Mini Lathe Motor Upgrade

I have a Chinese mini lathe and it has the usual issue of breaking motors and control circuits.

Finished three phase brushless DC motor Conversion lathe with current and temperature monitoring

Finished three phase brushless DC motor Conversion lathe with current and temperature monitoring

My lathe needed a new control circuit at £90 and a new motor at £86. Both +P&P.

Replacement Lathe motor

Replacement Lathe Speed Control Circuit

http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/C3-Mini-Lathe-Spares Now my first thought was “Are you taking the expletive?!” I could build one for a quarter of that… plus a solid weekend. You can buy much better technology for much cheaper. Brushed motors are archaic. Brushless are the future! I’ve been wanting to upgrade the motor and control to a three phase brushless DC for a while. This seemed like the perfect excuse. I did the research found out the original motor did 2500 rpm,  rated at 240VDC 2A. The upgrade motor for the C3 mini lathe is 350w and does 6000 rpm. I found a 149kv motor. kv means rpm/volt. At 24 volts this would be doing roughly 3576rpm which would be ideal. It also has a 8mm shaft so it would be a straight swap out for the original motor.

149KV Brushless DC outrunner

http://www.hobbyking.com/hobbyking/store/__37213__Turnigy_Aerodrive_SK3_6374_149kv_Brushless_Outrunner_Motor_UK_Warehouse_.html?strSearch=149kv I found a 25A 24v power supply to pump out 600W max.

AC-DC power supply. 24v, 25A

http://www.ebay.co.uk/itm/300944840662?var=600224822408&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 I found a 200A speed controller for £30. The power supply can only provide 25A and the motor should only pull 10A so 200 is massively overkill. Exactly what we want because it will add robustness to the system.

200A ESC three phase brushless DC motor controller

http://www.ebay.co.uk/itm/121344627700 The original motor had grub screws tapped into the motor casing to act as the mount. I would have to make a motor mount to hold the new motor so that the shaft is in the same position as the original. I started with the mounting plate. Used the one that came with the motor as a template, roughly measured the diameter of the motor and used the radius to define the depth. I found a sheet of steel just the right size for the lathe interface pate hold it in place, and marked up where the holes needed to be drilled for the bolts to go through. I welded these two together and test fitted it. So far so good. I know these motors can kick out rather a lot of torque so I’m keen to strengthen the mount and ensure it is robust. I chop out a couple of gussets and weld them on. The eagle eyed of you may notice that these pieces of steel are different to the others. When I was jigging up for welding I noticed they weren’t magnetic. Fear not! It turns out that stainless welds to mild no problem! After this I tacked on the captive nuts and cleaned up the welds so that it would sit flat in position. Test fit and all seems good.

Time for a test drive, I solder the appropriate bullet connectors on the wires for the Speed Controller and fabricate some spade terminals out of some copper pipe. Heat shrink it up and it’s almost ready to go.

Golden Bullet Connectors

http://www.ebay.co.uk/itm/261487688895?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 The speed controller needs a pulse width modulated square wave input to control the motor rpm. Luckily Hobby King do a servo tester for about £3.50 that will produce one for us from an on board potentiometer.

Servo Tester

http://www.hobbyking.com/hobbyking/store/__31878__Hobbyking_LED_Servo_Tester_UK_Warehouse_.html These servo testers need a 5V power supply to run. I get a 5V, 5A regulator off ebay for £9.

5v, 5A Voltage Regulator

http://www.ebay.co.uk/itm/261225817711?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Everything runs just as it should, jolly good! Time for some integration. I strip out the old circuit and make room for the new electronics. I wire the mains into the control box emergency stop switch, then out to the AC-DC converter I have out the back of the lathe. I remove the pot from the servo tester and wire in the one from the original lathe. Since it’s a Pot, it doesn’t really matter what the resistance values are, I check they’re reasonable with a multi-meter just in case.

I want forward and reverse to work so I wire in the original switch with two of the phases, if you swap the wires between two of the phases of a brushless DC, it will rotate in the opposite direction. To do this I bought some nice silicone wire for £7.50.

