Hardcoat Anodize Facts and Guidance

In an effort to promote better understanding of the Process of Aluminum Hard coating, we offer to our customers the following facts and suggestions:

  1. Hardcoat IS different; Hardcoat is NOT plating.
  2. Hardcoat PENETRATES the base metal as much as it builds up on the surface and the term THICKNESS includes both the buildup and the penetration.
  3. Hardcoating a shaft .002 thick will increase the diameter by only .002.  Plating the same shaft .002 thick would increase the diameter .004, since plating is 100% surface build up.
  4. Be positive before you machine your parts that you are allowing for the hardcoat buildup and not a plating buildup.
  5. When you call for hardcoat, the use of the term “Build up per surface” will make it impossible to misunderstand what you are requesting.
  6. Exacting dimensions can be maintained with the Hardcoat Process. Standard commercial tolerance is +.0005 on a coating thickness of .002.  For closer tolerance requirements, consult the finisher in advance.
  7. Allowing a tolerance on coating buildup means that you must machine closer than blueprint dimensions.  For example: A shaft diameter which is to finish at 1.500 + .001 and is to be hardcoated .002 thick (.001 +  .0001 buildup per surface), your planning should call out Machine to 1.498 +.0008”, your part will then be to finish dimensions after hardcoating.
  8. When a “V” thread is to be cut to allow for hardcoating, the formula is “Build up per surface”, multiplied by “Four”; this will equal the pitch diameter change.  A typical example is: Desired P.D. = .405/.4091 (7/16 N.F. Internal Thread) Coating Thickness .002 + .0002 (.001 + .0001 buildup per surface).  Minimum buildup per surface is .0009 x 4 = .004 P.D. change.  Machine P.D. to .4094/.4127.


To save time, trouble, and possible errors, information on the following four items must be known:

  1. Alloy
  2. Coat Thickness
  3. Masking Requirements (if any)
  4. Racking Instructions (if possible)
  1. ALLOY – Hardcoat can be applied to virtually any aluminum alloy.  However, since the coating builds up at different rates on each alloy in order to control coatings accurately, it is important to specify the alloy.  Also, some alloys require different procedures from others.  If the alloy is not properly designated, there is a possibility of damage.
  1. COATING THICKNESS – Hardcoat may be provided in thicknesses ranging from a few .0001’s to .008” or .009”, depending on the alloy and the application.  Like other coatings, Hardcoat changes the dimensions of the basic part.  One half of the Hardcoat build-up and one half is penetration, i.e., .002” hardcoat consists of .001” penetration and .001” added to the original dimension.  Therefore, in machining the part, it is essential to allow for the change and to request a specific coating thickness on blueprints and/or purchase orders.
  1. MASKING – It may be necessary to exclude (or mask) the coating from certain areas of a part.  If so, areas to be masked (threaded hole, bored holes, ground points, etc.) should be clearly specified. In designing for hardcoat, remember that masking is a hand operation which often, but not always means added total cost. For instance, even if Hardcoat is only required on one area of a part, it is usually much less expensive to permit the part to be coated all over if at all possible. It is also important to remember that masking on small parts may not be an effective solution since electrical anodic contact points of the rack leave small voids in the coating.
  1. RACKING – Firm electrical and mechanical “contact” must be made with every part to be Hardcoated. That is, each part must be “racked”.  Proper racking is a key to economical and effective processing of parts.  Since each rack contact point leaves a small void in the coating, it is essential that such contacts be made in non-critical area. Any guidance which can be provided as to where best to rack the part will aid in proper processing. Before designing a part for Hardcoat, if you have any questions as to how to handle your specific part, please give us a call. We will be glad to advise by telephone or make a personal visit.
Posted in FAQ.