Grizzly 60704 DC motor control


Hi Masso Forum,

A few months ago I converted a G0704 mill to CNC with a kit ( ). All is working well with my Masso G3 controller. But what I'd like to do now is control the motor like a spindle/VFR. I recently purchased a Variable Speed Controller card from CNC4PC as I'm a bit afraid to wire the mill's electronics directly to the Masso. Like the Masso the card can convert 5v signals to 0-10v to replicate potentiometer motor speed control and has opto-isolated outputs. But maybe there's actually a safe way to wire directly to the Masso (and return the card).

This mill has a forward and reverse, speed control plus a couple of gear ranges: 50 to 1,125 RPM LOW and 100 - 2250 RPM HIGH. I assume everything but the potentiometer and F/R switch can be left as-is. It's the Forward Reverse switch I'm having trouble figuring out. Has anyone wired one of these G0704s this way (or PM 30-MV etc.)?




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@phil_sf - I am very hesitant to provide advice as replacing the main Forward/Reverse switch will defeat one of the safety features of the machine.

The A1 terminal of the On/Off switch connects to an electromagnet that holds the switch in the On position (the other end of the electromagnet is terminal 14). If you have a power dip the On/Off switch will automatically go to Off.

The 3rd layer of the reversing switch (terminals 9, 10, 11 & 12) are connected to the On/Off switch presumably to interlock the On/Off switch should the Forward/Reverse switch be in the intermediate position (neither forward or reverse). If you attempt to change the motor direction while the motor is running the On/Off switch will release.

The 1st and 2nd layer of the reversing switch is presumably your classic DPDT reversing switch structure (search for "DPDT reversing switch" for details).

The CNC4PC relays are only single pole and therefore not suitable for directly reversing the motor direction. I would be using a pair of DIN mounted "Cradle Relays" to perform the switching. However the coils for the cradle relays are too low resistance to be driven directly by Masso, you will probably need to use the CNC4PC board relays to drive the cradle relays.

As for wiring the cradle relays so they can be used with Masso style separate Forward / Reverse outputs - I will need to have a think about this to ensure the design is fail safe when both Forward and Reverse outputs are high (ideally the motor should not run, alternatively run in the forward direction).



Doh! - A google image search turned up the attached diagram which is a Forward / Reverse circuit implemented with a pair of single pole relays (you only need the CNC4PC board - no cradle relays required).


  • Left relay in diagram is the Forward relay
  • Right relay in diagram is the Reverse relay
  • Use the A+ terminal from the speed controller board as V+
  • Use the A- Terminal from the speed controller board as V-
  • For forward = clockwise motor shaft rotation
    • Use white as motor positive
    • Use black as motor negative

  • For forward = counter-clockwise motor shaft rotation
    • Use black as motor positive
    • Use white as motor negative

NOTE: Please check motor rotation direction compared to mill bit rotation. Due to gears, pulleys and motor mounting direction "Forward" at the mill bit could be either clockwise or counter-clockwise at the motor.


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Hi ZombieEngineer,

Thank you so much! I'll be working on it this week.




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