Best way to true a motor shaft?

[Chalo]

I would like to know if anybody has trued or machined a motor shaft without extracting it from the motor. I'm looking for tips or observations before I attempt it myself.

Here's the background: Some years ago, I bought a Transmagnetics double stack 800W motor and controller from Thunderstruck EV. The motor is furnished with a plain 16mm (.630") shaft that protrudes about an inch and a half from the motor's face. I don't know what's the matter with mine, but it has visible runout-- enough to make me worry about cyclic tension problems for a belt or chain. It appears to be bent, because there is no discernible runout at the shaft root.

I would like to use the motor as a self-lathe and reduce the shaft diameter until it runs true. It's not clear to me whether I can clean it up by going down to 5/8" diameter, which would only remove .005" from the shaft. I think the runout probably would not be cleaned up by a .005" diametral cut, though I haven't put a dial indicator on it to find out for sure. My working assumption has been that I would reduce the shaft diameter to 15mm and maybe cut a Woodruff key seat.

Has anyone successfully turned down a shaft to a smaller size by running the motor against a grinding or cutting tool? If so, how did you do it and how did it come out?

 

[TMaster]

Have you checked the bearings?

 

[Chalo]

Have you checked the bearings?

They're not loose or binding. There isn't much else to know about them.

 

[liveforphysics]

This wouldn't be as easy likely, and near impossible to dial in 100% on, but I think you gotta bend it back to straight.
Reason I say that is because trying to make clean precision cuts on a flexible 8mm radius shaft that is mounted in a motor to keep it from being fixtured in a lathe chuck effectively just doesn't seem likely to go well to me.

It could be just because im a pretty crummy machinist, but everytime I try to make a cut in a good rigid lathe on something flexible and out of true, it just ends up being a cluster fubar when im done.

 

[Chalo]

This wouldn't be as easy likely, and near impossible to dial in 100% on, but I think you gotta bend it back to straight.

I wonder how well I'd be able to bend a 16mm shaft without knackering the bearings. I guess I'd have to extract the shaft first.

I really don't like to open high field strength permanent magnet motors if I don't absolutely have to. In my experience it's a hassle to get them apart, a great way to get to get pinched hard, and easy to get magnetic crud (filings, swarf, etc.) into the motor and difficult to get it out.

 

[TMaster]

If you can get a video of it spinning to show how bad it is that would be good. The dial indicator would be a plus. Then I could see if it's ok to run, or needs replacement.
If the shaft is hardened you will have real trouble trying to lathe it down. It would have to be taken out of the motor and go to a big machine shop that can do it.
I guessing this is a manufacturing defect, unless someone really dropped it hard.

 

[amberwolf]

I recall some discussions about doing this kind of thing over on the DIY Electric Car forum, but I don't remember the conclusions; probably from 2009 or 2010, early 2011 possibly.

I know that people there have used the motor as a self-lathe to trim splines off a shaft, among other things, so they could use different couplers on there instead. I *think* WoodyGB has a thread about that.

Major is probably the biggest motor guru over there that posts regularly. Not sure if Jim Husted ever came back (Hi Torque Electric membername) but he did all kinds of motor repair, upgrades, etc., and started some good threads that are still used even now, like this:
http://www.diyelectriccar.com/forums/showthread.php?t=7598

 

[Punx0r]

IME, unless the grinding tool is mounted very rigidly it will just follow the contour of the wonky surface you're trying to true. So you just end up with a smaller version of the same thing. Or worse.

 

[Miles]

How about bonding a sleeve on and turning that down to 20mm? It would, at least, give you the possibility of trying again.....

 

[liveforphysics]

How about bonding a sleeve on and turning that down to 20mm? It would, at least, give you the possibility of trying again.....

That's a good idea.


As amazing as it sounds, my guess would be the shaft was bent after it was in the motor, and the bearings where supporting the mass of the motor pushing against the shaft when it was dropped in shipping or fell of a shelf or whatever went wrong to bend it in the first place. My guess would be, if you took a steel shaft and machined a very nice slip fit or even slight tap-it-on interference fit out of something like 1.5" solid round bar pocket that fits over the shaft, you could both clearly see the runout, apply the force to bend it to correct the run-out with the bar (have the bar be like 2ft long or something for leverage), and my guess is that the bearing surfaces would not be damaged by it at all. Proper hardened quality bearings are so amazing at handling huge amounts of force without damage, it's kinda amazing how strong they are.

 

[hydro-one]

i agree with luke, it likely needs an equal and opposing force (my favorite tool is the hammer)

 

[Harold in CR]

I have done this on motors. I fix a piece of angle iron to a solid surface, like a stout workbench. Have it so the motor is nearly level with the table. THEN, turn the shaft slowly, while using a dial caliper fixed solidly to the bench. Mark the "high" side with a Sharpie. Use a piece of brass to sit exactly on the high side. The shaft should be sitting ON the angle right at the motor. Then, strike the brass with a hammer. Check the shaft for run out. If need be, strike it again, etc, etc, each time checking for runout.

With the shaft supported on the angle, it will take a LOT of stress off the bearings. Shaft may not be all that hard, so proceed cautiously.

 

[Chalo]

Sounds like a job for the lead hammer.

 

[nicobie]

Every time I've tried to grind or file on a running motors shaft it turned out a mess. I don't think the bearings on a motor are precise enough to act like a lathe.

Time to get out the dial indicator and start beating on the shaft. If you have a big lever like LFP was talking about I'd try that first.

 

[Harold in CR]

The big lever will just give you "leverage". How ya gonna hold the motor still ?? Quick sharp raps will bend the shaft before the motor raises up very much. Just don't over do it.

This is why the angle solidly positioned right at the motor, will transfer most of the strike to the shaft and may save the bearings. I never had to replace one ??

 

[Farfle]

Get out the dial test indicator, a good big hammer, a piece of ally tubing to slip over the shaft to keep the hammer from marring it, and a new front bearing. That's what I did for the shaft on a small DC motor. The shaft on it was 3/4" diameter and 2.75" protruding from the bearing.

 

[fechter]

Anytime you use a hammer, you can probably kiss off the bearing. A slow pry-bar kind of bend is much less likely to damage the bearings.

 

[Mundo]

I would think the shaft was bent close to the bearing, however, if it wasn't, I wonder if a long coupling nut might help straighten it out.
The longer, the better, you might have to make it.
Certainly help save the threads if it turns to hammer time.

 

[Chalo]

Anytime you use a hammer, you can probably kiss off the bearing. A slow pry-bar kind of bend is much less likely to damage the bearings.

I'm pretty concerned about the bearings in any scenario involving bending the shaft. It's 16mm in diameter. Even if it's annealed mild steel, the force required to bend it is probably well above the bearing's rating. That's why I was thinking along the lines of tramming it in on a mill table and traversing it across a fast spinning endmill, or maybe a grinding stone.

There are no features on the shaft. It's a plain round cylinder. Whatever I do to bend it is likely to leave some sort of impression.

 

[Chalo]

Get out the dial test indicator, a good big hammer, a piece of ally tubing to slip over the shaft to keep the hammer from marring it, and a new front bearing. That's what I did for the shaft on a small DC motor. The shaft on it was 3/4" diameter and 2.75" protruding from the bearing.

Did you knacker the bearing?

 

[Farfle]

The bearing still spun ok, but it was just ok to start with (water damage) so i guess it did not get noticeably worse.. I just replaced it anyway as its a pump motor that's in a cistern that's a pain to get to.

 

[fechter]

I'm pretty concerned about the bearings in any scenario involving bending the shaft. It's 16mm in diameter. Even if it's annealed mild steel, the force required to bend it is probably well above the bearing's rating. That's why I was thinking along the lines of tramming it in on a mill table and traversing it across a fast spinning endmill, or maybe a grinding stone.

There are no features on the shaft. It's a plain round cylinder. Whatever I do to bend it is likely to leave some sort of impression.

Something like a surface grinder would be good. Just cover any holes in the motor to keep crap out.

 

[amberwolf]

FWIW, if the shaft was bent from an impact in the first place, it's quite possible the bearings at that end are *already* damaged and may need replacing anyway.

 

[jerseyebiker]

Buy a new shaft honestly. There is no way to get the shaft back to normal. Its always going to have some sort of wobble. Its always going to put extra wear on your assembly. I used to roll a shaft in a lathe with a torch and a rocker but they still often break and most people dont own a lathe.

 

[Kirk]

I used to true shafts that were as much as .090 out. The trick is to put a dial indicator on the shaft and turn it to its highest point. Then apply heat to the top part of the shaft until its nearly Red. Take the heat away and blow some air on it from the compressor, to help cool it. Metal has a memory to it and will try to resume its original shape. The shaft would normally come back within .001-.002 in. It is much easier to do it between a set of V blocks on a level surface and can take a couple of shots at it. Heat is awfully tricky near bearings. Be careful.