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

David Malicky wrote:
... 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?

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
_____/ ._____/

.__________motor

Bob

In article , David Malicky
says...

Any other ideas?

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.

Jim

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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