Author Topic: Machining tolerance when anodizing aluminum  (Read 574 times)

Offline Mike K

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Machining tolerance when anodizing aluminum
« on: November 02, 2021, 03:22:46 PM »
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?

Mike

 


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

Offline BillTodd

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Re: Machining tolerance when anodizing aluminum
« Reply #1 on: November 02, 2021, 03:46:07 PM »
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
Bill

Offline Mike K

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Re: Machining tolerance when anodizing aluminum
« Reply #2 on: November 02, 2021, 04:31:03 PM »
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.

Offline BillTodd

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Re: Machining tolerance when anodizing aluminum
« Reply #3 on: November 02, 2021, 05:24:23 PM »
>> 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.

Bill

Offline Mike K

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Re: Machining tolerance when anodizing aluminum
« Reply #4 on: November 02, 2021, 05:28:20 PM »
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.

Offline WeldingRod

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Re: Machining tolerance when anodizing aluminum
« Reply #5 on: November 02, 2021, 07:09:36 PM »
Hard anodize shouldn't be especially expensive, and its -definitely- what you want for abrasion resistance!
Yes, the part ends up growing overall.  I have data at work if needed.  Converting Al to AL oxide in a columnar form.

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Offline philf

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Re: Machining tolerance when anodizing aluminum
« Reply #6 on: November 03, 2021, 05:02:26 AM »
You can always plug the tapped holes with a nylon screw.
Phil Fern
Location: Marple, Cheshire

Offline Mike K

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Re: Machining tolerance when anodizing aluminum
« Reply #7 on: November 03, 2021, 08:58:54 AM »
You can always plug the tapped holes with a nylon screw.

Nope, not nylon.  Not compatible with 15% sulfuric acid.

Offline philf

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Re: Machining tolerance when anodizing aluminum
« Reply #8 on: November 03, 2021, 11:33:18 AM »
You can always plug the tapped holes with a nylon screw.

Nope, not nylon.  Not compatible with 15% sulfuric acid.

Not even for the time it would take for the anodising? Wrap them in PTFE tape. I used to have a collection of polypropylene screws but I suspect I left them at work when I retired.

Many years ago I had some parts hard anodised and had to buy tungsten carbide taps to open out the threads again.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline WeldingRod

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Re: Machining tolerance when anodizing aluminum
« Reply #9 on: November 03, 2021, 11:33:58 AM »
Looks like 10 to 25 microns growth for colored anodizing.  20 to 70 microns for hard anodize. 
You are converting an equal thickness of the part into twice as thick oxide layer.  So, if you strip it off, the part shrinks twice the numbers above.

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Offline Mike K

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Re: Machining tolerance when anodizing aluminum
« Reply #10 on: November 03, 2021, 02:44:00 PM »
Looks like 10 to 25 microns growth for colored anodizing.  20 to 70 microns for hard anodize. 
You are converting an equal thickness of the part into twice as thick oxide layer.  So, if you strip it off, the part shrinks twice the numbers above.

Thanks!

Offline Mike K

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Re: Machining tolerance when anodizing aluminum
« Reply #11 on: November 03, 2021, 02:54:49 PM »
You can always plug the tapped holes with a nylon screw.

Nope, not nylon.  Not compatible with 15% sulfuric acid.

Not even for the time it would take for the anodising? Wrap them in PTFE tape. I used to have a collection of polypropylene screws but I suspect I left them at work when I retired.

Well, anodizing is one to two hours, depending on the anodizing depth desired.  I have no direct experience, mind you, but the online "Nylon Chemical Compatibility Chart" I found says that Sulfuric Acid (10-75%) is "D-Severe Effect".  Quesion, then, is: How much damage can be done in two hours?  I'd rather not find out; the dissolved nylon could wind up interfering with the anodizing process.  Polypropylene is listed as "A-Excellent".

https://www.calpaclab.com/nylon-chemical-compatibility-chart/
https://www.calpaclab.com/polypropylene-chemical-compatibility-chart/

Offline tom osselton

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Re: Machining tolerance when anodizing aluminum
« Reply #12 on: November 03, 2021, 08:02:03 PM »
Could you not drill a hole and fill it with silicone to make a plug it sounds easy or am I missing something?  :smart: