Potting electrical coil in steel

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: Harold_V, websterz, GlennW

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Potting electrical coil in steel

Post by Harold_V » Fri Feb 28, 2020 5:10 am

Hey, guys!
I've owned my Sag 12 Graziano lathe since 1967. It's been a pretty decent machine, with few problems. Unfortunately, clutch failure is one of them, albeit they are quite reliable.

When they fail, the clutch proper is generally in great condition, and mine is no exception. It could easily go another 50 years if the amount of wear I see is representative, but the coil has shorted, so the clutch is no longer useable.

Graziano is long gone, and I have not been successful in finding a manufacturer of magnetic clutches that can supply my needs. I'm giving some thought to rewinding the coil. I should be able to machine away the potting, at least enough to expose the surface of the wire. If I can, I should also be able to unwind the coil, so I can determine the length of wire required, plus that will allow me to determine wire gauge.

Assuming these things happen, one thing I don't know is how to pot the new coil once wound. Winding shouldn't prove to be much of a problem as I can use the Graziano to do that (it still runs, I just don't have one of the speeds).

Is there anyone on the board who has experience in potting such things? If so, this coil/clutch assembly runs in an oil bath, so that's a consideration for choosing the proper potting compound.

I'd enjoy hearing from anyone with experience related to this project.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
tornitore45
Posts: 1784
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Potting electrical coil in steel

Post by tornitore45 » Fri Feb 28, 2020 8:00 am

I can only offer my experience with power transformers in the 200W range.
We used a type of epoxy resin, gray, typical epoxy smell with the viscosity of warm honey.
We did not pull a vacuum except for HV transformers.
If you have the capability to pull a vacuum it may help to seal any inflow of oil. Still clean mineral Oil is a good bath for coils anyway.
You may be able to pull a vacuum with a vacuum cleaner and a large bag.

There are also rubber compounds that may work in this application.
Mauro Gaetano
in Austin TX

John Hasler
Posts: 1423
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: Potting electrical coil in steel

Post by John Hasler » Fri Feb 28, 2020 8:42 am

Found this thread on Practical Machinist:

https://www.practicalmachinist.com/vb/s ... er-350254/

You'll want a thermally conductive potting compound like the ones this company sells:

https://www.lord.com/products-and-solut ... apsulation

One reason for pulling a vacuum is that voids can cause localized overheating.

You might be able to release the block of plastic from the case coil and all with heat. If you section the coil you can count the turns.

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Potting electrical coil in steel

Post by Harold_V » Sat Feb 29, 2020 5:02 pm

Thanks for the advice. I hadn't considered that the potting may require being vacuumed, but I have a vacuum casting setup, so that won't be a problem. I expect I'll put it to good use.

I appreciate the links in regards to the Practical Machinist. I rarely go there, so it's unlikely I'd have found that information. I agree that the compound I use most likely would be best if it displays thermal conductivity. The clutch circuit is fused @ 6+ amps, so they most likely run warm, if not hot.

I think I have enough insight to pursue this project now. When time allows! I've been running the lathe without that speed for more than a year now. Time to get it done.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Hopefuldave
Posts: 148
Joined: Wed Jun 01, 2011 8:17 pm

Re: Potting electrical coil in steel

Post by Hopefuldave » Wed Mar 04, 2020 6:50 am

In case this helps...

I wound a new secondary for a welding transformer to convert to a 9 kva isolating transformer for my RPC, 3D printed the bobbin and used 6" rotary table to turn it, electric drill on the worm shaft and a lash-up of bearings and washers to straighten/tension the wire (3mm, possibly a bit OTT for your clutch winding!):
20190407_174707.jpg
The turns counter was a few Local Currency Units from a gentleman in China, made the process a lot more relaxed not having to tally in my head or on a piece of scruffy paper :)
20190407_174751.jpg
I bagged the bobbin and used some spiral cable wrap on the end of clear vinyl hose to feed the epoxy in / apply vacuum at opposite points with Lucky Bleeder, usually used for brake/clutch hydraulics, the epoxy filled the voids well! No pic, I'm afraid. Hardest part of potting was getting the bag airtight, once it was the vacuum pump pulled the epoxy through the hose (6mm / 1/4" bore) no problem, I stopped (clamped the inflow hose with a Mole/vise grip) once resin appeared at the vacuum hose but held the vacuum with the pump.

Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of Wise Men - Douglas Bader

User avatar
NP317
Posts: 2205
Joined: Tue Jun 24, 2014 2:57 pm
Location: Northern Oregon

Re: Potting electrical coil in steel

Post by NP317 » Wed Mar 04, 2020 11:03 am

Creative!
RussN

User avatar
tornitore45
Posts: 1784
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: Potting electrical coil in steel

Post by tornitore45 » Wed Mar 04, 2020 12:17 pm

I would like to know more about the wire straightener. I suppose it is free to move up and down to follow the wire position on the bobbin.
Mauro Gaetano
in Austin TX

Russ Hanscom
Posts: 1635
Joined: Wed Mar 15, 2006 11:10 pm
Location: Farmington, NM

Re: Potting electrical coil in steel

Post by Russ Hanscom » Wed Mar 04, 2020 12:33 pm

Might also check with a local electric motor shop about doing the potting, ones that rewind motors should have the epoxy and vacuum setup ready to do.

Hopefuldave
Posts: 148
Joined: Wed Jun 01, 2011 8:17 pm

Re: Potting electrical coil in steel

Post by Hopefuldave » Wed Mar 04, 2020 4:40 pm

The wire straightener wasn't that clever - I used a hand to guide the wire, but I can't see why the bearings couldn't move - the wire does a figure-of-W around the three bearings to take out any kinks, moving them closer increases tension as the copper has to go through tighter bends
20190407_174726.jpg
If it were to follow the wire, the bearings would have to be e.g. sandwiched between two plates free to move up and down AND the spacing would need to be adjustable, doable but not as simple as it appears :)

Dave H. (the other one)
Rules are for the obedience of fools, and the guidance of Wise Men - Douglas Bader

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Potting electrical coil in steel

Post by Harold_V » Wed Mar 04, 2020 4:56 pm

Clever! Thanks for the tips. My project is in the near future, so anything I can learn before the attempt is certainly welcome.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
Bill Shields
Posts: 5720
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Potting electrical coil in steel

Post by Bill Shields » Tue Mar 31, 2020 8:06 pm

I would suggest looking in Italy for something NOS before going to all this work...

did you try these folks?

http://www.gornatiservice.it/ricambi_en.html

willing to bet they know the company that originally wound the coils for that lathe
Too many things going on to bother listing them.

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Potting electrical coil in steel

Post by Harold_V » Wed Apr 01, 2020 1:31 am

Thanks, Bill. I hadn't found that source. I'll give them a go to see if they can help. I'm not all that anxious to try the rewind, but I'll do what I have to to get back the missing speed. Besides, all the clutches are identical, so if I can fix them, I won't have to worry about future failures.

I found the maker of the original clutches, but they are no longer making them. Strangely, one of the spares I have (with a shorted coil, removed from my machine years ago by Graziano) is marked. I hadn't noticed the mark, and the other one I have isn't marked.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

Post Reply