Silicone wire

http://www.ebay.co.uk/itm/301193422877?var=600257103735&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 The placement of the speed controller, voltage regulator, and servo tester mean the cables are a bit of a stretch so I bought some extension cables for £2

3pin Servo Extension Cables

http://www.ebay.co.uk/itm/191106121843?var=490285058155&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Quick test drive and everything works as it should. Now my main goal here is robustness and longevity so I want to keep an eye on the system and ensure I’m not straining it too much. The main worries are pulling too many amps from the power supply, getting the speed controller too hot, and getting the motor too hot. To keep an eye on motor and ESC (Electronic Speed Controller) Temperatures I buy two of these temperature sensors for £2 each:

Digital Temperature Sensor

http://www.ebay.co.uk/itm/141290372167?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 To keep an eye on the power usage and amps, I get a power meter for £10:

Digital Watt Meter and Power Analyser

http://www.ebay.co.uk/itm/301149839071?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Wire these in and were ready for the first test cut. I have an aluminium component in the lathe from the electric bike so I take a few cuts on this. No worries, much smoother than the original system. Now I need to neaten up those wires and mount the electronics. The easiest place is on the back of the chip guard. I line up the power supply and drill through the mounting holes placing a bolt in each hole as it’s drilled. I do the same with the voltage regulator. The ESC doesn’t have any mounting holes and it’s pretty late at night so I just duct taped it on for now. I’ll make a mount for it another time. Realistically when the tape fails. I got my bolts here: http://www.ebay.co.uk/itm/221307549544?var=520187427102&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

A2 Stainless Hex, Cap head Bolts

and nuts here: http://www.ebay.co.uk/itm/360650014234?var=630118289301&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

A4 Stainless Nyloc Nuts

Since I needed to check the motor RPM was as it should be, I bought an rpm meter while I was at it. It only cost £10 and I’m sure it’ll come in handy. Motor RPM was bang on the money.

RPM Meter/Gauge

http://www.ebay.co.uk/itm/291105855195?var=590257804048&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Since it’s been finished I’ve finished that part that was in the lathe, and one other job. So far the system I put in it holding up nicely. No signs of any issues. The only problem has been making sure I do the motor mount bolts up tight enough. They aren’t nyloc nuts so they have a tendency to vibrate loose. I might look into a more robust mounting solution another time, but for now, I have an electric bike to finish!

29 thoughts on “Mini Lathe Motor Upgrade

  1. Demis says:

    Nice and neat Job, I have a similar lathe and my dad’s motor packed up, I ordered a replacement. Unfortunately I did not know about your idea and it did not cross my mind.
    Have you got the drawing or dimensions for your motor mount bracket please?
    this would help other to make the right size should they do this upgrade.

    Kind regards

    • This is a one off built to fit my unit. If you follow my steps, you can make one bespoke for your machine. Mine is only the size and shape it is because of the bits I had in my scrap box. It’s a bit close to the motor can for comfort.

      If you would still like a drawing, I can do you one.

      Kind regards,

  2. jared says:

    Saw your upgrade for the mini lathe motor. I really like it and may try to do it with cnc control from Mach3. How well does it work with load applied? Does it have enough torque to turn large pieces? Thanks.

    • LetsBuildone says:

      It’s way better than the original. You could audibly hear the stock motor slow down as you load it up. The new brushless motor is much more stable. Better low end torque too. I haven’t managed to stall it yet. On the stock motor it was a regular occurrence.

      • Doug says:

        Just a couple questions about your terrific project. Doesn’t the outrunner style motor case “spin” while in operation? Second question is the ability for a static brushless motor to cool itself. Are these theoretical problems that just don’t occur or are you limited in how long you are able to run the lathe? I.E. Does this have an acceptable duty cycle?

        Thanks
        Doug

      • LetsBuildone says:

        Hi Doug,

        Yes, the outrunner case spins.

        I had similar concerns about temperatures. The motor and ESC both have temperature sensors. Neither have gotten more than warm yet. No duty cycle issues so far. The motor mount acts as a heat sink. I could weld on some finds and a fan if I was that bothered.

