Slow down a drill press: 2nd motor and use the existing motor as a countershaft?

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 the

1100rpm 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!

Thanks, David

Reply to
David Malicky
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Put a double-throw switch between the current on-off switch and the 2 motors. Then only one motor can have power at a time:

/ .__________ motor _____/ ._____/



Reply to
Bob Engelhardt

What you propose can work, and others have done it here.

You will spend a lot of time doing it and it will be fairly cumbersome when you are done - though it will accomplish the goal of getting the spindle speed down below 100 rpm.

There is another way that will cost a bit more but give you a much nicer result, and make the drill a great deal more versatile.

Replace the existing motor with a small three phase unit, and run it off a VFD that is powered from 120 volt single phase.

This is what I did at at work for my older SB drill press that *should* have a jackshaft, but does not.

I mostly keep the belt on the lowest speed, occasionally on the next speed up. But that setup will make the spindle tick over at about 60 rpm with full torque. Put an extra muffin fan on the motor to keep it from overheating at the slower speeds.


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Reply to
jim rozen

Thanks to all for your input. Yeah, I imagine that it will be an inverse relationship between (amount of work invested) and (cumbersomeness). I think I'll give it a little more work to get a feel for how cumbersome it will be. I considered a VFD initially and may reconsider soon. Thanks, David

Reply to
David Malicky

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