A fluctuation in rpm of say 1 ie; flickers between say 99 to 100 should be ok. Idealy the rpm should be constant since the movement during threading is a
cut at a constant feedrate. If the feedrate is not constant the thread will vary in pitch. If it varies too much then the following pass is modified to adjust for the varying feed rate. That's why you set vfd manualy to an rpm so the VFD is not fighting with the correction the program provides.
If you were to scribe a line, allowing for some time for accelration, say 0.5" ( 5 pitch diameters) and then contact with the material using a G01 Z 1.5 F... the scribed line should be constant. You would need to use a microscope to measuse the variance over a few inches. Scribe line is only as accurate as
the ball screw / screw moving the axis.
If as you say, the rpm goes out of whack, say it goes to zero or drops say from 100 to 20, the threading cycle will try to fix the the threading by adjusting the next cycle. I have tested it for extreme drops in rpm, almost a complete stop, and the threading recovered but the thread pitch was not in spec.
In general the more stable the rpm the better the cut screw thread pitch will be. Should you consistanty get something , say less you can always change the pitch in the wizard and it will be used, and the thread will be good. But that is just tricking the program pathing and is self defeating.
This is all in the write-up.
I think you have an index problem, or it can be the vfd's ability to maintain rpm.
I can't speak for version 57....I use 38 and never had a problem.
If you are measuring the rpm, check to see what the +- specs are on your measuring instrument.
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A test that can be done to check the acceleration / indexing is to mount a thin disc, the bigger the better, say 2" diameter, and do a thread.
You should see just one tick /scribe mark in the exact same location on the disc outside diameter. Under maginification the variance of the tick marks shold be almost zero / no distance between the marks. In fact if you try to do multiple start threading you will quickly find out just what your lathe can do.
RICH