Need Help!- Surface Speed changes, threading

[tds11223]

I threaded a part using 300 SFPM, had some chatter while sizing the threads, so I slowed the SFPM down and my threads were ruined(I did not take excessive depth cuts, I merely re-ran the threading cycle). Why does the SFPM need to be left alone if I re-cut the threads? I would think that the machine would have no problem syncronizing...

[camsys]

Did you do threading in G96 CSS? If you did,I don't think it a good idea to do so.

Threading always synchs itself with the RPM of the machine, and if you're
in CSS that will change as you go deeper into the thread, let alone all of the
retracts the cycle does.

Use a good SFPM, and translate that to RPM and run your threads.

[angelw]

Originally Posted by tds11223 I threaded a part using 300 SFPM, had some chatter while sizing the threads, so I slowed the SFPM down and my threads were ruined(I did not take excessive depth cuts, I merely re-ran the threading cycle). Why does the SFPM need to be left alone if I re-cut the threads? I would think that the machine would have no problem syncronizing...

Whether you used G96 or G97 is unclear from your explanation, but the way I comprehend your Post is that you cut a thread in either G97 mode with spindle RPM that equates to 300 SFPM, or you used 300 SFPM in G96 mode. After cutting the thread, you re-cut the same thread with a slower spindle RPM, either via G96 or G97. If that's correct, then yes you will wreck the thread every time you execute that procedure.

The reason is as camsys stated, the spindle rotation and the motion axis are synchronized during the threading operation. Lets say you were using 2000 RPM, and cut a thread. If you were to halve the RPM (1000) and run the threading cycle on the same threaded part, you would cut a second thread with the start indexed 180 degrees, a two start thread. The thread, of course would be ruined, as the second start would be tracking right on top of the crest of the previous thread.

From the above, it should be obvious, that if you vary the RPM by a small percentage (not 50% as in the explanation) then you will cut another thread indexed an equal small percentage of 360 degrees.

As camsys advised, you should not use G96 in conjunction with a threading cycle, because as the threading tool changes diameter, so do the RPM, and as a consequence, another thread is cut at a new index.

Regards,


Bill

[txcncman]

He posted to the Mazak thread. It can be assumed he used the Mazatrol canned threading cycle.

The reason you cannot arbitrarily change cutting speeds (RPM) is that nearly all machine controls use a timing point on the spindle to start the threading cycle. There is some delay in acceleration of the cutting tool up to the needed feed rate. Changing the spindle speed causes the spindle to be at a different rotation when the tool reaches the part (i.e. out of time). All of this is explained in every machine manual I have ever read. Direct RPM programming must be used. Constant surface feet programming cannot be used.

It is possible on every CNC lathe I have operated to "pick up" a thread lead after changing speeds. It is a little complicated to explain how to do it, but not that complicated to do (on external threads).