If you are concerned with maintaining an accurate speed, I'd highly recommend a Baldor Vector drive. I have a vectorless drive on one mill and a vector drive on the other, and for low speed tapping, you can get far superior results with the vector drive, because it will hold a speed, and provide the needed torque, too. The vectorless drive becomes very gutless at a low rpm command, and you cannot even count on it being any certain speed.
Of course, you will need to add an encoder to your motor shaft for feedback to the vector drive.
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