Letter 5060

Anodic coating thickness Statistical Capability. 

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I have experienced varying statistical capability (Cpk) on anodized aluminum coating thickness. Cpk values typically DO NOT reach the customer desired 1.66 value. I wrestle with words trying to explain why anodic coating thickness on aluminum does not distribute evenly and therefore cannot consistently achieve high Cpk's typical of dimensions produced by machining.

Is there a techical response to my customer's question? Is there written material from experts in the field that I could reference?

Ross Banks
IHC, Inc - Detroit, MI. , USA

Hi Ross,

There is a paper entitled "Evaluation of Anodizing Process Capability" that was presented at AESF SUR/FIN'99 in Cincinnatti. You will get enough information on your interest. Ling

Ling Hao
METALAST International - Minden, NV

Ross,

The anodic coating should distibute very evenly, provided that the cathode placement in your tank is correct, and that you are not anodizing dissimiliar alloys in the same load.

The 1rst culprit I would examine is your cathode placement. This may require some shielding of the cathodes..or the placement of more cathodes. I'm not sure if you are experiencing this difference in 1 load, or if its a difference from load to load. We typically hold a tolerance of plus/minus .00015 from the desired thickness.

Marc Green anodizer - Boise, ID, USA

Most likely, your capability problem is more measurement error than actual anodizing variation. It is normally assumed that the measurement technique is ten times more accurate than the variable value. If that is true, then measurement error will account for less than .33 or one standard deviation of your allowable Cpk. Have you done an R&R gage study? If you are using an eddy current gage to measure anodizing thickness, the error is probably significantly higher, eliminating room for actual coating variation. Unfortunately, eddy current is the only nondestructive method I know of for directly measuring anodic coatings. Some possible approaches for reducing this measurement error are:

1)Fixturing the eddy current probe contact to reduce operator variation.

2)Take multiple eddy current readings to reduce variation by averaging.

3)Use destructive cross sectioning and microscopic examination.

4) Use precise before and after dimensional values as a substitute for direct coating thickness readings.

5) Ask finishing.com readers for new and improved thickness measurement techniques.

Chris Jurey Luke Engineering & Mfg. Co. Inc. Wadsworth, OH

It might be of interest to find out if you are measuring your thicknesses from lot to lot or from the same lot. There are some important considerations to be determined when using SPC in chemical batch processing. The tank concentrations change based primarily upon the surface area of the work being put through the tanks. This means that in effect you don't have the same "setup" as you would say with a machine. The answer is to use moving range calculations and to use SPC as if the average of the process output may be expected to shift significantly from one run to the next. An excellent paper was published by International Quality Institute and I believe it was presented at the ASQ 54th Annual Quality Congress. The author was Davis Bothe. This paper can be used in conjunction with chemical batch processing. IX and MR dual charts are suggested, with a CPK setup index generated along with other special metrics which identify which part of the process to be targeted first for! improvement.

The measurement thoughts mentioned by Chris are quite valid and one of the ways one usually deals with measurement variation is to determine significant numbers, for example the last number in most Eddy Current devices usually jumps around a bit. Also I believe that anodize does cover pretty evenly all things being equal like anode to cathode ratios etc.

Marc's thoughts are important, I have worked at a shop where the anodize thicknesses produced at the parts on the bottom of the racks were much thicker than at the top. It turned out that the anode to cathode ratio and placement was greater and closer, respectively at the bottom of the tank to the work. The cathodes were subsequently adjusted to 4-6 inches above the work, loads were standardized etc..., cathodes and bussing were sized to the rectifier and load size, and the thickness differences became +/- .0001".

Ward Barcafer CEF
- Wichita, KS

Dear Sir/Madam,

I am a metallurgical engineer, I want to know the exact methodic details of microscopic anodising thickness measurement. Please be kind to let me know the details thereof. The exact query is that when the cross-section is seen under microscope at about 200 X magnification, two layers are seen at the boundary. The internal layer being uniformly gray coloured and the outer layer is non uniformly black. Kindly let me know whether I should consider the combined thickness of the two layers as anodised thickness or only either of them. Expecting a kind response in anticipation.

Yours faithfully,

Shridhar Garge
- Baroda, Gujarat, India

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