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I am using an old treadmill motor to build myself a variable speed disk sander. The motor is a 2 1/2 hp DC motor and I am using an mc-60 speed controller.

I am wiring it all up according to the wiring diagram, but my power cord no longer has a place to ground to. I built the body of the sander out of wood, and the only metal that it has is the small aluminum case for the speed controller which I believe is not suitable for a ground anyway.

What should I do with the ground? Does it cause any safety concerns if I just cap it off?

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    \$\begingroup\$ The motor case should be grounded at some convenient location unless it is in a double-insulated case. The aluminium case should also be grounded. Aluminium is a good conductor but due to surface oxidation you should use an earthing crinkle washer to pierce the oxide. \$\endgroup\$
    – Transistor
    Commented Aug 1, 2019 at 19:15
  • \$\begingroup\$ I believe your DC voltage max is about 50-60 volts ? I wouldn't bother with earthing on that side of your circuit. However what is the source of your power ? THAT power supply should be earthed. \$\endgroup\$ Commented Aug 2, 2019 at 6:57

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Any conductive surface which can be touched should be grounded. The reason for this is that if there were some fault in your device that made a touchable surface "HOT", then a fuse or circuit breaker would blow. So, for your sander, if there is some metal that can be touched, (the screw that holds the sandpaper on? The center of the sanding disk? the shaft of the motor?), you should connect it safety ground. If there is really no way to touch a metal surface in your device, I suppose you could not use the safety ground. (You have made it "double insulated")

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