricity using my dial gauge.
Using the compound set to 30 degrees to move the tool in.
I will review the videos and maybe that will show me. The "Ohhhh moment" as my son calls it.
Cross slide on the last 3 eh? didn't know that. That's why I come here.

my phone says i have to create more storage before I can post pictures but the narrative should give you an idea of what i'm doing.

Also, is there a class of tap that will give me the fit required for a suppressor to muzzle brake? I spoke with Victor Machinery and they say they have this or can get it but they want to know what class need. an "H" number or something like that.

Spring passes, or lack of, would be the typical culprit for what you describe.
I try to run one every cut and 2-3 on final pass.

Thread classes are A, B, & C
H is a designation for oversizing the Pitch Diameter, beyond the basic dimension.
This shouldn't account for a tapered or improperly cut thread.

Some Silencer Mfr's have drawings of their required thread specs, available on their website.

While concentricity is important, a tapered thread would/could be caused by the axis of rotation not being perfectly parallel to the lathe bed direction of travel. To check for this, turn a piece of metal that's chucked up the same way you Chuck a piece to thread it. Turn the entire length of the piece. If the diameter of the piece is different at the two ends then your axis of rotation isn't parallel to your lathe bed DOT.

Sergeant wrote: Cross slide on the last 3 eh? didn't know that. That's why I come here.

That's not something to "know", it's bad info. Yes, you can do it that way, but it isn't the "right" way, and isn't going to fix your issue. It's also not what most experienced machinists do. There's always some internet egg-spurt who'll say otherwise of course.

Just make sure you're running a couple spring passes; tapered threads come from deflection (spring) of the tool or the work, or from misalignment. Most likely it's just deflection in your case, unless the lathe headstock has been removed from the ways and been reinstalled poorly. It can also happen from tailstock offset, if you have the muzzle against a live center in the tailstock.

yondering wrote:

Sergeant wrote: Cross slide on the last 3 eh? didn't know that. That's why I come here.

That's not something to "know", it's bad info. Yes, you can do it that way, but it isn't the "right" way, and isn't going to fix your issue. It's also not what most experienced machinists do. There's always some internet egg-spurt who'll say otherwise of course.

Just make sure you're running a couple spring passes; tapered threads come from deflection (spring) of the tool or the work, or from misalignment. Most likely it's just deflection in your case, unless the lathe headstock has been removed from the ways and been reinstalled poorly. It can also happen from tailstock offset, if you have the muzzle against a live center in the tailstock.

Could also be caused from deflection of part if no live center is used, or worn chuck jaws that don't hold the part evenly (ie tighter in the back than the front), or from worn ways close to the headstock where 90% of the machine's work is performed, or etc.

You get the idea.

I would start with the easiest place first - tool deflection. Take lighter cuts and add in some clean up passes (spring cuts) and see if the taper clears up.

If that doesn't do it, start checking for misalignment and wear.

Thanks guys,
I will try these. The chuck has been removed and could have been installed wrong (by me).
I'll try these one by one.
Sergeant

I get that too. If I watch when the threading tool engages I can see the toolpost rock back a little. I don't get that with any other operation - or at least to a degree where it affects dimensions along the part. Only threading. To 'fix' it, my QCTP handle is to the left when tool is locked in - at about 8 o'clock if 6 o'clock would be pointed directly at me. I push hard against it. That extra leverage keeps everything the same thru the length of the cut.

I'll actually break down & buy a good name brand QCTP some day - until then I just tinker about with my ~$100 faux-loris.

CMV, in your case it's probably not the tool post itself that's flexing, but what it's mounted to (compound and maybe even the cross slide). Even the cheap AXA/BXA/CXA tool posts are fairly rigid themselves, much more so than the structure below them.

If you like to keep your compound ways a little loose, that's a source of deflection too. I keep the compound fairly tight for that reason.