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?
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?
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