Lapping Anodized Aluminum

I need to lap the ID of an aluminum piece out about half a thousandth.
It is heavily annodized, and has a matte, frosted finish. I think the
reason it is undersized is that the annodization built up more than
expected.
I have an expanding brass cylinder lap that fits fine, but I'm wondering
what sort of abrasive I should try. Given that annodizing itself is very
hard, I don't know if normal compounds will touch it. I'm also not sure
how fine I should start with. This is a one-off, so I don't want to have
to go out & buy several grades of diamond paste. I have a "sample" kit
of Clover lapping compounds, which includes silicon carbide, and possibly
aluminum oxide.
Any suggestions?
Thanks!
Doug White
Reply to
Doug White
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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
Reply to
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
Reply to
Gwhite
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
Reply to
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?
Reply to
Pete C.
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
Reply to
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.
Reply to
Pete Snell
True. I'm working on the assumption it isn't since hard anodize is more complicated and less common.
Reply to
Pete C.
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Reply to
dcaster
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
Reply to
Gwhite
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
Reply to
Harold and Susan Vordos
A paste of brown tripoli, and oil.
Steve R.
Reply to
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
Reply to
dcaster

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