Gear Lapping Compound and Grease

We just finished making a gearbox and are wondering what kind of lapping compound and grease we should apply to the gears before using the gearbox.

If anyone can help that would be great.


and a note to Andy, we got the CEC couplings to work! It was super easy too. We ran to the broaching place and had them do the couplings for us yesterday, 1/2" keyed hole now has an Al plug and a 3/8" hole and keyway! Thanks for the help!

You can order lapping compound from most industrial suppliers (e.g. see the MSC catalog under lapping compounds, McMaster-Carr, etc.). If they can’t deliver quickly enough for you, you can also use valve grinding compound in a pinch (available at most large auto supply stores: Trak Auto, etc.).


What grade is reccomended for regular steel spur gears?

Also what type of grease should i use?



Why are you “lapping in” the gears? All you should need ids some white lithium grease. If you set-up the center to center distance properly, and the gears are meshing squarly, you shouldn’t need to do any “lapping”. Using only grease, the system should be smooth and roll with little or no friction.

Don’t lap in the gears! If it doesn’t run smoothly when you put it together, then something is wrong. Lapping it in will undercut the gears. Works fine if you’re in a pinch (ie, it’s week 5, and it’s that or no robot) but I wouldn’t do this to say the tranny of my car.

-=- Terence

thats not always the truth…

A light lapping compound wont hurt the gearbox. It will just remove any junk from the gears as well as helping them seat better. You gotta remember nothing is perfect.

It is true that the gearbox will technically “lap” itself as it wears in, but this is just remove any possibility of the gears binding in the new gearbox…

I was asking, as per Andy Baker reccomendations…

Maybe he will chime in???

BTW. We DID end up using a light diamond lapping compound before we put the gearbox under load. Then cleaned the lapping compund off and applied grease.

We noticed it helped ALOT. Our gearbox was CNC’d, but it wasnt perfect. Even with the lapping compund it wasnt perfectly smooth. We let the competition do that for us. Our drivetrain was the most efficient at the end of the third day of use. It had worn in just enough to make everything smooth. Since then it hasnt worn in much more. We make it a habit of keeping the gearbox covered, cleaned, and lubed. Its been working great.

This year I used absolutely no lapping compound in our gear boxes. Last year we had to, because we didn’t use great care to make sure the gears were meshing squarly. This year, I made a fixture that holds the gears true so I could bore a hole to bush them. I made sure the mounting locations were within .0002. Putting the gear set together yeilded the desired results -extremely smooth power transmission. I have about 50 hours into the entire fabrication of the gear boxes this year. Believe it or not, most of the time was spent “tweaking” a little here and a smiggin there. To make every thing work as smoothly as possible, youu need to spend ttime finding where all the little binding spots are. Ultimatly we are trying to utilize as much output we can from the motors. Any friction is wasted power.

The best advice i can give, makee sure that your gears have bores that are absolutly true to the pitch dia. and your centers are held as close as possible. We used Pic Stainless Steel gears, which have a very smooth tooth profile. White lithium grease is all you need as long as your machining is held to a high tolerance.

When I was a kid, I used to race Tamiya 4 wheel drive cars (1:32 scale). Some of my friends graduated to 1:12, and 1:10 stuff, but I never had the money to do that. Besides, I’d rather build a robot :-). There were lots of mods people do, such as drilling out gears, hollowing out chassis, etc, to get that extra speed advantage - and that applies to all scale racing.

I knew a guy that put lapping compound inside his tranny on his RC car, to “lap them in”. Within a few hours, he was back at the hobby store - his gears had turned into mush. I don’t know what sort of grit he used, but needless to say, the only thing that I ever put on my gears after that was “speed oil”. (A mixture of sewing machine oil and light lithium grease - one of those tricks that your friends show you and you just stick to it … )

-=- Terence

use some ghee hey it worked for us

Okay this thread has gotten past the point I originally intended…

Lapping compound on plastic RC car gears, or even gears with a small face width isn’t smart if you use a heavy lapping compound.

Remember there are hundreds of different grades of lapping compound varying in abrasiveness.

I just wanted to find a source on the compound itself. I found it and used it, and for my purposes it worked great.

I have talked to many engineers, and they suggested in many cases, when dealing with machined gears used in gearboxes, lapping compound can be used to help gears mesh better than they would otherwise in their freshly machined state.

I’m sure I will get hundreds of different opinions, but for this particular purpose when I used it I saw no negative effects while using the compound. If I do in the future I will let you guys know.

I just looked back on the technokats 6 motor drive train files from last year and noted Andy Baker’s recommendations to use lapping compound. I may have this out of context, but I am pretty confident he used it for the exact same thing we did.

… Use lapping compound to help get the gears to mesh. Clean off the lapping compound and then grease the gears before operating under a load. …

Anybody else care to share their feelings towards lapping compound?


We didn’t use lapping compound this year… mainly because the mesh of our new gearboxes was pretty good. We would run them for a while NON-GREASED, to work themselves in… then once they are worked, we would add grease.

As for when to lap gears, all I can suggest is to do it when you have a bad, tight mesh. I would start with the finest lapping compound you can get and work up from there.

I am definitely not an expert with regard to lapping gears… so if any of you have more experice, please correct me.

Andy B.

Just a bit of advice. Run them in UNLOADED without lubricant. I would recommend greasing them before running them under load…particularly if any two meshing gears are of the same material together…ESPECIALLY if they are stainless or aluminum. We galled a stainless gear by running it under load without lubricant. I’ve seen aluminum gears do the same thing.

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