Rack marks and MIL-A-8625 anodizing

A. Rack or contact marks are required for anodizing. MIL-A-8625 addresses this issue in Para. 3.13.1 Contact Marks and Para. 6.14 Size of Contact marks. You will need to work with your anodizer to find an acceptable racking method.

A. MIL-A-8625 (as does the AMSA8625) allows for "rack marks" and permits touch up with "chem film"...see MIL-C-5541 (or AMSC5541 ...by brushing the solution (aka "Alodine", "Iridite" or other proprietary chemical solution...see the QPL. You can download the "MIL" specs and QPLs from DODISS / "ASSIST"). But it sounds like your subcontractor is trying to pass off scratches for rack marks. The rack marks should be reasonably consistent part to part. He may truly be scratching your hardware. Check your hardware geometry against his racking procedure and handling practices. Are you doing chromic acid, sulfuric acid or hard-coat anodizing? That is, which "Type" do you spec?

A. Depending on the finish in question. Sulfuric Type II, Sulfuric Type III (Hardcoat), or Chromic Type I, your racks marks can and will generally speaking vary.

At least with hardcoat anodizing (Type III) the surface area needed for sufficient current requirements can be pretty substantial depending on the size of your part. Most rack marks are going to be approximately the size of 1-2 pencil lead points in width, if the racks are titanium.

I've used 1/4-3/16" aluminum also on very large parts, and the rack marks associated with them can be just plain horrendous in size but, because of the size of the part, they are still within MIL spec. I believe somewhere in the specification there was a way to determine a pass/fail on rack marks such as X number of marks in a Y * Z square would pass or fail. But its been a few months since I read the spec, so I could be wrong. My suggestion is first talk to your supplier, see if there is an alternative method for racking. If he suggests the purchase of racks to solve the problem, weigh the cost and see. Most "stock" racks can satisfy 90% of racking requirements, and they are not all too overly expensive and the costs associated with your purchasing them can be absorbed quickly, and the benefits noticed immediately upon the change because rack marks can AND SHOULD be hidden as much as possible if the situation presents itself as being able to be done that way. But not everything is perfect, if it cannot be done to eliminate the problem, see about ways to at least minimize it.

A. A picture of the part would help a lot. Have you contacted your customer to inquire where racking marks would be acceptable? Are there threaded holes on the part that you could use to make contact, either with Ti or Al bolts?

Anodizing racks are available in many, many, different configurations, both in Ti and Al. The larger rack making companies will also custom design something for you, if necessary, after discussing with your customer on where marks would be acceptable. It's a resource you should consider using.

----
Ed. note: Readers may also be interested in letter 6109, "Hard Anodizing racking problems"

A. Can you use the two diagonal holes as a jigging point for Aluminium plate. Finally hole touch up is covered by MIL-A-8625.

"6.10.1.1 Holes. In the case of small holes and tapped holes, coating thickness can vary from no film to a full normal coating. Holes, both tapped and not tapped, over 1/4 inch should be anodized. Parts with Type II coatings, external or internal, with a total tolerance of 0.0004 inch or less, if lapped, honed or stoned to size after anodizing, should be subsequently treated with QPL-81706 materials to provide surface protection.
Discoloration on the surface that has been sized is acceptable (see 6.6). The designer is cautioned to require adequate thread and hole sealing operations in subsequent assemblies as may be required to produce the necessary corrosion resistance."

  Hi Loka. Thanks for helping but, the component which Nigel is actually having trouble with doesn't have any holes :-(

Regards,