The dimension change is inevitable with a hardening process. And don't worry about messing up part, last year i made axles out of aluminum bars by milling the key slot with my max NC cnc milling machine and when i was setting it up i adjusted for the width of the material, but forgot to adjust for the width of the endmill so they didn't fit into the sprockets

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Whats the difference between mechanical engineers and civil engineers?
Mechanical engineers build weapons, civil engineers build targets