        This outrunner specifically has an impeller built into the case to move some air through it while stationary, though I’m not sure this is necessary. I over rated all the components on purpose.

        All the best

      • jared says:

        I bought all the pieces you listed here and assembled them. Tested them out today with the motor in the bench vise. My motor works fine, but I did notice that I can get the motor to noticebly slow down (and stall at low speed) just by grabbing it with my fingers. Not as much torque as I expected. Is yours this way, or did I do something incorrectly? (Actually, mine is a 24V, 30A power supply.)

    • Hi Jared,

      There is a significant reduction in the gearbox within the Lathe. The torque will be improved when it’s fitted.

      The upgrade motor for the lathe is 350w. The motor I used is capable of putting out up to 2250w. That’s almost 6.5x as much power. The power supply will only kick out 600w however. But that’s still almost twice the upgrade motor for the original unit. The 350w motor runs at 2500(rpm)=261.8(rad/s). 350(w)/261.8(rad/s) = 1.34(Nm). The new motor runs at 3576rpm @ 24v from the 149Kv quoted. 600(w)/374.5(rad/s)= 1.6Nm. That’s ~20% more torque.

      As for why you can stop if with your fingers:

      Motor torque is proportional to current multiplied to the torque constant of the motor; Kt. The current is limited by reducing the PWM duty in this instance.

      If you start the PWM from 0 until it just starts to spin, then it will be at the point where it has barely enough current available to produce enough torque to overcome it’s own internal friction. Any extra you add with your hands will stop it. This is true of any motor.

      Take the 310 kW, 600 N·m, motor out of a Tesla Model S, run it at 0.1% duty PWM (gross simplified torque estimate: ~0.6Nm), and if it spins at all, you could stop it with your fingers. Humans can apply about 5Nm with a screw driver.

      Hope your conversion goes well 🙂

      • jared says:

        Thanks for the breakdown. That helps a lot. Just an FYI, I was considering using this setup to upgrade my mini mill. The torque issue had me really concerned about using it to turn endmills. My motor would run at 4130 RPM’s as clocked by a similar tachometer to yours. I am considering purchasing as additional power supply and running them in series to get a full 48v (30A). This would allow me to gear it down even further to add extra torque, no? 7000+ RPM’s would give more room for a better gear/belt ratio.

        I did study some of the numbers around the motor (apparently not enough). I figured two 24v, 30A supplies would be 1440W, which is theoretically 2hp. What I expected was the motor to compensate for the load by drawing more current (using the same voltage) and kicking back up to speed while handling the load. I was under the impression that low speed=high torque and high speed=low torque.

        I noticed in the description of your build you stated the motor would only pull 10A.

      • Is that 4130RPM on the motor shaft, or at the tool spindle? And is that the original motor, or your new BLDC?

        Your plan about going for double voltage and lower gear ratio is smart. Amps make motors hot. Higher RPM, lower torque, gives the same power, with less motor heating. You may find that you physically can’t fit big enough pulleys in, to get the gear ratio you need to make the spindle slow enough. It might pay to check that before you commit to buying parts?

        That is pretty much how it works; If you have it running at 100% duty (4130rpm). And you apply a retarding torque to the shaft, then the rpm will drop, and the torque will increase. This is a typical power curve for a BLDC motor: http://www.linengineering.com/_images/BL24B28-04_NEW_HR.jpg

        Torque is across the bottom, the grey line is at about 35oz-in torque. You read the motors performance at that torque by the intersection of the grey dotted line, with the curves, off the side axis. So at 35oz-in torque, current draw is about 3.5A, 115W, 4000rpm, 83% efficiency.

        You can see, that by moving the grey line to apply more torque, it reduces rpm and increases current draw. Current, and torque, will continue to increase until; either the power supply can supply no more amps (often burning it out), you burn out the motor, or you burn out the speed controller.

        You are correct, for the same rated power, low speed=high torque and high speed=low torque. As a general rule, the lower the Kv of the motor, the higher the Kt, and hence, the more torque you’ll get for the same amps, and at lower rpm.

