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My lathe up and quit

DPittman

DPittman

Ultra Member
My variable speed dc motor lathe was acting a wee bit particular lately with a bit of variation in speed control. And now this morning while free running it, it sped up and down quite radicaly. I turned it off and now it won't start again. No motor buzz or anything.

The speed is controlled with a potentiometer and I have had one of those be faulty on me before but in that case it was a dead spot in the pot. The variation in motor speed now was occurring with no adjustment to the potentiometer.

I'm thinking maybe the control board or the motor is faulty? My first suspect is motor brushes. Fuses are fine.

Any suggestions?
 
Test dc voltage coming out of control if you push start and have pot on full blast. For 120v power should be about 90 and for 240 should be about 170.

If it is brushes, and just a dead spot in them, you should be able to manually turn the motor and get a spot where it runs or buzzes. If brushes burn up, assuming it’s not brushless DC, the motor should smell hot or burnt / arc-y.
 
Well that shot a good part of the morning. I got er fixed, it was the motor.
I had replaced the brushes on this motor a few years ago and remember it was pausing on start up and then eventually quit. I put in new brushes and it seemed fine until now. The old brushes showed very little wear and no other signs of trouble were evident.

I had ordered a brand new motor from Busy Bee a few years ago as a back up and I am glad I had it on hand.

Oh by the way, I did try turning the motor over by hand and still no start and it didn't smell burnt either. I'll eventually pull the motor apart and inspect.
 
John Conroy said:
Nice! Usually when I buy a spare part for something" just in case" the something is long gone but the spare hangs around collecting dust for years after.
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Yes me too, this was the exception.

Because parts take so long to get for these Busy Bee asian machines, I wanted to have that component on hand "just in case". I have a few other new items that will likely be forgotten about and thrown away when I'm dead and gone. Maybe I should put better notes with the parts.

It turned out there was a bad brush in the old motor. The commutator was very carboned up but was smooth and undamaged. What makes brushes go like that? They certainly were not worn down much. The last ones I replaced looked fine but new replacements fixed the problem for a few years.

I turned the old motor winding in its own lathe with the new motor just to clean up and inspect the commutator.

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On my 601 it was always the rear brush that gave the problem, I’m thinking weak springs they actually heated up enough to fuse the brush locking tab to the threaded cap making it impossible to unscrew. The 601 motor cover has a few vents and I 3d printed a cover to hold a computer fan to draw the air through and never had another problem with it, another thing I did was to cut brass spacers to go between the brushes and the cap so they will not fuse again.
 
Weak springs are common failure point. They heat up and lose temper. Wires look heat discoloured soldered to brush. At least it’s running!
 
Tom O said:
On my 601 it was always the rear brush that gave the problem, I’m thinking weak springs they actually heated up enough to fuse the brush locking tab to the threaded cap making it impossible to unscrew. The 601 motor cover has a few vents and I 3d printed a cover to hold a computer fan to draw the air through and never had another problem with it, another thing I did was to cut brass spacers to go between the brushes and the cap so they will not fuse again.
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That's interesting. On the bad brush on my motor there are wear marks on one side that might be indicating the brush was jumping/vibrating around? The brass brush guide/holder appear to be fine (other than carbon dirt) .
How do you determine which brush is front and which is rear?
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Well it's been a good day for me as I learned something new again from the good folks on the forum!

Indeed my bad motor brush has a bad spring. It's obvious it got hot. What came first a bad brush or the bad spring resulting in a bad brush?

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It looks like mine did that’s why I hooked up the fan. I also bought a few sets of brushes as they order them in it takes a couple months.
 
The fan would keep the springs cooler, allowing them to keep their springy temper. Copper getting that colour means big time heat. Torch hot.
 
DPittman said:
About the only thing for my lathe I don't have on hand that I couldn't easily access, make or repair myself is the lead screw.
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That's fairly typical. The lead screw is usually longer than the bed although it doesn't always have to be threaded along the entire length.

I got rid of the half nut on my Gingery and turned the lead screw on the South Bend. There are times when I miss the hand crank to move the carriage over faster. I'm running 1/2" 10 TPI ACME. So if you did have to make a new lead screw you just have to find someone with a bigger lathe.

Or switch to a ball nut and screw and use CNC.
 
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