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Internal threads warping after paint.

Internal threads warping after paint.

Internal threads warping after paint.

We are using aluminum cast 108,113 or 319A. We are machining a 3/8-24 tap thru about an inch of material. We have tried using an H5 and H7 tap both work fine before being sent out for Flat Black paint but upon return they need to be chased. We tried to use H9 and H11 tap which leaves the thread too large.
My question is, does anyone have any suggestions? I understand the deformation is happening because of the heat(400 degrees +) for the pre-treating and baking in the paint process. I am hoping there may be another option for painting? Possibly an additive that can be applied to the casting material? I am hoping this forum can give suggestions that I can research and ask the questions that I don’t know enough to ask yet.

Thank you,

RE: Internal threads warping after paint.

Stress relief of the casting before it gets sent out for coating, or machining of the part after it comes back.

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