Rigid tapping and thread milling on a PM25 LinuxCNC mill

Rigid tapping and threading milling are attempted on a Precision Matthews PM25 LinuxCNC CNC conversion. To allow rigid tapping, a spindle encoder is added by attaching a rotary encoder to the top of the drawbar. The differential signals from the encoder are converted to single-ended and then passed to the fourth axis encoder input of a Pico Systems Universal Stepper Controller. LinuxCNC is the CNC controller. With a 2500P/rev encoder, rigid tapping works well at 400 RPM, but the required sampling rate is too high to support 800 RPM. At 400RPM, the stock PM25 spindle motor does not have sufficient torque in the low end with the high speed belt speed to reliably form tap 1/4″-20tpi but cut threads are reliable.

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