Is there a prize for the worst thread ever cut HELP!

[th62]

I'm having problems cutting a decent thread. I've never had any problems in the past with previous lathes. My current lathe, when I first got it, was cutting terrible threads; however, when I included a helix angle on the tool bit all was resolved. Yesterday I tried cutting a 1/2" UNF thread, to say it was abysmal is downplaying it - see pictures.

The tool piece is ground at 60 degrees, I've tried side rake from around two to 6 degrees. I have also tried grinding the tool bit with negative helix right up to around five degrees - all to no avail. After all angles have been ground, I finish off by flatting the tip slightly and finishing with an oil stone. I have spent the day trying to figure out the problem - all to no avail.

My method is: set tool with fish plate and angle the compound slide at 58 degrees (normally at 90 degrees when parallel with top slide). I then touch the tip of tool on work and set cross slide to zero, back off the top slide and move carriage to start of work. Wind in top slide to zero, engage lever and make first cut.

At the end of thread, stop motor, wind out cross slide one full turn, switch motor to reverse and return carriage to start of work (no threading dial). Wind cross slide in one full turn to zero mark, wind in compound (tried .02, .04, and .06 mm) and perform second cut.

Continue as above until finished.

Lathe speed is 150 rpm. As I'm threading to a shoulder, I really can't go any faster.

This is the only lathe I've ever cut a helix angle on the cutting tool.

I've also tried using my diamond/tangential holder, first grinding the tip to 60 degrees. The thread is marginally better, but nothing I could bring myself to actually use.

As you can see in the pictures, the steel is tearing on both leading and trailing cut as well as the top of the thread. The thread form is abysmal.

The form of the thread when cut using the tangential was passable. but again the finish was abysmal. I tried a reasonably heavy cut with the tangential first off, but even finer cuts didn’t improve the finish.

What am I missing?

[scottyd]

What sort of angle have you got on the top face of that tool?

It could be the steel youre using too. Is it mystery meat or some sort of really ductile stock? that can make life miserable with lots of tearing like what you are experiencing, particularly at lower speeds. Adding the tailstock in might help a touch too.

[th62]

No top rake and the steel I'm using is standard bright. The overhang is only around 25-30mm, but I'll add the tailstock tomorrow - can't hurt.

[GSRocket]

I have absolutely no idea, having a total of about three hours of turning experience but I'm wondering
if the cutting piece is extended too far out of the holder causing it to flex.
Perhaps raising the holder and lowering the cutting tool would help...maybe.

[cba_melbourne]

Maybe some cutting oil would help?

[morrisman]

Just wondering how sharp that tool is ? You need to hone the tool edges on a arkansas stone . If you can see a reflection on the tool edge , this means it isnt sharp enough . Mike

[th62]

I've tried shortening the tool overhang, made no difference. Also tried with coolant, cutting oil and also my cleaning fluid (oil/turps mixture). No luck there either.

Not familiar with 'Arkansas stone', I have a very fine oil stone, which I used today, out of exasperation. To tell the truth, I've never used an oilstone before.

I have a piece of black on which I tried different things, I'll have a look and post if I can find it.

Did anyone spot a problem with setup or method?.

[familyguy]

I've seen that effect in some of my past experiences.

Try taking a second or third pass at the same depth, if you find more than a smidgen of steel being removed then it possibly might be irregular flex in the system causing the work and tool to deflect away from each other and then spring back only to be deflected away again, at least that's what I put it down to. A sharper tool or smaller cuts with multiple passes could help along with cutting fluid or your choice.

[morrisman]

Not familiar with 'Arkansas stone', od?.

This may help you
https://www.woodworkforums.com/showth...light=arkansas

these hard arkansas stones are available on ebay Mike

[th62]

This is a piece of black I machined half a dozen threads on using different tool geometries and using a live centre, they're all much the same - orrible.
The second pic is of the compound slide angle table: when parallel with the cross slide it reads 90 degrees, so I swung it anti clockwise around 28 degrees, not the 58 degrees I quoted at the start, I assume that's correct?

[krisfarm]

th62,
That looks like a nasty piece of steel you have,the material is being torn out not cut. The cut shown in your photo looks way to deep to me. I would hone the side faces and top of your tool with a diamond lap until you cannot see the edge, reduce your speed down to 100 RPM or less use a live centre and plenty of heavy cutting oil and reduce your depth of cut down to .002" to start and finish with .001" cuts and do a spring cut every second or third cut. Good luck.
Bob

[TKO]

Set your tool with fish gauge after you have set your compound slide to the correct angle which
ever thread you are cutting ,I think perhaps you are using the wrong compound deg angle .
you are cutting an inclusive angle of 60 deg so try 29.5 deg on your top slide to the lateral angle of the thread, and the tool will be entering the cut at he correct angle , and take smaller cuts.I hope this will help.

Eddie

[pipeclay]

Looks as if the problem lies in either the material or the lathe ,as you have had no problems in the past cutting threads.

[simonl]

Hi.

While the incorrect compound angle may affect the shape of youf V form, it does not contribute to the poor quality cut. If/when I get results like that I would be lookjng at the tool, tool post, DOC and lastly the spindle bearings.

Simon

[Pete F]

I think you're trying to baffle yourself with things like helix angle, as it is not important if cutting standard pitch V threads. The depth of cut looks "ambitious" for that material. Grind a standard 60 degree tool with no back rake and sufficient side clearance. 28 degrees in feed sounds fine, but try it again with smaller feed and just plunge straight in if you wish to eliminate compound angle. It looked correct to me however, just looking quickly at the pictures as I'm away at present.