Unimat SL-100 motor?

[Murphy]

I'm resurrecting a old Unimat SL series lathe, and after having a motor bracket made in steel to clamp where the crummy factory one went, I need to figure out what to use for a replacement motor, as the factory ones weren't good for much more than keeping my coffee warm.

Thanks for the help, guys!

Murphy

[Bob Ford]

You might try Sherline http://sherline.com/prices3.htm #33050 $210 motor with speed control.

Bob

[Murphy]

Bob, I'm looking for an inexpensive motor, not providing some sod's retirement!

A Sherline lathe goes for about 550, and I should pay 210 for a 'effing motor?

There was a posting on Ebay about a inexpensive motor for the Unimat lathes, and I was (and still am!) trying to find out if anyone here has done anything of that nature.

[Evan]

I replaced the motor on my Unimat with a DC motor from a cheap air compressor that failed (the piston) after a year. There should be plenty of these in the trash. It even has a rectifier board so a light dimmer will work as a speed control.





Or, you can put a stepper on it and use it as a 4th axis on a CNC mill...

[loose nut]

sewing machine motors are about the same size as the original motor don't know about the spec. though,

[Murphy]

Evan, I damn well LIKE that!

I've got a 1/2 hp DC motor here, 3" dia. x 4" long, speed tops out @ 8000 RPM!

Big question is this - is that going to be TOO MUCH horsepower for that little bugger?

Of course, second question is that it's got a 1/2" dia shaft, so I'm debating getting a motor pulley and opening up the bore to work for this job.

Cheers, Allen

[darryl]

Allen, that motor sounds perfect for the Unimat. Sizewise it will probably fit without much problem, and as long as you can drive it from some kind of speed control you'll be 'golden', as they say. At that size and rating, I'm assuming it's a PM motor, so it should be good for lots of torque and long periods of running without overheating.

That won't be too much power, and in any case the power you use will be determined by the job at hand. The motor is likely going to be capable of more torque than the belt can handle, so belt slippage would become the deterining factor in how much power can be used by the spindle. The motor itself will probably want about 20 to 40 watts continuous just to turn at some reasonable speed, so that doesn't represent that much power. I doubt that in any normal turning operation you'd be using more than about 100 to 200 watts, so that means (to me anyway) that you're right in the ballpark with that motor. If it's vented, you could cover the vents with tape and monitor it for temperature rise, or use a shield of some kind.

It's also likely that it's designed for cw rotation. No problem using it ccw, though you'll have to put up with brush noise for awhile. It's unlikely that the brushes have a set direction, except for being worn in for cw. If you're industrious, you could always seat the brushes for ccw.

I have a motor very similar which I'm going to use with my Unimat. This one is rated 6000 rpm at 90 vdc, but for the most part I don't intend to run it that fast. I would probably want to run the belt on the large diameter of the spindle pulley, and the same on the motor pulley, for about a 1:1 ratio. That might mean a different belt, but an o-ring will work fine there. It should be good for 4-5000 rpm without 'growing' very much.

[Murphy]

Darryl, looks like we're both on the same page here!

You're right, it's a PM motor, made by American Bosch, Part # 2229027MO30MM, and the seller on Ebay had the bridge rectifier available for another 7 bucks!

With shipping, I'm out $50, but I suspect I've got one hell of a motor for that money.

Now, to see if the drive pulley will work with a 0.475 hold bored through it.

I suspect there's enough meat there to work well, just have to decide if I want to use a setscrew or a roll pin to hold on the pulley.

As to the belt 'growing' problem, I bought the 4mm polyurethane belting and plan on making my own. Upside is the belting sticks to the pulleys so well, it'd make a octopus go home and cry!

If it's cut right, it doesn't 'grow' either. I have 1/4" belting like this on another machine and I'll never go back to a neoprene O-ring again! The instructions in a Small Parts catalog went as such:

Run the belting around the desired pulleys and make your belt 8% shorter. Light both ends aflame, let melt, and press together, with a angle iron or aluminum channel to hold them square until cool. Trim and use.

Who am I to argue with success?

