Mach3 VFD PWM signal...

[Doddy]

Having just come in from the shed doing very much the same thing:

BOBs, or cheap ones at least, may not have a linear output from the PWM drive. I had very much the same experience as the OP.

You absolutely should be able to measure the voltage at the VFD with a DVM/DMM/etc. If you can't, then I'd look for possible dry-joints (as you prod the terminals with the meter leads it may crack the joint open). Try prodding each terminal in turn with a single lead to rule out mechanical play. Other than that - hand-held or bench meter? (I'm wondering if a bench meter if you've got a grounded common line... unlikely, but I can't think of any explanation why, having confirmed not on current-input on meter, why you couldn't read the voltage at the analogue input to the VFD).

What I can recommend as a very cheap alternative solution is one of the 2-pin USB/RS485 adapters (I picked up 3 for less than a tenner, UK) - use one to control the spindle (direction, speed, on/off) digitally, using the Mach3 plugin. (assuming HuanYang VFD).

Or, google mach3 linearity.dat