Distortion budget: how it propagates through the build

On a 500 mm closed-die cavity that has to land 0.05 mm flat, the cavity face and walls carry 0.20 to 0.30 mm of finish-grind stock after heat treat on grindable surfaces, and 0.10 to 0.15 mm of EDM stock per side on internal features that only EDM can clean up. The grind reclaims heat-treat distortion. The EDM brings the cavity to final form.

The bolt pattern, dowel locations, and parting-line datums hold to print at finish machining. These surfaces want symmetric quench fixturing and a grind of the datums after heat treat before the cavity is finished, so the datums establish the coordinate system for everything downstream.

The outside reference surfaces that locate the block in the press carry 0.05 to 0.10 mm of finish-grind stock. They are not where the cavity tolerance lives, but a die that runs out of square wears one corner of the cavity faster than the others.

The nitride-growth allowance is the one that does not appear in finish-grind stock because grinding through the case is not an option. It appears in the EDM operation as a deliberate undersize on every nitrided dimension by 0.05-0.15% of the local dimension, depending on the nitride vendor's documented growth. This is the part of the budget that the build has to plan for at engineering time, not absorb at QC.