On this Lathe (TCL150) the way the tool turret and tools are designed the Surfacing tools come into contact with the workpiece from above. This is where the reverse rotation of the spindle motor is needed.
I assume also that accurate control of the spindle speed is required where 'Threading operations' are carried out ?
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Of course. If you always use reverse then my Krypton extra circuit would work. If you require both forward and reverse, then the G-code solution would be best - that I cannot advise on.
Inherent in the PWM filtering is a ripple on the DC. I adjusted the values of the filter (R, R*C) and the PWM period as a compromise to achieve satisfactory operation. In our spindle driver there is an output when the speed is stable, and it showed that the speed was continuously hunting - trying to follow the ripple. But the motor sounded fine, so it is a reasonable compromise.
Krypton