Mini lathe blown fuses and Motor overheating

Hi guys I need some help. I have one of those chinese mini lathes CJ0618 model 300 mm with 400 watt motor. I have had it for over a year now and when it came it had sustained some damage to the control box and change gear cover. It was just the plastic shell that was broke so got replacement gear cover but the control box was broken where the bolts attach so I had to improvise a strap to hold it to the lathe. There is a slight gap where swarf could get in so that might be the problem.

Anyway lately the motor has been overheating and smoke came out a few times but yesterday I was turning a piece of 50 mm EN19T steel ( I know, I know its too much for this little lathe to handle). This is the biggest piece of stock I have turned to I took it easy at around 1100 to 1300 rpm on the high setting (knob turned about half way), so because the motor was heating up I would turn up the speed after each cut to try cool it but each cut was slowing down the spindle, I use indexable carbide tips and taking cuts of between .075mm to 0.1mm ( 3 to 4 increments).

So then yesterday after a cut I turned up the speed to try cool down the lathe and I heard a pop and saw a flash coming from the commutator end of the motor. Lathe stopped and the fuse in the control box was blown. The pop sounded more like a match stick being ignited. I took the control box apart and there appears to be no obvious damage, no black marks or blown components. I took the motor out and had a look, no obvious black burn out marks but I have not looked at it with multimeter yet as I left that at my friends house. The motor did have blobs of black stuff in there kind of looked like burned plastic blobs but felt very light and crispy. Other than that there are no obvious signs.

I changed the brushes in the motor and put a new fuse in. The machine turns on with no prob but as soon as I turn the potentiometer the fuse blows again.

So I guess I want to know what to do next. Am I right in assuming the motor has an internal short from the windings insulation melting? Is there a test I can do to find out if its the motor or the control circuit? Bear in mind there was no swarf in the control box but I could use a new one to prevent that happening in the future.

Thanks for your patience and sorry for such a long question.

An overheating motor for no good reason (like the lathe spindle bearings seizing) has a shorted coil or two, which may have taken out the control box.

Check the resistance across the brushes as you turn the rotor from segment to segment.

Hi David

Ouch! sound expensive. The use of the low speed recomended with the motor running at higher speed.

One improvement I have posted repeatedly is to fit a DC Ampmeter on the motor leads.

Mark the Guage with Motor Max Amps.and adust the cutting load to a reading of less the Motor Amperage.

Posted elsewhere Amp Meter

Eric

Hi guys, thanks for the replies. I dont think it will be that expensive, new 400 watt motor is £100 but I found just the rotor  for £60 so thats not too bad. I think its looking like its the motor but I have to wait until friday to get my multimeter back and check for sure. The question now is whether or not the control circuit is affected, if it is I did find a replacement for around £50 which is not too bad unless this happens again and it becomes a regular expense 😖. The Ammeter in the motor line is a great idea so I think I will do that. Amadeal have a replacement control box but its the one with the space in the top for digital speed read out which my lathe does not have so I guess I could buy that and use an ammerter in there instead.

Edited By JasonB on 27/08/2020 16:11:21

Hi David

Keep us posted on your lathe motor repairs alway intrested in how thing get fixed.

Eric

Oldvelo your link is not taking people to where it should.

Hello David,

brushed motors have usually a on time of 10% in a hour. If you do not cool down the motor the windings overheat then again. And this with the addition of metallic dirt is the dead of the motor.

Werner

It is possible to check the operation of the control circuit by fitting a 60 watt INCANDESCENT lamp (if you can still get one) in place of the motor. The speed control should vary the brightness of the lamp if working properly. There have been a whole series of posts regarding the CL300 lathe and its clones. Find these by putting CL300 in the search box at the top of this thread. Good luck.

Thanks guys. Yes I did read about the bulb test but I heard 100 watt bulb. I guess the wattage doesn't really matter so long as its not more than 400 😄. I will keep you updated as things progress. First port of call is motor commutator test with ohm meter to look for internal shorts then the bulb test to see if controls are ok.

I just remembered a few days before that the speed control was getting a bit weird. The lathe would rev up by itself for no apparent reason and one time when I turned the pot to off the lathe turned on by itself and revved up full speed. I think that indicates a Problem with the pot but it may have been a bit of swarf, as I said the control box is damaged so its possible some got in there.

Carbides and superglue are not the answer to everything. Hailstorm will start. STOP using carbides on baby lathes. Test IS 60W blub for control board. Worst case scenario, motor AND control board, going on £300 - £400 to replace. Use HSS and lighter cuts.

Regards Ian.

How much lighter can he go, cuts were only 3 to 4 thou which even a mini lathe should be able to handle with HSS or **GT inserts.

Yes I was taking light cuts. The EN19T steel was harder than the previous stock I machined and that usually machined very nicely with nice strings coming off. This time the lathe really struggled, slowing down.

I only have a 100 watt old style bulb. Will that be ok to use for the test or should I try to find a 60 watt. The control circuit is £50 . A whole new control box is £135 and the brand new motor is £100 but only £60 for just the rotor which is the part I need. I guess the 300 - 400 you are quoting is for professional repair? I can do it myself if I get the parts so it will be much cheaper that way

Edited By JasonB on 27/08/2020 16:11:39

reason I use indexable carbide tools is because I do not have a grinder and have no experience grinding my own tools. It just seemed more economical for me and was working fine up to this point. I felt like the problem was more to do with the 2 inch thick 200mm stock of hard steel I was turning which was too much for me to expect of my baby lathe.

But are they the polished ones for aluminium ?

There may be a control board issue, but I know from experience that running mini-lathes hard and slow burns out the motors.

Neil

No I am using dcmt070204 inserts for steel. Yes I feel like it is just the motor because the control circuit is rated for 6 amps and is protected by a 5 amp quick blow fuse (F5AL250V) which blew so I am hoping that did its job and protected the circuit.

100 watt bulb will be fine.

Good luck..

Bear..

Oh thats good. Thanks

If in the UK with 240vac filament bulbs almost any wattage will do because the motors run off 110vdc or so. The bulbs can't be overloaded with that. The bulb should respond to the control, but won't reach full brightness; no matter as long as it follows the pot up and down.

Another test is to disconnect the motor and run it off a 12v car battery. While it won't spin at full speed, it should turn without getting hot.

Helps isolate if the problem is the controller or motor or both.

Dave

That's half your trouble, you need dcgt as suggested by Jason they are a lot sharper and therefore need less power. They work fine on steel at mini lathe feeds and speeds. Just don't use then on brass.