On 2009-07-04, Michael Koblic wrote:
DoN. Nichols wrote:
[...]

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.

I cannot imagine that a lot of torque was required of Singer sewing
machines. What hapens to torque at the lowest speeds?

Enough to handle sewing through leather to make thumbstraps for
English system concertinas. (Granted, you are part of the feedback,
adjusting the pressure as needed to maintain the speed which you can
work with.

[ ... ]

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.

I got one of them drills, too. But again, what happens to the torque at the
low speed if using a rheostat? On mine there is a slow start feature which I
take to be nothing more than a rheostat. I can stop the chuck by hand at the
low speed.

At absolutely the lowest speed, yes it is easy to stall. If you
have a foot pedal, you are adjusting the pressure to maintain the speed
you want, so it is no problem.

And remember that you are also *first* using the maximum *belt*
speed reduction to keep the motor's speed up a bit.

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 got a 13.6V 20A power supply - that's 1/3HP.

Just 13.6V or variable voltage all the way down to zero? If the
latter, you can use it with a 12V drill motor to get variable speed.

A TRIAC based speed controller will need AC on the input, but a
DC motor is normally also a Universal motor -- unless it has a permanent
magnet field -- which you are more likely to find in a good servo motor,
which is overkill for the task. So -- with a TRIAC based controller
(which could include something as simple as a lamp dimmer) you want 120
VAC input, and a 120V DC/universal motor.

[...]

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.

63 rpm. But I take the point about the torque.

Likely too slow for that small a machine -- especially if you
don't have a threading setup with halfnuts and a dial to say when to
engage the halfnuts.

[ ... ]

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.

This would mean turning the spindle shaft down to at least 3/8". I do not
think this is possible. AFAIK the spindle is 5/8" with a 5/16" ID. Some of
the plain bore pulleys have large enough bores but maximum of 24 teeth or
so.

Look at the hub diameter in the pulley data. You should be able
to bore it out to fit the existing spindle. I would *never* consider
turning down the spindle to fit the pulley -- always modify the least
expensive and easiest to replace part.

And you won't want to be stepping up speed to the spindle, so
the larger pulley will be on the spindle, thus with the largest hub
diameter too.

Or -- go for the taper-lock type hubs where you can change hubs
to fit the shaft once you have the right pulley.

It is beginning to feel like trying to make a silk purse out of a sow's ear.
Some limitations will have to be accepted I think.

:-)

12" swing,
Increased rigidity,
The Holy Grail...

How about 18" swing or larger? :-) It all depends on what you
want to make. In this case, how large a dial do you want for your
sundials? You *could* use a gap-bed lathe for that -- but there can be
problems getting the gap insert back in precisely enough so it does not
affect accuracy of turning close to the headstock.

I do know that I occasionally find projects which would go
better with a larger lathe -- but I don't really have room for one.

Enjoy,
DoN.

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