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Measuring the ano layer with a Micrometer

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  • Measuring the ano layer with a Micrometer

    After hearing about the 720 rule, I decided to see how well my setup would do at trying to grow 0.001" of anodized layer. The most accurate measuring equipment I have available at the moment is a 0-1" micrometer, which can measure to the nearest 0.0001".

    I started with an aluminum rod that was 0.2538" diameter, and after 60 minutes of anodizing in a 1:1 electrolyte bath at 12A/ft^2 at 61F, I dyed the part, sealed it, and measured it.

    OD after anodizing is 0.2546", a growth of 0.0008" for the diameter, or 0.0004" per side.

    However, assuming a TypeII anodized layer is 2/3 above the original surface, 1/3 below, this would yield an anodized layer thickness of about 0.0006".

    If the layer is 50% above, 50% below the original surface, this would yield a layer thickness of about 0.0008".

    So... my estimate of this layer's thickness is 0.0006"-0.0008". I'll be sending this along with some other test pieces off in the next few weeks for thickness testing with an eddy current thickness gage. We'll see how it correlates.

  • #2
    I haven't been able to get repeatable results using a micrometer to measure coating thickness. The problem seems to be the normal variation in metal thickness over its surface. We're talking about ten thousanths of an inch here. When you measure different points on the sample you get different readings. +/- 0.001" isn't unusual. Unless the test piece has been precision ground flat and parallel this problem arises.

    The micrometer is a Mitutoyu 293-761-30 5 1/2 digit electronic, rated for 0.00005 inch accuracy/resolution. I have confirmed this accuracy using precision gauge blocks. These micrometers are cheaper to buy than you would think, this one is listed in the MSC catalog for $136.00. The gauge blocks cost much more than that.

    The ambient temperature and how dry the sample is also effects mechanical thickness measurements. The sample should be BONE dry. BTW, dryness also effects the eddy current meter readings.

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    • #3
      Wow, that's a pretty good deal for that mic! I'll have to pick one up the next time I order from MSC.

      I can't wait to see some numbers from the eddy current gage.

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