Distortion budget: how it propagates through the build

Add the worst-case numbers and a 500 mm H13 cavity could see 0.20 mm machining movement, 0.75 mm heat-treat change, 0.25 mm nitride growth, and a few microns of EDM-stress release. That is over 1.2 mm of total movement on a part with a 0.05 mm flatness tolerance.

The build still works because the operations are not independent. Stress relief between rough machining and heat treat removes machining stress before quench can warp the part. Finish grinding after heat treat removes heat-treat distortion before EDM begins. Stress relief after EDM dissipates recast stress before nitriding pulls on it. Each intermediate step is a reset that prevents prior distortion from compounding. Skip a relief, and the prior distortion arrives at the next operation as a starting offset.

The right mental model is not addition. It is a series of operations where each one's input is the residual movement that was not removed by the prior reset, and each operation's output is the new movement plus whatever the reset failed to catch. The cycle traces and the witness coupons are the only way to measure where the resets actually landed.