On 2009-07-04, Michael Koblic wrote:
DoN. Nichols wrote:
Ah. Did not think of No.1.

You should *always* consider what might fly out when you could
be in the path.

As my old boss used to say: "You can recognize it only if you think of it."

Well ... when anything is not firmly secured (and not just by
friction between two pieces which *could* shift) consider what would
happen should it slip. The better your imagination, the less likely you
will be hurt in reality.

Someone else suggested a light dimmer which is typically quite
inexpensive -- and as for motors -- a cheap hand held electric drill
could be clamped onto a shaft in bearings to drive at a lower speed.
Or -- you could salvage a motor out of a vacuum cleaner or a kitchen
blender or anything else with a DC/universal motor. (Just look for
brush holders to verify that it is not AC only -- and the holders may
be hidden inside, as is common in cheap hand held electric drills.)

1) I might be wrong but the DC motor controllers usually have a feed back
which will increase the power in response to loading conditions. I do not
see a light dimmer doing it.

The fancy controllers do -- by various techniques, not all of
which are immediately visible like an encoder or tach on the shaft.
Some controllers apply power to the motor winding in pulses whose width
are varied to change the power to the motor. During the moments between
those pulses, the motor is acting as a generator, and the motor can see
what the speed is by the voltage generated.

But consider things like an old Singer sewing machine motor and
controller (such are are on the model 221 portable). It is simply a
DC/universal motor and a foot pedal which controls the current to the
motor (AC, but DC would work just as well). The foot pedal is simply a
stack of blocks of resistance element between two electrodes. The
harder you step on the pedal, the harder the blocks are pressed
together, and the lower the resistance, so the faster the motor runs.

You don't *need* the kind of regulation which the fancy
controllers give for this lathe. And you could use a pedal speed
control from a Dremel (from before Dremels came with built-in speed
controllers). I can't find the Dremel ones on eBay at the moment, but
this looks as though it might work as well. It is poorly described, but
I expect a controller for a DC/universal motor.

Ebay auction # 260291509189

2) A router speed regulator is a thought, however, typically these motors
turn at 30,000 rpm give or take so major gearing would still be required.
And I am still not sure about the feedback thing. Some routers have it built
in now (EVS).

The speed regulator is based on the speed a given motor is
capable of. It won't make motors not designed for it turn at 30,000
RPM. Find a 120V motor which is closer to 1000-2000 RPM and see what
happens.

3) I have a DC motor from an old Sears drill sitting somewhere. As it stands
it would also need the whole gear box to get the speed down to something
useable. The speed regulation with it is basically the slow start. When it
is turning slowly there is hardly any torque which brings me back to (1).

This sounds good -- until you mention later that it is a 12V
motor, not a 120V motor.

4) It is a thought to make a controller along the lines in (1) for the
motor. However, given that the motor is a 12V one I usspect that the current
at the lower speeds would be appreciable.

You want a drill which plugs directly into the AC line, not one
which runs from batteries.

[...]

I was thinking of just getting two more Taig pulleys ($24.49) and
making *two* countershafts.

Two extra pulleys (and an extra belt) would make *one*
countershaft, not two.

How is that? One pulley=motor. Belt to second pulley=countershaft 1
(reduction by factor of 3) Belt to third pulley=counteshaft 2 (reduction by
factor of 9). Belt to spindle pulley (reduction by factor of 27). A single
coutershaft would require pulley 2 and 3 to be on the same shaft with no
reduction of speed between them.

Oh -- you are thinking of two belts on each pulley except the
end ones. That restricts you to combinations which don't need the same
groove for both incoming and outgoing power. And I think the lowest
speed with triple reduction would be both too slow for the size of the
machine and those tiny belts could not handle the transmission of power
over the last two stages.

Use timing belt pulleys. Figure the largest diameter which
would clear the base when mounted directly on the spindle, and then
look for the smallest pulley of the same pitch which will mount on the
countershaft. At a guess, you might be able to get a 5" diameter pulley
plus the belt on the spindle (make sure that it is available with a hub
which matches the OD of the spindle at that end), and assuming 5 teeth
per inch of circumference, that would be about 78 teeth. Then a smaller
pulley with 12 teeth would give about a 6:1 reduction in a single pass.

O.K. Looking in McMaster Carr's web site, I find 1/2" wide belt
pulleys with a 0.200" pitch (MXL series). Let's see the largest which will
fit within 5" diameter.

The range for this size is 60 teeth max and 10 teeth minimum, or
6:1 ratio.

OD Teeth Bore Cat No Price

0.87" 10 3/16" 57105K11 $7.40
1.13" 14 1/4" 57105K14 $7.51
3.80" 60 5/16" 57105K33 $17.15

So -- if you need to fit it on a 1/4" shaft, you will need at leat 14
teeth (60:14 ratio, or about 128 RPM for 550 RPM in.

If you can turn the end of the shaft down to 3/16", you get a full 6:1
ratio, or 91 RPM.

These (and others) are on McMaster Carr's catalog page 1044 via
the web. (You'll also need to select a belt to fit including the proper
spacing between pulleys.) The pulleys which I have listed are acetal
plastic, and I would suggest that you go for the steel ones listed a bit
later in the page for stronger gears. And you'll probably need to pin
the hubs instead of just use setscrews to get enough strength with the
small diameter shafts.

[ ... ]

And bear in mind that the speeds I give above are with no
slip. It will be slower with normal slip. Fore example, the 1650
RPM motor would be 1800 RPM with no slip. (900 with no slip becomes
something like 825 RPM with slip, and 600 with no slip becomes 550
RPM with slip.) That 550 RPM would get you down to 316 RPM with
your existing gearing.

Ah, with slip I can get down to zero, no problem...

Yes -- you can get down to zero -- but between zero and a bit
below the nominal speed with slip you will have no torque to work
with, so the descent from normal slip to zero will be very sudden.

Also do not forget the smoke...

A lot of smaller motors are "impedance protected", and can sit
there stalled forever without letting out the magic smoke. :-)

Enjoy,
DoN.

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