Unimat SL lathe motor conversion.

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Hi Animal, Amazon have the Comsew motor $354 shipped from the USA. I could almost buy a new lathe cheaper than that. But it would need more bracket work, pulleys, etc. I guess it is a very good solution, but I was simply explaining my choice, for £20. And very little work. My finished motor installation looks like the original as it is mounted in the original casing.
My choice is Not your choice obviously, but others may find it of use.
Thanks for the alternative suggestion.
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Thanks Mark.
I am running at half speed. Not checked the voltage. (Musically, half speed is the same note, one octave lower than full speed). It is a useable hum, not too noisy. I have ordered a 40mm fan for computer cooling, that will blow air through the motor. I'll be able to feel the output temperature to monitor it...
If the half-speed relates to 1/4 power, then the 125 watts at 6000rpm should not be too much torque for the frame... I think? I understand that it is the torque applied at the tool that distorts the frame and loses accuracy. Max torque seen by the frame during cutting, is based on max torque from the motor (at the set speed) with the ratio of the belt drive applied.
Really, the lathe sees the torque applied by the tool, so as long as I don't apply heavier cuts, or higher feed rates, I should not twist the lathe during cutting. I was able to make a 0.010" cut down to a parallel 0.40" pin, in mild steel, so it does what I could do with the original motor.
Thanks for your advice,
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Wow $ 200.00 price difference between US & UK . Monitoring the heat coming off the motor is a good idea , but ou need to know if the fan is really cooling the motor . I would monitor the actual motor Housing temp . All the air coming off the motor is just air from the motor , the actual heat is in the metal of the motor housing/body & that is what you really want to be concerned with . Experience tells me if your running that motor at the lower end of it's spec's it's going to run warmer than at the upper end of the spec . If you really want to cool it with a fan CFM is your friend .
Hi Animal, Not sure about "CFM" - Computational Fluid Mechanics? - perhaps?
And YMMV has completely passed me by...
But that aside, in the industrial world, if temperature management was a real issue on something needing a long lifetime, I should probably use thermal sensors in the armature? - As it is usually in the core of the armature where the heat is trapped to raise the local temperature to break down the lacquer insulation, causing shorts and arcing that fail the windings. (Instantly! in my experience!). But with a £6 motor that is not an option. I have hidden it in an enclosure - the old SL Motor casing. That is probably worse than running at half speed (quarter power?). But forcing some air-flow through the casing will extract heat very effectively from the armature, and perhaps remove some of the ionised gases from the commutator, to prolong the life of that and the brushes? When designing High Power generator busbars (500MW and upwards, Just 2000A or so), we made designs that were sealed, keeping dry, conditioned air in the enclosures, and air cooled or water cooled depending on the needs of the application. The air-flow through the enclosures (and conductors) was usually less than 1m/s, as that was adequate to keep the air "disturbed" and extract the necessary heat. Faster flows were only required on rare installations, for particularly long runs, where the air collected more heat. It didn't improve the local heat extraction considerable. So computer fans extract hundreds of watts of heat at very little air flow, which is what I need.
I have considered the heating to be an I-squared R calculation (Or V-squared/R), so running at half speed, quarter power should be less heat generated than at full power, surely? - What have I neglected/missed? - I am glad of the attention you are giving this, as your "Buddy-check" asks me the questions I have usually ignored.
On another topic: The original SL motor (Universal motor = series field coil - configuration) had a double capacitor wired inside the casing.

0.01microF plus 2 x 2500pF.
The device had 3 wires. One = Earth, the other 2 went across the armature brushes. I suspect this was for commutator arc reduction (RF Noise reduction)? And the Earth, between 2 x 2500pF capacitors, was to keep the armature (Brushes) close to Earth potential.. (an HF earth bond?).
I GUESS the 2 x 2500pF capacitors are in series, but the total is in parallel with the brushes (armature). The mid-point of the 2 x 2500pF capacitors is bonded to earth through the 0.01micro-F capacitor.

I'll try the new motor for radio noise (Yes, I still have a radio, for listening to broadcasts and music!).
Should the new motor need RF suppression? - As it takes up to 220V at 100Hz as spikes of chopped and rectified sinewaves? And I can see some arcing at the brushes? It is however almost completely enclosed by an Earth screen - the SL motor body - with just a few slots in the ends for the cooling air-flow.
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Hi Ken

Yes these probably are suppression capacitors "Universal" Motors due to their design are notorious for producing RFI and are usually quite heavily suppressed

Permag motors produce less but they do produce some and I suspect yours probably already has suppressors fitted but nowadays they are tiny and hidden inside
Even in China there are RFI/EMI standards.

The motor casing does a good job of reducing direct radiation but there is another route by which it can "escape" and that is up the supply cables they act as aerials and propagate the noise. This is where the capacitors come in. As you have surmised they provide a low impedance path to earth for the RF noise and effectively short it out.

Electronic speed controllers can also produce RFI, as I know all too well having worked on theatrical gear.

With a small drive like this I doubt you will have much trouble but if you do small mains filters are cheaply and easily available.

Best Regards Mark
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Thanks Mark.
My larger lathe (750W PM Motor has a mains filter at the input - before the variable voltage control. Presumably to stop lathe noise from feeding back into the house!
Thanks for the advice.
Not sure about "CFM" - Computational Fluid Mechanics? - perhaps?
And YMMV has completely passed me by...

