Allowance for anodizing

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
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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.
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    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.
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wrote:

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
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"Karl Townsend" wrote in message

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

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.
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On 23/02/13 06:35, RogerN wrote:

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.
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On Feb 23, 4:34pm, David Billington

r machining and will anodize at home

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

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
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