Ano
Article · Masking
How to Keep Threads Bare Through Anodizing
Mask them, machine them oversize, or chase them afterward. Which one is right depends on the coating thickness and whether a bare thread compromises the part's corrosion protection.
Why Threads Are the Usual Problem
Anodizing builds on both flanks of a thread at once, so the effective pitch diameter changes by more than the coating thickness suggests. A tapped hole that gauged correctly before finishing can refuse the same gauge afterward, and on a Hard Coat it very often does.
The Three Options
Mask or plug them. The most common answer, especially on Hard Coat parts. The threads stay bare and gauge exactly as machined. The trade-off is that bare aluminum in the threads has no coating protection, which matters on parts facing corrosive service.
Machine them oversize. Cut the thread larger by the expected build so the finished dimension lands correctly. This works well and requires the machinist to know the coating thickness before cutting, so the drawing has to state it.
Chase them after finishing. Acceptable on plenty of parts and unacceptable where a bare thread would compromise protection, since chasing removes the coating you just paid for.
What Masking Costs and Why
Masking is manual work performed on every individual part, twice: once to apply and once to remove. That is why it is the single largest controllable line in most finishing quotes, and why a part with eight protected features can cost a multiple of the same part with none.
It is also why a suspiciously low quote is so often a quote that did not account for the masking you asked for. When comparing finishers, check that everybody priced the same protected features, because a quote missing them is not cheaper, it is incomplete.
The practical saving is not to hunt for a cheaper masking rate. It is to review which features genuinely need protecting, since some get called out as bare from habit rather than function, and every one you can release saves work on every part in every future order.
What Does Not Work
Saying nothing and hoping. A drawing with tight thread callouts and no coating allowance is the most reliable predictor of a first article problem we see, and by the time it surfaces the parts are finished.
Mark the protected features on the print, or note them in the request for quote. A marked-up PDF is completely sufficient and it is what most customers send. See masking, plugging and assembly.
Common Questions
Is Masking Threads Expensive?
It is quoted with the job, because it is manual work performed on each part twice, once to mask and once to unmask. It is also the largest controllable cost on most finishing quotes, so it is worth reviewing which features genuinely need it.
Can You Mask Blind Holes and Bores?
Yes, with plugs sized to the feature. Blind holes have a second consideration: trapped air can leave an uncoated void, so the racking has to account for drainage.
Is It Cheaper to Mask Threads or to Chase Them Afterward?
It depends on the part and the volume, and chasing removes coating you paid for. On a corrosion-critical part chasing is often not acceptable at all. Send the print and we will tell you which approach suits it.
What Tolerance Can You Hold on a Mask Line?
It depends on the geometry. A sharp mask line on a flat face runs considerably tighter than one on a curved surface. Send the print and we will tell you what that specific feature supports.
Should Threads Be Masked on a Type II Part?
Often they do not need to be, because Type II builds far less than Hard Coat. On close-gauged threads it still matters. Tell us the thread callout and the class and we will say whether it needs protecting.
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Have a Part That Needs Finishing?
Send the print and the count. Standard turnaround is 3 to 5 days from our dock in Paoli, Indiana.