Stripping anodizing will increase dimensions?

[00_buckshot]

I've read in several places now that stripping anodizing off of aluminum will actually reduce it's overall dimensions and size from what it was originally. For example, I have an AR15 lower that has an original hardcoat anodized layer. I've read that a hardcoat thickness of 2 mils will actually consume 1 mil of aluminum. So if and when you strip it off your piece will actually be 1 mil smaller overall. I'm not sure if the thickness on an AR lower is 2 mils but this still makes me wonder. Will stripping open up pin holes enough to make them loose after reassembly has been done? Thanks for the help.

[NeoMoses]

Yes, it is possible to change dimensions by stripping. I've never dealt with hardcoated items, only Type II anodizes. With these, I've never changed tolerances enough to be measured by a 0.001" caliper. For my stuff, this has not been enough of a tolerance issue to worry about.

However, I can imagine that many things such as guns may have tighter tolerances, so this might become an issue.

[Fibergeek]

00_buckshot, remember me?

In this situation for once, things will tend to go in your favor, assuming it really is 2 mils thick:

1. It's not likely to occur and dangerous to attempt removing all of the anodize from the pinholes.

2. If you removed 1 mil from the surface after stripping, you increase the pin hole diameters 2 mils. You add .3 to .7 mils when you reanodize. this decreases the hole diameter .6 to 1.4 mils. Since this is type II anodize, about 2/3 of it is above the surface. The reduction in diameter is .4 to .9 mils. This results in the hole diameters being 1.6 to .4 mils oversize. The milspec tolerance for the holes in a M-16 lower is + 1.5 mils - 0.

3. Since because of 1. (above) you probably won't encounter this situation anyway. Another reason why you want to leave the existing anodize in the holes. It won't show anyway.

4. In practice, it's usually necessary to ream the holes after reanodizing, back to the original specs.

[00_buckshot]

When using a chemical stripper how do you keep the pin holes from loosing their anodized layer? Do you mask them or not even worry about them? Thanks for all the help.

[Fibergeek]

I have found the safest way to strip a high dollar item is to alternate chemical stripping with beadblasting, you take off the anodize in layers. It's slow, but its safer. When beadblasting avoid holding the blasting gun right over the pin holes, just sweep across them, trying to blast the holes clean can enlarge them.

Masking the holes before chemical stripping would be a good idea, rubber cement seems popular as a masking material. Test it first to be sure that the stripper won't attack it.