View Full Version : Polishing gears
08-31-2001, 02:50 AM
I'm in the process of making a very small, precision gear drive. It is a replacement for a commercial one used in a telescope drive system.
I have several cluster gears... my definition of that, is a larger gear and a smaller gear, bound together on the same shaft. The larger gears are made of aluminum and their smaller gears are brass. They connect in line so as to create a 60:1 gear reduction overall.
My question relates to how to best 'polish' the faces of the gears to make them as smooth as possible. The smallest iregularity in any of the tooth faces will effect the constant speed because the drive motor is only a small 3v motor running at 7.5 rpm most of the time (NO torque to speak of).
I have begun the polishing procedure by taking some jewelers polish and mixing it with some teflon based and/or lithium grease and applying it to the gears. What I really don't know is, what kind of mixture ratio of polish to grease I should use. Too much grease, it seems to me, and there would hardly ever be any polishing going on.
Should I apply the polish dry? A mixture of mostly polish and a little grease, lots of grease and little polish? I just don't know.
I cannot tell you the grit of the polish because it is simply listed as 'jewelers polish', but I'd guess that it's in the smaller micron size.
Any ideas? Or is this the wrong type of question for this fourum? If so, I truly apologize.
08-31-2001, 05:00 AM
You could buy some commercial lapping compound and lap the meshing gears together. Set them up on the bench and apply the lapping compund to the teeth and then run them together until you get the finish on the tooth faces that you desire. I would start with a 600 or 1000 grit and go from there.
Maybe some of the clock building guys could help you on this...
Hmmm...I'm not sure you'll get very far by running the gears together with lapping compound, as in theory at least all you should be getting is rolling contact between the gears (assuming an ideal involute tooth form). There should be no -- or minimal -- sliding contact, so what's going to do the lapping?
You say "very small;" what is the diametral pitch of the gears?
08-31-2001, 05:40 PM
They are 48 pitch gears. The smallest being 8 teeth at the beginning of the gear train, down to the largest being 44 teeth at the end.
Even if the gears were 100% involute, which I seriously doubt from the looks of them ans their size, all I'm trying to 'polish' off, would be the small irregularities that would keep them from being perfectly involute and smooth.
As for putting the gears together and running them with a lapping compound, that's exactly hat I'm doing. The only difference here is that I'm using a jewlers polish grade of compound. That's all I've been able to locate in this area. The question still is, would I do it dry, or in a mixture of grease or oil, and if so, how much? I figure the lubricant, just because it's such soft materials that I'm trying to buff to a nearly-shiny surface.
Any ideas considering the modifications I made to the conditions in the first post? Thanx.
09-05-2001, 12:59 AM
The "Clover" brand of Silicon Carbide Lapping Compound is available from any Loctite distributor. It is available in 50-1000 grit.
Diamond pastes to 50,000 grit are available from lapidary suppliers and is sold by gram weight concentration. However, diamond probably will embed itself in the brass. That is how they facet diamonds - a copper alloy lap charged with diamond paste.
Lapidary suppliers will also have different abrasives such as Jewellers' Rouge, Tripoli, etc. They sell bulk SiC abrasive for tumblers as well which you could mix with regular grease to make your own lapping compound. (slightly more grease to grit)
www.kbc.com (http://www.kbc.com) also carries diamond paste and Clover brand lapping compounds.
09-06-2001, 11:44 PM
I would think that the lapping compound would at least give you a break in feel even without a good work surface scrub. Auto parts supply may have some valve grinding compound---I'm not positive on that and you may not be able to get a fine enough grit to suit this application. One more thing to consider is Brasso polish. It will cut very fine and I've used it like jewlers rouge on a buffing wheel---this makes a mess by slinging it everywhere.
09-07-2001, 06:44 AM
I think it is a big mistake to use the lapping compound myself. Lapping compound embeds itself into the softer material. I am afraid all you are going to wind up with are worn gears.
If you can do it they need to be shaved. If you could come up with a worm of this pitch, preferably hardened steel, grind slots lenth wise, and then run worm against gear on center allowing it to free wheel, then transverse face.
My wild idea for today, it is not original by the way, seen this in a book somewhere. And they do make gear shaving machines that operate similarly.
Terrell E. Koken
09-07-2001, 01:45 PM
Involute curve exactness mainly means quiet running of the gear train. However, if you want to make the best fit, you should scrape the gears in. How you'll do this with 48-pitch gears I don't know; maybe a stereo binocular microscope would help. Marking them is another problem; this should be done with the gears mounted on their axles, in-train, using a standard high-spot compound, but not very much of it since the teeth are awfully small. For a scraper, maybe a dental pick?
Personally, I'd just put up with some irregularity, or else make or buy 16-pitch gears that could easily be marked or scraped.
Planning a Springfield mount, myself.
09-08-2001, 05:17 AM
If you are already making your own invoulute cutters for specific tooth counts you could do this:
Make your steel cutter and also a pure copper one as well. The copper involute "cutter" will not need any teeth or relief as it actually will become the "lap" for your gears. Using a small ball bearing mounted on a shaft to "charge" the Diamond (3,000 or higher grit) paste into the lap as it rotates. Use water as a coolant and gently recut the gear with the Diamond lap. You may have to recharge it from time to time. A lapidary store can explain the Diamond charging process to you in detail.
This should produce exceptionally smooth gears. I would consider using Bronze & or Steel for the gears instead of Brass and Aluminum. Bronze is tougher and Steel can be hardened and polished.
If you go with the Rouge mix it in with a lithium based grease - it will help keep the grit where you want it.
[This message has been edited by Thrud (edited 09-08-2001).]
10-12-2001, 12:55 AM
I agree with halfnut. It's almost impossible to get all the lapping compound off the gears when you're done. It embeds itself in the metal. I made that mistake with some slot car gears one time. Put lapping compound on and ran slowly for five minutes, cleaned it all off carefully, and then proceeded to wear the gears out in about two minutes of full speed running. Some slot cars use 48 pitch btw.
10-28-2001, 02:11 AM
I once had to mfg. a custom gearbox for a highly precise naval instrument. A rush job, had no time to order or make precision gears. Used stock commercial gears which required boring to fit special shafting. In order to avoid excessive lapping time I placed each gear in a "pot" large enough to allow pins which touched the tooth face, at or very close to the pitch circle of the gear tooth and which engaged the ID of the pot securely enough to permit boring the gear to required size. The resulting gear train ran so smoothly that lapping was almost unnecessary. Used either 3 or 4 pins for the gears as needed for a symmetrical placement in the pot. Might have used small pieces of drill shanks, I have forgotten.
11-10-2001, 08:29 PM
Just remembered. Use Timesaver Soft Metal Lapping Compound for the brass gears. This lapping compound is made for lapping babbitt, brass, bronze and aluminum. Timesaver Compound will not imbed into any metal surface and will not continue to cut. Timesaver Products Company-629 W. Washington Blvd. Chicago Illinois 60606.