Here are a few circuits I came across on the web, which may be useful in the future.

 

• Some people have reported problems with the x3 spindle drive burning out. Rather than replacing with a direct replacement. The circuit opposite could be used, to provide a simpler solution.              

• If you need a PWM output to control a mill spindle which needs a standard 10 volt analogue voltage, you could try this circuit below.

 

I have not built or tested any of these and make no recommendation as to how effective they are, but you may find them useful.