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Is it the machine coolant that is etching aluminum?

We have had a product, machined from 6061 alum. sent out for blue anodize-52 pieces. Our vendor claims when the rack of parts came up out of the dye tank-some looked good,some did not plate well.The parts that were acceptable for the color kept the press fit hole diameter perfect(.442)The others lost .004 from the I.D..Everyone wants to blame the plater and say they just left them in the tank too long.The vendor claims he had all the parts on the same rack and processed them as he normally would a billet 6000 series aluminum part.One thing we are looking into is did the machine shops coolant have an effect on the surface that led to this material removal-some parts being exposed to the coolant longer than others -- explaining how some did not lose press fit dia. Any ideas?

Thanks,

Greg Freeborn
- Guilford, Connecticut
2003

6061 aluminums do not anodize well at all. If you want a good look to your finished part you have to use 2024...........

G. Sclippa
- Livermore, California
2003