Ed Huntress wrote:

"Pete C." wrote in message
...
Ed Huntress wrote:

"Gwhite" wrote in message
ups.com...
On Sep 13, 7:56 am, "Ed Huntress" wrote:
"Doug White" wrote in message

...





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

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...

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.

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.

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