        I based 10A off the original motor being 240w. The new motor at 24v, 10A= 240w, so that would be equivalent power. The parts are rated for that kind of power, if I put too much power through, I’m going to break something. I’m tired of replacing snapped belts and pulleys with a sheared off key tooth. That happened far too often with the old setup. I’ve converted it from that weird, unique, Chinese pitch belt, to a standard one, with aluminum pulleys. So that I can replace the belts far more cheaply, and the pulleys won’t sheer key teeth. So far so good. No more breakages. I might put on a reset-able fuse, to limit the current and protect the components.

  3. Mikko Rintakuusi says:

    Hi! How did your upgrade work? How much torqua motor provide with low RPM? I have Einhell MTB3000 minilathe with similar 400W DC motor and I would like to upgrade motor to brushless motor with more torgue? Can you make me parts list what I should boy on this project. Some of your links are dead. Thank you!

    • Hi Mikko, It’s significantly better than the original. With the stock motor, I would stall the motor occasionally, and you could hear the rpm drop as you loaded it up. Now, torque is no longer an issue. The limit is not the motor or circuitry anymore, it’s the tools, material, and structure of the unit. I’ve had to strengthen the tensioner because it’s so torquey.

      Ok, so for you, Search for a low KV motor. Power=angular velocity*torque so; low KV means high torque. High power means high torque. Large diameter means higher torque. Heavier motor means Higher torque. You’re aiming for around 150kv, >500W motor.

      Then you need to check what the rpm of the stock motor is, divide this by the kv of your selected motor; what ever this number comes out to is what voltage you want to power your unit with. Find a PSU that delivers either slightly less, or slightly more than this. 12v, 24v, 36v, 48v, are common PSU voltage steps. For example: 6000rpm, 149kv motor= 40.268v. So you would pick 36v or 48v. 36v if you want to play is safe, 48v if you’re feeling like an upgrade with the risk of breaking parts of the lathe if you push it too hard. Ensure the motor can take that high a voltage.

      At 48v, 400W would require 8 amps (36v 11.1A, 24v 16.7A). Pick a power supply that can deliver this voltage at more than this current. You want a bit of head room. so at 8A you’d want a 10-15A power supply to ensure you aren’t stressing it. (11.1A; 15-20A power supply).

      The same goes for your motor controller. Make sure that is can take more volts and amps than you need it to by at least 20%. This is safety factor to ensure you don’t blow any of the bits up. some people use smaller safety factors of 5% or 10%. Others larger, 50% or 100%.

      If you pick out some parts, I’ll double check them for you to make sure they’re about right.

      Let me know how it goes!

  4. Doug says:

    Thanks for the reply,
    I’ve done some reading and see that Hall Effect Sensor BLDC motors have better performance at lower RPM’s compared to sensorless (back EMF) systems.
    Is that a problem for you?
    Doug

  5. rocketpatel says:

    I have made a motor mount from mild steel. I didn’t leave much clearance around the motor so I am getting kick back at low speed. I may have to remake the mount from nonmagnetic material. Damn.

    • LetsBuildOne says:

      That’s an interesting development. I never thought of that. If you use a non-magnetic metal, won’t the induced eddie currents in close conductive material have a similar effect? Mine has a lot of kick. But I put that down to the motor being so torquey.

      • rocketpatel says:

        That’s good point about eddie current as well. For now, I have not built a non-magnetic mount. My plan is to go with the mount I have already made and if I feel that kick-back is too much and/or torque is too low. If I ever make the mount again then I better choose non-magnetic materials for the sides close to the magnets. Which will mean a hybrid mount, motor mount plate made of metal and sides made of plastic.

        By the way, thank you very much for this article. It made my life very easy. One change I made: I bought an ESC with built-in 5V BEC that way I don’t have to power the servo tester. Just plug the powered 3-pin connector front he ESC to the output side of the servo tester.

  6. James says:

    Will this motor work with the original controller and power supply?