CFM is cubic feet per minute. A common way to measure airflow in the US. There has to be a metric equivalent.

At the car dealership there is a window sticker that tells about the car you are looking at including its fuel efficiency as miles per gallon. YMMV is translated to mean "Your milage may vary" which means that their estimate may be far from reality.
Thanks for the translation.
Now I have received my little fan (today's post) I shall see if it mounts internally, instead of my first idea of external to the old SL motor casing. Possibly, it may even mount on the end of the new motor body, so all the air blows through the internals of the motor. A 40mm diametrically fan, on a 42mm diameter motor seems workable....?
We shall see!
Well , now I'm at a loss " Buddy-check " ? Good luck & have fun .
it may even mount on the end of the new motor body, so all the air blows through the internals of the motor.

Blowing air through the motor rather than around it would certainly give the best cooling if you could arrange it.

Best Regards Mark
Hi Animal: a "buddy-check" is when someone reads something you have done (a letter, calculation, etc.) and queries what you mean by a part they don't understand. Or it could be checking a machining set-up, or anything else? ("Buddy" = colleague, mate, partner, or other volunteer - or people on the thread who ask "why?", or for more explanation about something.). Because you and others asked, I thought more about what I was doing. In Design offices where I have worked it was common practice to get anyone who could read drawings just to glance over a drawing and check it looked sensible. A crazy bit of practice, but almost everyone found something to query, usually the mistake the author had missed! Book authors call the "Buddy-check" Proof-reading, I suppose? - I thought the term originated in the US of A? Or possibly Military?
And you have just done a "buddy-check" of what I wrote!
Mark, I have tried the motor with the fan externally mounted, (Just with Bodge-tape temporarily). Seems to work fine. With 10 minutes machining, previously, the housing felt warm to the touch, but with this tiny fan, the outlet air did not feel like it was warming-up at all., and the casing stayed cool. I'll add an internal baffle to reduce "blow-by" air around the outside of the motor, and encourage more air through the motor itself, as that should improve cooling further. i may even paint the outside of the motor black, and likewise the inside of the casing. (It is amazing how you can increase radiant cooling with just "black! - or insulation with "silver", white, etc.).
Photo of the temporary mounting of the fan (Bodge tape). Depending on "other projects", sometimes my "temporary" becomes "years"... But until I have a job needing a good half-hour or so of running, I don't really know if this should become permanent, or not.

And another picture of the belt drive, before I make a triple pulley to utilise all the change speeds.

These photos remind me how I am not the world's best photographer!
Thanks Dave, but "no thanks". I have an 8 in swing Chinese lathe used for most work, so the SL is used rarely, and only for tiny jobs where it beats the bigger machine. It came with loads of tooling and fixtures - most I have never used. But it is fine on the simple brass parts I make below 1/4" diameter.
But when I burned out the motor, it was an opportunity to find an alternative, and I just thought it was an alternative to the other options that other owners have tried.
My conversion had just a couple of objectives,
  • Relatively low cost,
  • Relatively simple,
  • Possibly with variable speed?
  • Modern parts, if practical,
  • Continuous rating equivalent to the original torque.
  • No bigger (physically) than the original motor
So my finished conversion has turned out to cost: £7 motor, £6 electronics, £3 fan.
With the fan I am comfortable that it is OK for continuous running.
It has modern parts.
I can use the variable speed, but to control torque vs speed I probable will make a pulley for the new motor, so I can use all the gears.
It was simple enough for me to do.
And I reckon it is better than the big, noisy universal motor I had tried.
Hope that helps?
Latest news = no news, it continues to work, and with a new motor belt that is slightly longer, so the tension is reduced, the Motor noise is quite low, so no bother at all now. I suspect the higher tensioned belt was causing something to touch inside the motor, as the end brackets are plastic and not so stiff.
A point to watch for anyone else using a cheap motor.
Also, I didn't make another multi pulley, as that would have meant running the motor even slower than at present, all the time, and as it is I have 3 speeds plus variable speed going even faster than the lathe did originally.
On groups.io there's a Unimat group. There's a Unimat group, and a Unimat3 group if you are lucky enough to possess one. In the files section of that group you'll find a LOT of legal PDF Unimat books. You'll have to join the group to get access to the files section, but I think that's relatively painless. (Don't know about that, I've been a member of that group since it was a group on Yahoo - kind of dating myself there aren't I?) If you ever want to make any mods to your Uni, odds are that somebody there has already done it and will be willing to help you along.

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Hi Steamchick.

First, what can you say about long time (?) use of lathe with new motor.

Here is my take of Unimat motor.



It is a 200W spindle at 48V/12000RPM but at 24V it is just right at 6000RPM.
Pulley is custom made and is fixed between a shaft shoulder and chuck cap.
Heating is no problem.
It is speed controlled.
Some equivalent spindles are listed below:

Your choice of motor seems also smart -to me. I thought at it somehow; and even thought buying a a second hand cordless tool - 24V could offer such a motor and electronics at cheap. It seems they have the same motor as yours and can take enough abuse. And if their plastic housing can take the torque, for sure Unimat SL can take. Anyhow, the belts and their wise use limit both harmful torque on lathe and excesive heating on motor.

Sorry; re-reading, I see 240V instead my first rush reading of 24V on your motor, but otherwise they seem to share the same frame -775 motor (more or less).


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