we use Delta ASDA B2 servo motors.
in turning it is very precise.
maybe we understand where the problem lies.
as spindle motor we use asynchronous + VFD.
to transfer the motion, between engine and gearbox are trapezoidal belts (original of the lathe).
probably the type of engine / transmission is not suitable.
I say this, because there is an oscillation of the rpm, caused by the trapezoidal belts.
from the various tests, we came to the conclusion that by starting the thread cycle, a certain number of laps is taken to activate synchronism.
this is maintained perfectly even in the presence of speed oscillation.
the problem is created when the cycle must be repeated several times.
precisely due to oscillation, data other than the first cycle is examined, causing damage to the previous thread.