    • LetsBuildOne says:

      No it won’t. The replacement motor is three phase brushless DC. This is not compatible with a brushed DC motor controller. It is also 12-45v DC, 70A. Where as the original power supply was 240v 1A.

  7. Nitrous says:

    One silly question. Is “pot” really short for potential divider where you come from? How exactly do you abbreviate “potentiometer”? 😉

  8. rocketpatel says:

    In India too, it’s called pot.

  9. Nitrous says:

    check your description of that “pot” means in the context of your instructable.

  10. Rodrigo says:

    Hi! Great tutorial! I´ve got a Sieg C1 lathe, and I plan to upgrade it to CNC.. One question: By changing the motor to Brushless, can the CNC software (MACH3) control it´s speed, in order to be able to make threads using CNC? Can this brushless motor have it´s speed controlled electronically precisely? Or it should be done with a Servo motor instead? Thanks from Brazil!

    • LetsBuildOne says:

      Hi Rodrigo, In theory, yes. It could. They are powered off a three phase signal. The frequency of the applied wave dictates the rpm. It wouldn’t be too complex to calibrate this to the actual rpm. You could also use hall effect sensor to form a closed loop control circuit.
      Usually stepper motors are used for CNC machining though, because of their accurate control of spindle position. But on a lathe, gears connect the chuck to the feed, so you can cut threads without needing CNC control. If this is a posibility, I’d recommend this.
      Otherwise, there is a reason stepper motors are usually used. You could use one of those instead. With a three phase motor, I can’t guarentee it’s accuracy. Good luck!

  11. Rodrigo says:

    Thanks a lot!! I already did some research and I´ll assemble a simple hall effect circuit yet on the spindle, despite not upgrading the motor right now… The problem with threading is that as soon as I change my lathe to CNC, I´ll loose the gears to thread… I´ll have to leave it to the stepper motors to control both axis.. But I´ve seen it´s possible to thread, as long as the Mach3 knows exaclty the speed, with the hall effect sensor! I´ve searched online and it seems that this motor is discontinued… I´ve came across this version on ebay… it has 192kv…

    http://www.ebay.com/itm/RC-Turnigy-Aerodrive-SK3-6374-192kv-Brushless-Outrunner-Motor-/111860247041?hash=item1a0b63ea01:g:SIoAAOSw5IJWgJ6A

    What do you think? Is this one an option? Do you know of any other options? Thanks a lot for you quick answer!!

  12. Rodrigo says:

    Hi! I´ve been researching more and more….

    Here goes my questions… so many!

    My lathe is the smaller version of the C2/C3. It´s the C1. It´s motor is this one: 4000 RPM 230V, 0,9 A, 150W. Shouldn´t it be 207W (P=IU)?

    http://www.arceurotrade.co.uk/Catalogue/Machine-Spares/C1-Lathe-Spares/C1-19-Motor

    It has a thick frontal mounting plate, so I guess in upgrading to the Brushless, the casing would be easier to be done, right? Just mill a thick frontal mounting plate.. Could it be done in thick aluminum? Would it be strong enough?

    The timing belt and pulleys are plastic (17T, 10mm wide) on the motor and 34T on the Spindle, so it reduces the speed from 4000 to 2000 RPM on the stock motor.

    So, if I upgrade to the brushless 149kv one, at 24V it would give 3576 RPM. Using my plastic original pulleys it would give the max speed of 1788 RPM, less RPM but more torque than the original.

    Would 24V at 1788 RPM would be more than enough turn aluminum and maybe some steel?

    Or 36V at 5364 RPM on the motor, and 2682 at the spindle would be better?

    Problem is it´s hard to find around here 36V high AMPS power supplies, and they are expensive..

    Another thing:

    Are there replacement metallic pulleys/belts, so I can make it…. Stronger? I read you did replaced yours… Where can I find these metallic sets? (pulleys and timing belts). They must be the same as mine (17T, 34T) or If I can´t find exactly the same, could I use different ones but keeping the ratio (20T, 40T, for instance)?

    Thanks a lot! I´m in the process of doing your mod in the next months! But it takes time to buy stuff abroad and get them to Brazil…

    Cheers!

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