Allen

[Murphy]

Here's the same motor with a longer shaft:

http://www.kansaswindpower.net/motors.htm

American Bosch These new American Bosch motors are rated 12 to 120 volt, 800 to 10,000 rpm, .5 to 5.5 amps, reversible, continuous duty, bronze sleeve bearings, 50 in. oz. Torque. 3” diameter, 7.5” long including 7/16” x 2 3/4” shaft with 5/16” threaded end. A low cost motor for ceiling fans, circulating fans, sewing machines, small water pumps, etc. Motor part number 2229027M030MM. Experimenters can use with wind and water power.
F205 6 lbs $25

[darryl]

I'm wondering what you might consider your needed rpm range at the spindle? Because the pm motor is going to be more torquey than the original, it won't slow as much under load, so it can be used at a lesser rpm to start with. Running it at rectified line voltage is pretty fast already, and if a speed control is used, it might start by rectifying and filtering the line voltage. In that case you might be able to get more than 10,000 rpm out of it. It's going to draw a somewhat constant idle current just to spin, so the faster you have it turning, the more power it's going to use and the hotter it will get, just turning. In part this is why my preference would be to run it slower, but 'gear up' a bit more to the spindle. The extra torque a pm motor gives can handle this, and the slightly slower operation will keep things cooler and vibration down. That may or may not be a problem, but it's something to consider. I use a pm motor on my tool post grinder, and I'm actually using nearly a 2:1 overdrive to the spindle. Running the motor at roughly half the speed cuts the power quite a bit, but it's still enough. The motor barely gets warm, and only after tens of minutes of continuous running. It has no cooling fins, fan, or vent holes. It's about the same size as yours, and with the same type of magnets, and two pole, so it's got ballpark the same power as yours.

You wouldn't want to overdrive your lathe to this extent, but you can go up from what looks like about 1 to 1.5 to about 1 to 1. (if your high speed ratio hasn't changed from what my Unimat has).

In any event, the speed control will let you run the high speed as slow as you wish, and you still have the belt ratios to play with anyway. You might not ever use the second (backgear) pulley again.

Speaking of speed control, I wonder what you'll be using- I'm going to look into this because I need to make up a couple of those now. Currently I use a variac or a power supply with voltage taps, and while a bit crude it's effective and not prone to failing. But speed regulation is poor, and I would now like to upgrade this. I'm hoping to find a circuit that does a decent job with a pm motor off line voltage, without paying a relative fortune for it. After all, a light dimmer to control 3/4 horse (600 watts) is pretty cheap, and to control a dc motor the circuit wouldn't have to be that much different. Some of the prices I've seen for dc motor controllers are up there, beyond what seems realistic to me. Maybe someone else will have a circuit that is tailored for the job, and can be built up at minimal cost.

[Murphy]

I've been kicking around the same thing as to speed control and came to a cheap solution: put a single diode in the AC feed that can be switched out so you have full wave or half wave feed to your rectifier bridge.

With that circuit, I'll have a low range and high range speed with a flip of a switch. The electronic techs used to do this to give their soldering pens a dual heat capability, and you would NOT believe how much beer I had to buy to get this trick out of them!

[darryl]

Well, I did some searching, but I couldn't find a circuit that's already tailored for those or similar motors. Seems odd to me with all the similar surplus motors out there. Anyway, it would seem the best way to run such motors is via a PWM output and adjustable frequency, and it should have a current sensing input so the circuit can compensate for motor loading and thereby keep the speed consistent with the control setting. You won't need super accurate speed control anyway. It's not like a tape deck or whatever, where any speed deviation would be unacceptable. So- I found one circuit that does have current draw sensing and a frequency control, but it's made for 12 to 24 volts. That can be overcome, but as such it will take some development time. Being able to adjust the frequency is important so the motor doesn't develop high voltage pulses due to the PWM action. With any luck, the frequency required won't manifest as an annoying squeal as in cordless drills at part throttle.

I suppose one could hack a treadmill system to be able to use the motor driver circuit, but that doesn't meet my ideal- I'd rather have a stand-alone circuit purpose built to run just these motors, and then any time I put another one into service, I just duplicate the circuit again.

Yes, controllers can be purchased, and will probably have a safety feature or two that might not make it into a home-built controller- Personally, I'd rather build it myself from parts on hand- I probably have all the parts I would need- but at this time I'm not into developing the circuit. Nor can I say what it might cost overall to build one- certainly if one's time is worth anything it would be cheaper to just go out and buy one.

Having said all that, sure the diode trick will get you a slower speed than 'full tilt', but it will make the motor hum probably, and speed regulation will be non-existent. It almost seems that to avoid electronic problems and still have a means of changing speed, a variac would be hard to beat.

If I do come up with a suitable circuit that I use, I'll be making a pc board design to build it on, and if anybody's interested I'll post it.

[Evan]

I haven't done much machining on the Unimat since I put that motor on but the testing I did do showed that the motor is so much stronger than the original one that even when slowed down with a dimmer it holds speed very well. The load presented by the lathe is small compared to the torque the motor generates even when running at a reduced phase angle. A dimmer designed for ceiling fans will work better as it controls the phase angle differently than a light dimmer and is more like 60hz pwm.

The problem with regular dimmers is that to produce very low speeds the phase angle has been reduced to the point where the motor is only receiving low peak volts. By chopping the phase at both ends the average current is smoothly reduced but the voltage is maintained.