On Sat, 23 Feb 2013 00:35:01 -0600, RogerN wrote:

I have some Aluminum parts, 7075 T6, for machining and will anodize at
home using the sulfuric acid / battery charger method, or a variation.
For the tight tolerance parts, do I need to machine, ream, oversize to
allow for the anodizing? If so, how much? I've seen recommendations of
.001"-.002" per surface, is that about right? For example, one of the
hole sizes in the drawing is .251" +/- .001, the instructions by others
that have built these parts say to use a .251 reamer, so that's what I
bought. But now reading the anodizing info I'm wondering if I need to
ream to .253 or so?

What I learned from hanging out with the mechanical engineers at work is
that yes, hard anodizing grows the part. How much depends on how thick
the anodizing layer is.

I'm not sure if what you're doing is hard anodizing or not -- I think
hard anodizing is done at near-freezing, and I couldn't tell you for the
life of me what the reactants are.

For really precision surfaces they specify that areas of the part be
masked off before going into the anodizing tank. I don't know how how
things are masked -- but now you know it's possible.

I believe that the folks who make AAO cylinder sleeves (aluminum with
hard anodizing) post-machine the anodized surface to smooth it and to lap
it to size. But I dunno for sure.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com