I have a Chinese mini lathe and it has the usual issue of breaking motors and control circuits.
My lathe needed a new control circuit at £90 and a new motor at £86. Both +P&P.
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.
Step 1: The Parts
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.
I found a 25A 24v power supply to pump out 600W max.
I found a HobbyKing 200A ESC 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.
Step 2: Mounting the Motor
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.
Step 3: Making Your Own Spade Terminals From Copper Pipe
I needed some spade terminals. Had none in stock. I used some micro bore copper pipe to fashion some. They worked pretty well!
Step 4: Putting It Together
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.
These servo testers need a 5V power supply to run. I get a 5V, 5A regulator off ebay for £9.
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.
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
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:
To keep an eye on the power usage and amps, I get a power meter for £10
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=520187... and nuts here: http://www.ebay.co.uk/itm/360650014234?var=6301182...
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.
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!
Step 5: Results
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.
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 fins 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.
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.
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.