In all but the rarest applications, coating thickness will be under .001".

I've never masked threads on an anodized part. The coating isn't thick enough to effect fit.

For an interference fit bore, specify an extra .0005"-.001" of clearance if you really want, but depending on the diameters you are talking about, another .001" may not even matter.

Anodizing is NOT plating.

It comprises conversion of metal to its oxide.

Mike Halloran
Pembroke Pines, FL, USA

Anodizing is not plating, but it does alter part dimensions.

Thanks jgKRI. My experiences has also been for threads it doesn't make much difference since there's clearance in the threads anyway. For bores though the the 0.0005" can make a difference. I'm usually operating in the 1/2-2in diameter bore range. I had a supervisor who just oversized his bores a little bit, but it was hit or miss.

And, yes, I realize it's an oxide buildup rather than a secondary material adhesion, so it's not plating, but most shops I've come across either call it or list it under "plating services."

I think maybe what I was trying to get at is, is it worth adjusting the hole size so I can get a harder surface to seat the bearing against? Or is there ever an issue with galvanic corrosion with steel bearings on aluminum bores.

With regard to your specific questions about the tradeoffs of the bearing bore being anodize or not- I don't think we can answer that for you; we don't know enough about your assembly.

I would agree that .0005"-.001" could be significant in the 2" range.

If it were me, I would specify an extra .0005" to .001" in the ID.

This is under the assumption that A) your anodize process is well controlled (for consistent thickness) and B) .0005" either way won't wreck your fit.

You might like to have a look at a standard like MIL-A-8625; it tries to address most of the issues you mention.
Fatigue properties are generally reduced by the brittle anodizing.
A recommended allowance for the expected dimensional increase = 1/2 thickness of the anodizing...(but, as jgKRI said, depends how well the process is controlled).
Rather than the normal sulphuric type, you could consider the thinner chromic type (I), or, the thick "hard anodizing" type III followed by grinding/lapping/stoning to size.
If the interference fit is done using force, then the recommendation is to rather go for the 2nd option above (because if too thin, the brittle layer will get damaged/crack).
The std gives typical thickness ranges, but note that type I can be as thin as .00002 or .0001", and type III as thick as .0045".

Regards
G

Also specify dyed, not clear, anodize, so you can verify that it was done.

Mike Halloran
Pembroke Pines, FL, USA

if you want to anodize the bores you can use Mil-A-8625 type 1, 1B,1C and IIB thickness range from .00002 to .0007 total build up will be between .0004-.0014, you can always ask your vendor to control dimensions while processing.

Generally speaking in aerospace, the effects of anodizing bare* hole edges/lips and hole-bores can be very disruptive and damaging. Major fatigue life reductions have been noted.

MIL-A-8625 spec, and all design manuals, generally recommends masking fastener holes and most small pass-thru holes. At a certain large diameter [++], open holes with smoothly radiused edges/lips can be anodized successfully.

'Hard' fasteners [Steel, CRES, Ti, I718, etc] installed in close-tolerance or [God-help-you] interference-fit... bearing against an anodized surface and sharp hole-lips, tend to have relative fast and irregular crack initiation. BAM... fatigue life goes down the toilet. I HAVE SEEN THIS ACTUALLY HAPPEN in structural parts.

* Bare = 'as machined' surfaces. Carefully radiused hole edges and shot-peening provide significant benefit for crack initiation retardation. However, shot-peening and anodizing have unpredictable effects on hole tolerances and quality... which is especially necessary for lose-tolerance/interference fastener installs.

I always mandate the following for a durable fastener installation.

Finish-machine the Part, EXCEPT for all bushed, fastener and small pass-thru holes, which can be pilot-drilled [at least 0.016 undersized]. Exception: only pilot drill holes that do NOT require mate drilling/fastening on installation... such as structural spares. Precision located holes [lugs, pivots, pass-thrus, etc] can be pilot drilled... omit all others that could have significant variation from airframe-to-airframe!!!

NDI the part.

Shot peen if mandated [preferred for durability critical parts]. Masking piloted holes is NOT required.

Anodize MIL-A-8625 Type I, IC or IIB. Masking piloted holes is NOT required.

Apply epoxy primer MIL-PRF-23377 Typ I class C1, C2 or N to all surfaces. Masking piloted holes is NOT required.

Finish ream/bore the piloted holes to drawing tolerance.

Radius hole edges/lips, R0.01-to-0.02 all holes up-to ~0.375; and ~R0.01-to-0.03 for larger holes.

Brush-apply chromate conversion coating [CCC] per MIL-DTL-5541 Class 1A to all bare aluminum holes and a lips.

NOTE.
And sometimes... I add the following... Swab-wipe holes/edges liberally with fresh epoxy primer; then wait ~5-to-10-minutes. Then gently wipe-off the primer from holes/edges using a fresh swab [preferred] or cheesecloth-pull-thru [etc], leaving a barely perceptible primer finish [translucent tint] in-place.

