Motor Brushes

[Doris]

I have my late Nan's sewing machine, which I think is either from the 50s or 60s with a sew-tric motor. I've been using it for several years since she passed and now the motor is playing up. I thought I would check the motor brushes and they are indeed worn down to just shy of 3mm.

I've spent the past year or so on and off looking for these replacement brushes to no avail. They seem to be in metric now or for old Singer machines. These are 12/64" or 4.74mm square.

Is it possible to take a close matching sized one and sand it to fit perhaps? I seem to remember hearing carbon is brittle but haven't tried actually working it before. I presuming I'm not the only one to have this problem with old motors.

I am happy to just replace the whole motor but as its only the brushes that are the problem it does seem a bit of a waste to scrap the motor because of this. Also I am a little skeptical that a replacement motor will last just as long.....

 

[Tris]

Given the fact they wear down in use without (normally) breaking I don't see any reason you couldn't sand them down. There are muktipacks on some sites for a fiver, so plenty to experiment with.

Usual caveats apply, just some random person on the internet etc

 

[guineafowl21]

You can sand oversize ones down to an easy sliding fit. However, there are lots of different compositions, resistivity, etc, so the new brushes may not work so well.

I usually measure the running, no load current with the old brushes so I can see if the new ones are in the ballpark.

Lots of info on brushes:
https://us.mersen.com/sites/default...2-ptt-carbon-brush-technical-guide-mersen.pdf

This includes a guide to grinding the correct curvature, by wrapping the commutator in emery and turning it against the brushes.

 

[Amplidyne]

I usually clamp them lightly in soft jaws, and file them. Don't see any objection to how you do it really, sand or file. They're pretty tough actually. .Just my observation, but the small ones often seem to be of a softer compound. Make sure the comm is clean. In the past when I've made brushes fit they've been fine.

 

[ChaiLatte]

These are 12/64" or 4.74mm square.

Is it possible to take a close matching sized one and sand it to fit perhaps?

If you could find 3/16" ones, they might substitute for the 12/64" ones.

Sanding is fine. A ceramic tile is brittle, as is a piece of glass, and an abrasive process works on these.

You might have to pay attention to any non-carbon parts of the brush to make sure they were also left small enough not to jam in the holder. In particular, if there is a spring, see how fat it gets when compressed (rather than just measuring it in its relaxed state).