I have a Delta 10" variable speed drill press that runs 500-2100rpm (variable "diameter" pulleys), and I need to slow down it down to drill for metal in larger diameters--ideally, as slow as ~50rpm, but I'll take what I can get. I would like to have it capable of both the slow and fast speed ranges w/ minimal changeover, and I'd like to keep the existing VS pulleys as they work quite well. The existing motor is a standard 1/3hp 1750rpm induction motor. The drill press (and motor) is from China, but seems much better built than the HF variety. Here's my idea, which I've started a little work on...
There just happens to be enough room for another pulley on the existing motor shaft, mounted between the VS pulley and the motor casing. And I happen to have a GE 1100rpm 1/3 hp induction motor. I plan to mount the 1100rpm motor behind the existing motor, and drive the existing motor shaft (via a belt and 1.5":6.0" pulleys), which would then drive the spindle via the VS pulleys. Thus I would use the existing motor as a passive countershaft/intermediate shaft for the low speed setting. This gives a slowest spindle speed of (1100/1750)*(1.5/6.0)*500rpm = 78rpm, and a fastest speed of 330rpm. The existing motor/"countershaft" would be running at a steady 275rpm (1100*1.5/6)). I would de-tension the belt for the 1100rpm motor when running the existing motor for high spindle speeds (else it would be at 7000rpm!). For safety, I am thinking the tension device for the1100rpm motor could trigger a switch that would shut off power to the existing motor when the 1100rpm motor belt is under tension. Or a Sprague clutch? Or hope I *always* remember. Any other ideas?
A couple other questions...1) When it is in the low speed setting, will it cause any problems for the existing induction motor to be passively driven at 275rpm? (I don't think the new belt loads will be a problem for its bearings, since this load is in the opposite direction as the existing VS belt). Should I have its power wires open circuit, shorted, or through a resistor?
2) I understand when VS pulleys are set for low speed, they do not really give all the spindle torque they should since a lot is lost to friction and slippage. Anyone have enough experience with them to predict whether I will have enough spindle torque at 78rpm to actually drill holes?3) In low speed mode, drilling 1" through aluminum, or 3/4" through steel, say, will the frame of the press likely be too flexible to keep the bit from chattering, since it was never designed for that? Anyone have experience w/ a similar conversion?
Any other input is appreciated!