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?

RogerN

On Feb 22, 10:35*pm, "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?

RogerN

http://www.anoplate.com/finishes/anodizing.html

"RogerN" wrote in message
...

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?

RogerN



The anodize thickness may be 1 mil, but most of the coating grows into the
existing surface. The dimensional change is closer to 0.3 mil. Hard anodize
can be much thicker.

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?

RogerN


Roger, thanks for the heads up. I'm planning the same project and
didn't think of this. After watching eBay forever, I scored the tap.
Now you got me worried it will be undersize after anodize.

If you're interested, find me a piece of T7075. I'll drill and tap.
You can also drill and ream a couple holes in it with whatever reamer
you have. Check before and after anodizing the scrap part. Only way to
know for sure.

Karl

"Karl Townsend" wrote in message
...

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?

RogerN


Roger, thanks for the heads up. I'm planning the same project and
didn't think of this. After watching eBay forever, I scored the tap.
Now you got me worried it will be undersize after anodize.

If you're interested, find me a piece of T7075. I'll drill and tap.
You can also drill and ream a couple holes in it with whatever reamer
you have. Check before and after anodizing the scrap part. Only way to
know for sure.

Karl

I did a little cad work to make it so I can bolt an AR-15 receiver to a
1-3/4 bar stock with a flat milled. I plan to have 2 dowel pin holes and 2
tapped holes in the areas to be milled out later. On the 1-3/4" stock, mill
off .246" leaving the thickness 1.504 on the D shape, this puts the center
of the buffer tube at 0.629" (0.875R - 0.246) above the deck per drawing. I
just have to make sure the piece of stock is running true in the lathe. I'm
thinking of running this on the CNC lathe, it threads fast and repeats.
Anyway, I can cut a little oversize to allow anodizing, since the buffer
tube threads shouldn't be subject to much wear, I wonder if it would be
easier to mask the area.

I'd guess running the taps and reamers back through the holes after
anodizing wouldn't be good for the cutting tools.

RogerN

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

First: I have never anodized anything.

Your reference to a battery charger says "electrolysis" to me. In which
case, the interior of a hole might be very poorly anodized due to the
lack of a direct path to the electrode.

I'm not saying that it won't happen, but you should confirm it.

Bob

On 23/02/13 06:35, 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?

RogerN


I've had decorative anodising down professionally once and they
mentioned that they did a reverse current etch first which removed a
small amount of the surface, 0.002" ish from memory but it was a few
years ago, then the normal anodising to end up with nominally the same
size. The items in question were shelf support brackets so size wasn't
an issue but it might be worthwhile looking at the process.

On 2013-02-23, anorton wrote:

"RogerN" wrote in message
...

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?

RogerN



The anodize thickness may be 1 mil, but most of the coating grows into the
existing surface. The dimensional change is closer to 0.3 mil. Hard anodize
can be much thicker.

However -- it is tricky to anodize *inside* a hole -- the
smaller the hole, the more difficult, so your reamer probably does not
need to be changed. If you want to be sure --plug the ends of the holes
so you can't anodize in there.

The problem with anodizing in a hole is that the current tends
to be mostly a straight line sort of thing -- it bends corners a bit,
but likely won't anodize very deep into the hole -- just at the outer
end.

Do you really *need* the anodizing in the hole? If so, you need
to put an electrode down in the hole, and do something to keep fresh
chemicals flowing through during the process. And certainly you don't
want to run the reamer in after the anodizing -- anodized surfaces will
wear the reamer rather quickly.

Good Luck,
DoN.

--
Remove oil spill source from e-mail
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

On Feb 23, 4:34*pm, David Billington
wrote:
On 23/02/13 06:35, 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?

RogerN

I've had decorative anodising down professionally once and they
mentioned that they did a reverse current etch first which removed a
small amount of the surface, 0.002" ish *from memory but it was a few
years ago, then the normal anodising to end up with nominally the same
size. The items in question were shelf support brackets so size wasn't
an issue but it might be worthwhile looking at the process.

Where I was working, they did anodizing for motorhome trim. So not a
real tight tolerance thing and not hard anodizing. You can't do that
with just sulphuric acid anyway and the aluminum alloy types that will
respond to that process are limited. First bath was an alkaline soap
to remove oil from bending. Second bath was an etch with hot
saturated caustic mixture which removed the remaining oxide. So that
right there would change precision dimensions anyway. Any
electrolytic process tends to overdo sharp corners and under-process
the inside of bends and holes. Just the way electrons like to group.

EPA is all over hard anodizers, it involves chromic acid which I guess
will kill you at ten paces. Anyway, they're trying to eliminate same
along with chrome platers.

If you've got holes or threads you want kept at a precise dimension,
mask them off would be my advice. Plug the holes with silicone plugs,
lacquer or something similar.

Stan

On Fri, 22 Feb 2013 23:52:19 -0800 (PST), jon_banquer
wrote:

On Feb 22, 10:35*pm, "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?

RogerN

http://www.anoplate.com/finishes/anodizing.html


Per the above web site you are looking at less than 0.001" change,
most likely only a few tenths.

If you start at 0.251 you should be fine.

Remove 333 to reply.
Randy