Making Commutators

If you're thinking of casting the items, you might do well to investigate the investment casting system that jewelers use. The equipment is not very expensive and you can cast items that will need very little machining, probably nothing more than removing the sprue and assembling, then the final machining of the commutator after assembly. Without seeing what you're up against, it would be hard to say with certainty if this would work, but my feeling is it would.

Good luck!

Harold

Yep. Now that I've read it again, that's the impression I get, too. Who said I jump to conclusions? :-)

I think I'd have to agree, a mold and plastic press could cost a ton of money, never to be amortized unless demand was high. On the positive side, though, it sure is nice to have everything under one roof, whereby you're always in control. I've always subscribed to that theory, but it can be very limiting because you get spread too thin.

Considering the commutator is silver, I can't help but think the product is not a run-of-the-mill item and might well be worth the investment of hard tooling to do the job in house.

Harold

Jeff Wisnia wrote: (clip) unless he's talking tens of thousands, I doubt if he can amortize the injection molding equipment, molds and learning curve hours very quickly. ^^^^^^^^^^^ When you walk through a busy shop, with all their opertions under one roof, you are seeing one of the sucess stories. When you get a cheaply printed flier in the mail, announcing the bankruptcy/auction of a well equipped plant, with lots of nearly-new modern equipment, you are seeing another, fairly common outcome.

I don't think it's done that way. I think the segments are made first, either a rolled billet that is then cut into pie-slices, or the individual slices are cut from a turned solid blank, with the dovetails and OD turned on the lathe. Then, two tapered discs of some insulating material are used to hold the segments by the dovetail. Mica or other insulating material is placed between the segments as it is assembled.

I suppose if the ID had the right configuration cut into it, it could then be molded onto the shaft and cut apart, and points on the ID would keep the segments from moving and shorting to each other.

Jon

I really don't think injection molding is the only acceptable process. Pouring in a fiber-filled epoxy where other parts have been made such that they form a dam for the epoxy should be doable. After it sets, you then cut the segments.

Really, there might not be much special tooling required. Some sort of piece pushed onto the shaft that acts as a dam, the commutator material pushed down on that, pour in the epoxy, and then slip on a temporary cover that centers the commutator on the shaft.

Jon

John,

The vendor changed production facilities since my last buy to another part of the state (i.e. not out of the country) and the reason they give for the price increase is "the guys ar our old plant were idiots that lost us alot of money". That may be true, but I find it hard to believe that it was to the degree of the price increase (about 650%!!). The real reason, if my guesses are right, is that the new plant does not do silver parts.

The bottom line is that we bought the comms from the old facility for about 40 years. One move and one year later, and the price jumps dramatically. They don't want my business anymore, it seem to me.

Also, with respect to the injected material, I asked and received (much to my suprise and to my vendors credit) the certs on the material that is injected. I don't have it in front of me now, so I can't tell you exactly what it is. I do konw, however, that it has worked for us for a long time and I will try not to change it unless I have to.

The application, by the way, is neither high speed (600 RPM at the most with about 99% of the duty cycle under 100 RPM) nor high temperature (maybe 150 F max). Also, the maximum voltage generated through the comm would be about 4 volts DC.

I'm no expert, but I would say those are not harsh conditions!

Anyway, thanks for the reply, John.

Regards,

Pete

Are any of the employees who used to work at the old plant still around? It might be time to hire one as a part timer or a consultant. Those money losing idiots just might be able to help you bring you commutators in house.

In addition to Ted's questioning the use of silver (in a forty year old design it may be worth taking another look at this) I'd suggest going to your commutator vendor and asking them what they ar currently producing that is close to what you are doing now and can you modify your design to take advantage of something they are allready tooled up to do. It sounds to me like when they moved plants they probably ditched a lot of low volume tooling and machines rather than move them, unfortunately you were probably the one customer that kept a lot of the old stuff around. I'd also look at some other motor vendors, they are usually eager to try to adapt a design they are allready building to replace what you have been doing. Just go in with the specs and physical dimensions and ask if they have anything close. Sometimes it's just a matter of making a longer or shorter version of a standard motor. Call around and see.

Dave

Excellent idea. Thanks.

Impossible, unless your rotor is turning at 6,000,000 RPM (which is also impossible, IIRC). ;) It's true that silver oxide & sulfide are conductive, whereas copper oxides and sulfides are not. But it takes an intricate electric path to make any significant inductance, and it takes a wide area for capacitance to make any difference (greater than simply an open circuit).

Should use platinum then, since it's hard and doesn't oxidize. Silver tarnishes easily and seems to me a bad idea.

If it's for a sensor, just how much current are you pulling?

Tim (sorry to jump in on a thread without reading much of it..)

-- In the immortal words of Ned Flanders: "No foot longs!" Website @

Well, there you go. Unless you can modify the contract, you've got to do what it says.

You were told wrong. Both materials do suffer increased resistance when they oxidize, but the rubbing contact of the brushes removes this as quickly as it forms, so it doesn't really matter for rotating machinery. It can matter for non-wiping switch or relay contacts. In those cases, a non-corroding gold flash coating is used to prevent the oxidation.

DC tachs are fairly common, and they normally do not use solid silver commutators. Most use ordinary copper commutators, same as a DC motor, and it works just as well too because of the wiping action of the brushes, as previously mentioned. They don't start to get noisy until the brushes are nearly worn out and can no longer maintain sufficient contact pressure.

Of course none of this matters if your contract requires silver. But frankly, whoever specified silver in the first place didn't really know what he was doing.

Gary

This sounds like a very unusual application. I've seen plenty of sensor signals running thru slip rings, but not a commutator. In any case, high quality slip rings for low level signals have been made of silver since dirt was new. Some of the best have mercury wetted contacts.

It might be worth contacting some of the mfrs of slip ring assemblies, which are usually low volume or custom made. Even if they can't make the commutator, they may be able to suggest a source. One place I've gotten good service from in the past is IEC Corporation in Austin TX. It's a small, family-run business.

Ned Simmons