Lapping Anodized Aluminum

Anodizing is, itself, aluminum oxide. I think it's unlikely you'll be able to do anything like *cutting* into that surface using other than diamond. You probably could *scrape* it off with other abrasives, but you'd just be tearing the anodizing from the base aluminum.

Just an educated guess...

-- Ed Huntress

That was my concern. Somewhere I have some diamond compound I used to lap a small carbide expander button, but my recollection is that it was extremely fine. My concern was that it might take quite a while to lap the aluminum piece, which is about an inch ID and 2 inches long. I may just have to try that.

Some of the Clover compound is aluminum oxide, and I don't know how well lapping with an identical hardness compound would work. I suppose I could try that, and then switch to the diamond if it doesn't work.

Doug White

Lapping with equal-hardness compound does work (that's how they do diamond-to-diamond lapping), but it's reported to be slow and sensitive to pressure. I have no experience with it myself.

Diamond, in general, cuts fast, grit-for-grit.

Please let us know how it works out.

-- Ed Huntress

Might be easier to strip and reanodize the part properly. Even then, the anodizing layer is normally supposed to be pretty consistently .001" thick, with half of the thickness growing into the part and half growing out from the part. For your bore, if the anodizing is done properly you would expect the bore to shrink be .001" total. Was the original part properly sized to account for the anodizing layer?

Unless this is a "hard anodize," which runs up to 0.003" or so. When he said "heavily anodized" I wondered about that but didn't ask.

-- Ed Huntress

I've had to do similar work to the OD of anodized pieces. Hard anodizing is hard but not *that* hard. Give it a quick rub with carborundum or silicon carbide, and I'll think you'll find it works out alright. Go really slow and use a lube. You probably find that the initial work will make a big change, as you take off the high spots, but it'll go a little slower after that.

True. I'm working on the assumption it isn't since hard anodize is more complicated and less common.

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OK, I'll start with Silicon Carbide and see what happens. Because the finish is sort of frosted (it may have been bead blasted before annodizing, but it looks about as rough inside as out), I expect I will mostly be knocking off the tops of the frosting.

Any idea what grit I should be trying? I can't find the Clover kit for sale anymore, and I don't remember what grits it has available in Si Carbide. I'll dig it out when I get home today and see.

Thanks!

Doug White

Silicon carbide is far harder than aluminum oxide and will easily remove the amount you desire.

If it was up to me, I'd use a Sunnen hone with a fine silicon carbide stone-----or diamond if I had it available.

Harold

A paste of brown tripoli, and oil.

Steve R.

You should be able to find valve grinding compound at an auto parts store. They usually just have coarse and fine grades for grinding valves. I would try the fine first.

Dan