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Machining tolerance when anodizing aluminum

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Mike K:
I made a bunch of aluminum QCTP holders like below.  And now that I've used them a bunch I want to anodize them to prevent further wear.  They have about two thou (0.002") oversize clearance on the 1.25" steel toolpost and I'm wondering if anodizing will change that...Increasing or decreasing the tolerance?  Anyone have experience with this?



EDIT: Sorry, I guess this could have been put into the "Finishing" section.

Anodising will increase the thickness of the oxide layer that is already on the aluminium parts the outside dimensions  will get bigger and inside ones smaller.

Screws will often become too tight so are usually blocked with plastic screws or tapped oversize with special taps .

By how much the dimensions will change depend on how the part is anodised; if you're looking for wear resistance then hard-anodising will add a thick layer of perhaps 100um (~0.004"),  but that can be expensive. Most anodisers will only do colouring which could be as little as 10um or less

Mike K:
Yeah, that's where I'm confused.  I konw the oxide layer will grow, but my understanding of it is that it grows *into* the material.  Your saying that screws can become too tight seems to be at odds with that, though.

And that could be a problem, because giving threaded holes some wear resistance is something I want.  Though my major concern is the wear of the large 1.25" hole from repeated toolholder changes.

I'm not doing hard anodizing, just the regular DIY type of anodizing with a quick etch, anodize for 1-2 hours, dye, and seal.  I'm going to try the dye, but it's not that important for me as I just want it to distinguish one toolholder from another.

>> I konw the oxide layer will grow, but my understanding of it is that it grows *into* the material.

It grows both ways the process converts aluminium to aluminium-oxide  , the oxide swells as forms crystals so takes up more room.

Your screw holes may not be a problem , easy enough to re-tap them.

If you're DIYing  then It may be possible to increase the layer in the bore by having an electrode in the hole , so concentrating the field there.

Mike K:
Electrode in the bore is a good idea!  Thanks.  I think I'll also plug the screw holes...I'd rather have them untouched than ruined or dulling my good thread tap.


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