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Making thread gage for pre-anodizing of internal threaded part

 April 3, 2010

I am making a set of aluminum parts that will be Hardcoat anodized by the end customer.

They have a non standard stub acme thread and although I can get number values for the range of thread tolerances I am still having trouble. I can't seem to fix on a proper set of numbers to make a go/no-go gage for the internal thread. I have lots of numerical values for min and max tolerances but I can't quite come up with absolute numbers that make sense to me.

Is there a source, short of a purchased gage, that will help me figure out how to make gages that will allow enough material to compensate for coating thickness? I actually have numbers but don't know for certain if they are right as we have never had to do this before. Everyone I know seems to have a different idea of what to do...

The thread is 1.8 in dia 16 pitch stub acme.

Charles Blair
machinist - Lawrenceville, Georgia

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First of two simultaneous responses --

The first question you have to answer is how thick your hardcoating has to be. Then calculate that half of that thickness "bites" into the base aluminum and the other half "grows-up" changing your dimensions twice that half. Internal diameters are reduced, external ones increase.

Guillermo Marrufo
Monterrey, NL, Mexico
April 7, 2010

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Second of two simultaneous responses --

Threads should not be anodized, especially not hardcoat anodized. Since the coating goes approx. 50% under the original surface, then on the sharp edge of the thread the whole edge is converted to very brittle 100% aluminum oxide, which breaks off when you thread in. Again, any sharp edge becomes pure brittle aluminum oxide.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

April 7, 2010

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While it is true that anodizing builds up to half the thickness requirement it is important to remember that a thread pitch diameter has 4 surfaces. That means if your thickness is .002" you divide that in half and multiply time 4 to get the change in the pitch diameter. Or just multiply thickness by 2.

Tim Hamlett, CEF
- West Palm Beach, Florida, USA
May 13, 2010