Threading fit issue, tapered?

[Huub Buis]

If the tenon has no taper, then threads shouldn't be tapered, again unless something is moving.

I have never had the end of the thread at the chuck side to be more tight. This could be the result of the tool is moving away from the stock. The tool pressure can be quit high when doing threading. I can imagine, the tool pressure could be large enough to:

• turn the cross slide spindle slightly when threading. Lock the cross slide during threading and check the position of the dial before and after the threading pass.
• push the tool back in the tool holder. Put a dial indicator at the back of the part (so it won't get in the way) and check the tool position before and after threading.
• rotates the tool holder (I am not familiar with your tool holder) if the top clamping nut is not tight enough. Put a dial indicator at the back of the part (so it won't get in the way) and check the tool position before and after threading.

For all these causes, the thread should be OK if you do a few cleanup passes.

Normally you only check the horizontal alignment of the head stock using a thick bar. If the head stock is pointing upwards or downwards you could get strange results when doing coarse threading. Check the vertical alignment of the head stock the same way you did when checking the horizontal alignment. Just put the indicator at the top of the bar.

I am very curious about the actual cause. I hope you figure it out.

 

[tobnpr]

Insert (carbide) tools are rarely sharp on the cutting edge and that results in increased tool pressure which can result in taper. I use HSS tools from Aloris that are sharp and easily sharpen able and adjustable for helix. They are available in several widths and pitches (width of flat on the end). They are sharpened ONLY on the top. and last for years in regular frequent use.

I've had no issues for years on a smaller/less rigid machine using both partial and full profile inserts.
I've become partial to full profile, just turn the tenon to correct OD and it's gotta be right.

Is it possible your radial alignment is out just a smidge? I know just enough about the TBAS to be dangerous...

 

[jareese]

I've become partial to full profile, just turn the tenon to correct OD and it's gotta be right.

How do you use the full profile inserts? I know nothing about them. Do you stop after a certain number of passes, or a certain point in the insert, or just go to proper depth and done?


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[Ischgl99]

The way I saw it descibed and how I do it is to turn your tenon slightly larger than your finished major diameter, make passes until it starts cutting the OD and then stop when the OD measures within specs for the major diameter. Since it is cutting the full profile, once the major diameter is correct, your thread is correct. There may be other ways to do it, but this is the only method I have seen.

 

[tobnpr]

^^^ That's it. Easy peasy. Reduces the OD as it cuts the thread profile. You can keep the OD on the large, or the small side of the tolerance of the class of thread you're cutting. No more wires...

ETA: Plus, root and crest have correct radii. No more sharp thread crests.

 

[Jake M]

Agreed - I am using long reach indicators.
I will try some different threading processes and see where I can find improvement.

Like I said - all threads are within spec 3A to 2A, (as far as fitment, not measurement other than OD) it's just me being me about it.

I've got a thought here. You're having an unplausable (not impossible, just implausable) problem. Tapers only on one particular thing that you're turning?

Thread mic's are an obvious answer (or wires, they're cheap but they're awkward at first, but it gets better, and you either get them right or they don't work at all, so no mistakes).

My other though though, why I'm posting, is that threads which are made to a stupid tight tolerance, (as I'm prone to do when I match them up) they tend to tighten up more, the more threads you engage. It "acts" like a taper, but isn't. It's the absolute slightest difference between being cut on one machine or another. It's the stacked resistance from fresh cuts that aren't polished/burnished yet, it's the resistance from a peak or a crown or a root that's "touching" but not interfering at first.

Anyhow, my suggestion is NOT FOR EVERY PART, but for something you have in hand, you could make up a "test guage", a ring that properly (by your standards and your methods, engages a few threads only, see how that runs down a male thread. Match it to the threads you're making, exactly how you do them. If that runs free, but a long thread binds up, it's NOT taper. Or a male test thread, with a few threads at the beginning, then undercut however long it needs to be. Then you can determine 100 percent for sure if the binding up is actually taper in the thread pitch, or if it's other "real world" shenanigans going on, which is managable, but never can be totally eliminated, and is why all threads need "some" amount of clearance.

I'm not there measuring your work, and you think you've got taper only on threads. So I'm NOT AT ALL ruling that out, But short of actual thread mic's (which is NOT a bad thing to have, just sayin'...), making such a "short" gauge, literally just three or four threads, to a tight (by your methods) tolerance can prove, beyond a shadow of a doubt just exactly what error you are chasing. That's all it needs to do, so like I say, this not a gauge for EVERYTHING you do, it's a gauge for one part, (or two gauges for two parts), and it's going to give you a 100 percent accurate direction in which to chase down the problem.

This advice is NOT from an expert, but from a guy who hand grinds tools (by choice), and has learned many hard lessons about root and crown "discrepancies" messing with logic, reason, and common sense.

 

[mksj]

Downside of full profile is it is specific to only one TPI, and may include separate inserts for outside, inside, right hand and left hand for lay down inserts.... With partial threads inserts they cover a wider TPI, but you do need to knock down the thread crests with a file and maybe a bit more cleanup of the threads. It also may be material specific, insert type and technique. I am still using the same Carmex lay down insert for years after 100's of threads in various materials and breaking one tip by accident, so I am on the 3rd insert tip. I consider that very cost effective. Previously with cheaper TPMC style inserts, I barely made it through one thread w/o it breaking.

 

[tobnpr]

Downside of full profile is it is specific to only one TPI, and may include separate inserts for outside, inside, right hand and left hand for lay down inserts.... With partial threads inserts they cover a wider TPI, but you do need to knock down the thread crests with a file and maybe a bit more cleanup of the threads. It also may be material specific, insert type and technique. I am still using the same Carmex lay down insert for years after 100's of threads in various materials and breaking one tip by accident, so I am on the 3rd insert tip. I consider that very cost effective. Previously with cheaper TPMC style inserts, I barely made it through one thread w/o it breaking.

True, but in my applications (smithing as well) there's just a handful of commonly used thread pitches for both ends of the barrel. That said, I've used both and if I've got the female part in hand I often prefer to "thread to fit" so I don't need to worry about whether those threads are in spec.
If I'm threading to a print/spec I prefer the full profile. No right or wrong, just preference...