Hi,
when threading Mach requires an accurate speed, that is not to say it has to be exactly 400 rpm say, but if it slows slightly as the cutting forces act on it to 390 rpm it needs
to know that it is 390 rpm, not 392 or 387 but 390 rpm. The reason is that Mach needs to synchronise the Z axis movement to the rotational speed, any error will result in a thread
with the wrong pitch.
An asynchronous motor will slow under load even if the VFD provides fixed frequency input voltage. If the load is but a small percentage of the spindle ultimate torque
then the speed change will be small, a few percent. Provided your index pulse remains active a small change in speed is not disasterous to your thread, Mach will accomdate small
changes by tweaking the Z axis advance rate.
If there is a large change in speed or it changes speed quite rapidly then Mach struggles and wont cut good threads.
You may have noticed that on the Ports an Pins/Spindle Setup page the opportunity to operate your spindle closed loop. Mach is not a good candidate for closed loop operation
because its so slow to respond but its still better than nothing. The speed is measured by the index pulses, per normal, but if the speed slips below target Mach will increase the PWM output
voltage which will in turn cause your VFD to up the ante. The feedback loop is slow to respond so this works well only with spindles of significant inertia and plenty of torque relative to the
cutting torque. If you expect this arrangement to be able to maintain spindle speed when the cutting forces are as much as 50% of your motors max torque you'll be disappointed.
I recently bought a second hand Allen Bradley AC servo of 1.8kW as a spindle motor for my mill. Its speed, and even more impressive, position accuracy is a real eye opener. Its not that its a great
deal bigger than your induction motor but rather the servo drive/servo combination have a feedback refresh rate of 20kHz. This is broadly true of any feedback control system, assuming the motor has
the torque to drive the maximum expected load it will do so accurately only if the feedback refresh rate is fast enough. For this reason you may have seen large production CNC mills that have
20 plus horse power spindles do a good job of rigid tapping, one) because they have plenty of torque and two) they have very high refresh rate feedback systems to take advantage of that torque.
If it is within your means replacing your spindle motor with an AC servo would be a very strong performer for you when threading. You might be surprised how economical second hand servos and drives
can be and some of the Chinese offerings are very good value for money.
Craig