Match/mate step-drill/ream fastener holes when in-position on the structure. Disassemble and deburr all hole-edges/lips.

Brush-apply ‘no-rinse’ CCC to bare aluminum edges and edges/lips.

Re-assemble parts and Install male fasteners, bushes, inserts etc, ‘wet’ with sealant MIL-PRF-81733 or AMS3265 or AMS-S-8802 [etc]. CAUTION there are special techniques for placement of the sealant, just prior to fastener install.

OH, YEAH... never forget to specify that fasteners be installed using a typical cross-pattern sequence.

But, hey... Its still Your call...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

The required fit on a non-rotating bearing OD can be line-to-line or tighter if the loading changes direction. Also if the bearing is in a rotating hub. If I end up with clearance the bearing outer race will creep ( not spin ) and the race and bore will proceed to wear each other out.

I would not want to lose control of the fit.

I agree with that line of thinking Moose, but without more details of exactly what this part is and what it does and what the loads are, we can't know if that level of caution is warranted or if the additional costs of two machining setups (machine -> anodize -> machine again) are warranted.

These are all great responses and I appreciate the detail.

@WKTaylor I've never heard of the epoxy primer surface coating, is this acting as a chemical surface passivation? Also, I often will lead a hole (the lip) with a chamfer to help with bearing insertion, but are you saying you recommend a fillet instead?

Does anodizing decrease fatigue life only in sharp corners, and does it depend on the MIL-A-8625 Type? A colleague told me they switched from clear to black anodize surface on a part and started getting failures. My guess was that it was actually switching from Type II to Type III?

Certainly, I generally follow manufacturer recommended fits based on rotating/stationary components to prevent creep. I have not, yet, gone to the level of specifying the anodize thickness, nor shot peening. The ano is easy enough, and I'll look into shot-peening.

cybanical...

Corrosion protective coatings that are 'most effective' that are very thin, adherent, flexible and 'continuous' [unbroken]. In General: a controlled radius is best; a chamfer with the two-sharp edges 'broken-smooth' is good; a 'break-sharp edges treatment is barely good-enough; and a sharp ragged/irregular edge is horrible . 'You can't paint a sharp edge was drilled-into me early-on [KISS]: I have evolved that to the 'You can't effectively coat a sharp edge' version.

MIL-A-8625 Type(s) I Chromic acid anodize [CAA], IC [proprietary anodize methods/materials meeting category criteria] and Type IIB [thin-film sulfuric acid anodizing [TFSAA] produce the 'least effect on crack initiation' for open surface/edges and radiused [sharp] edges the least potential for premature fatigue-crack initiation. for each type of anodic finish, there are recommended minimum edge-radius requirements... which I usually specify a 'bit-more generous'... expecting mechanics/techs to do, but in a somewhat irregular fashion.

Shot peening the bare machined surface smooth's-out the machining ridge/waves and leaves it in a compressed state... which has benefits for all 'brittle' anodic coatings: grossly retarding crack initiation thru the compressed zone. One little known benefit of shot peening [done correctly] is that the process MANDATES that 'sharp edges' be radiused or chamfered BEFORE peening to prevent the sharp edge from 'rolling-over' when exposed to the kinetic-shot at an angle to the sharp-edge. This results in a consistent, relatively uniform peeing coverage transitioning from one surface [A] to another [B] at sharp off-angle, etc. Where this benefits anodizing is that the anodic coating [on the shot-peen-toughened surface] 'flows smoothly' in a continuous/unbroken manner on the toughened surface.

Likewise the corrosion protective primer-finishes can smoothly/adherently 'flow' around from one smooth surface to the next with a relatively uniform coating surface-radius-to-surface.

jgKRI... I too am NOT wild about multiple machining operations... that is why I like piloted holes precisely located within the first machining stage. These precisely located holes are relatively easy for a competent shop to 'pick-up' by hand or drill-press reaming using a piloted reamer [RE NAS897 or NAS898]. Obviously, if a part has outstanding tooling and locating points, then these [piloted] holes could be omitted and done completely/efficiently during this final operation by guide jig/fixture or NC operations [restored to tooling-points], no-sweat.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

I agree WRT to your preferred order of operations. Since you come from the world of making things flight safe, my suspicion is that safety routinely outweighs cost in that equation.

@WKTaylor, is there another coating process for aluminum, particularly 7075 series that can be both cosmetic and have a lower effect on fatigue life? Or just suck it up and consider the expected load capacity/fatigue reduction?

here's a reference I found regarding fatigue life for anodized alloys.
http://dspace.dsto.defence.gov.au/dspace/bitstream...

cybanical... maybe, maybe not.

7075-what form*, what temper? *form = Die forging, hand-forging, sheet, plate, bar/rod/wire, extruded profile?

Primed and painted or left 'as coated'?

What is the intended purpose and operation environment?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]