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Effect of black oxide on tight tolerance parts


Q. I am in need to have some bushings reworked that have ± .00035" inside tolerance. They are already black oxide coated, and the rework will remove the coating in some cross holes intersecting the tight tolerance bore. Would it be reasonable to specify that these parts be coated again, or will the cleaning and recoat process affect the bore size?

S. Sheputis
- St. Louis, Missouri


A. I wouldn't worry about the black oxide affecting the inside bore of your bushings. There will be more dimensional change from the rework. Black oxide actually "becomes" the steel and penetrates about 20 millionths into the steel. I don't think you will see any adverse effects from having the parts refinished.

Dan Brewer
chemical process supplier - Gurnee, Illinois

Black Oxide causeds gear teeth dimensional change

June 9, 2016

Q. Having trouble nailing down material removal during black oxide process of a gear made of 9310 material. The bore and journal have no size movement, but the gear outside diameter and measurement over wires (pins placed in between gear teeth 180 degrees from each other) shrinks, by up to .0009" The OD of the gear is 1.500" and M.O.W. is 1.5710. Each measurement shrinks .0008" to .0009" for these two measurements, while the bore and journal diameter do not change. This is very consistent over the 20 piece samples we have done 2 times. Any ideas?

Erik Goelz
- Cudahy, Wisconsin

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