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-----:Hardcoat Anodizing a Fixture Plate with 4000 Tapped Holes
Quickstart:
In the anodizing of aluminum the components are immersed into a tank of acidic solution and connected to the anodic (+) positive pole of a power supply. The current separates some of the water in the solution into positively charged hydrogen and negatively charged oxygen. The oxygen is attracted to the positively charged components and converts the skin of the aluminum to aluminum oxides.
Readers new to anodizing of aluminum may wish to view our "Intro to Aluminum Anodizing".
It is usually considered a bad idea to anodize tapped holes, not only because of the effect on dimensions, and the brittleness of the coating, but because as the anodized coating builds on the surfaces they crash into each other creating problems.
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Q. Hi,
I found your website through some research online. I have a question regarding a type 3 anodization of a MIC6 aluminum CMM fixture plate.
CMM = Coordinate Measuring Machine
I machined my own 34"x 35"x3/4" Thick, MIC6 Aluminum CMM Fixture Plate, which has 4,480 1/4-20 holes. I want to anodize it like the big manufacturers of CMM fixture plates do. I recently asked a manufacturer how they have theirs done, and they said the following: "We use a Nituff/Teflon Type 3," and, "I won’t be able to go into detail about the thread, but I can just say that the full threads are being anodized."
They say the threads are anodized, but a local anodizer told me that you cannot apply a type 3 anodize to a 1/4-20 tapped plate because the threads will shrink too much from the surface growth. Is this true? What must one do to successfully apply type 3 anodization to the threads? I already machined my fixture plate with a lot of holes... Maybe I should've used a looser fit/different class of tap prior to anodization?br>
Thank you for your time,
Chase S [last name deleted by editor]
Pewaukee WI
February 13, 2026
A. Hi Chase.
Usually, aluminum parts avoid tapped holes if possible; and if there must be tapped holes, stainless steel pems are inserted instead.
Neither of those solutions is practical for a fixture plate with thousands of tapped holes, however.
Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert
You'll love this book. Finishing.com has sold a thousand copies without a return request 🙂
I think the right thing to do in the situation that you are in might be to hardcoat anodize the surface of the fixture but to chromate conversion coat the threaded holes. Please search the site for "chromate and anodize".
It's possible to do either process first, but it involves masking, and you'll have to practice on scrap to learn how to do it right.
Perhaps more practical for 4000+ holes -- even though it's usually not a good idea to anodize tapped holes -- might be to chromic acid anodize everything (chromic acid anodize is much thinner than hardcoat), then grind the chromic acid off the surface, and hardcoat anodize. The chromic acid anodizing in the holes will serve as a masking agent preventing hardcoating of the tapped holes. Please see topic 295/19, "How to Mask for Hardcoat Anodize"
Hopefully a reader experienced in CMM fixture plates, or at least some hardcoated product with a lot of small tapped holes will chime in 🙂
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Hi Chase. You're in quite a pickle l have to admit. You are also intaking bits and pieces of the whole story or multiple stories.
Mic 6 is not always fun to work with. If memory serves, it is a softer material and we used to refer to it as garbage material. It coated quickly and would also smut up if not processed correctly. For reference purposes it might be likened to using osb wood or plywood to make cabinets rather than a nice piece of cherrywood.
Enough about the material. Threads can indeed be adjusted prior to coating to allow for dimensional surface changes to allow for you to achieve your intended result.
Nituff is pretty much somebody's trade name for the hard anodize process with teflon coating. (35 or so years ago i knew who called their process by what name, but not anymore.)
As for complete coating of threads, unless i skipped over it, i did not notice if the holes you drilled are dead holes or thru holes. Each one has its own unique circumstances and how to address them. Dead holes will generally not coat thru to the bottom although you will have a better chance if the plate is anodized tilted to some degree as opposed to completely vertical. The tilt allows for the air pocket to flow out of the hole and solution to enter and do its thing, but again that is depending on the hole size.
Lastly, you speak of hardcoating but not addressing how much thickness. Anodizing thickness will generally decrease as it enters a hole and that is relative the depth and diameter of the hole. .002 thickness is standard.
Is coating absolutely required in the holes and is coating required on the entire depth of the thread. Uneven or incomplete coating will cause an ( loosely defined) out of round type condition.
Ted provided some excellent alternatives for you, but none are easy and all are time consuming.
Retired after 35 yrs in the trade - newport, tenn
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