Allowance Is Decided by the Process Chain, Not by Habit
Allowance belongs to the step before it
The allowance on a plate is not a single number; it is a different number at each stage. Stock removed by rough milling needs to leave enough for semi-finishing and for the final grind or fine mill that produces the dimension. The allowance that matters is the one at the step preceding the finishing operation, and it is set by the expected distortion of the preceding step, not by a general rule.
Rough milling releases internal stress in the plate, and the plate moves. A plate roughed on both faces and then finished will bow between the two operations; the finishing allowance has to absorb that movement as well as the tool marks.
Where allowance causes trouble
Thin plates and large plates are the two problem cases. A thin plate has little stiffness, so clamping force and residual stress dominate and the allowance has to be larger relative to thickness. A large plate moves more between operations, and the bowing shows up as a flatness failure after the final pass rather than as a dimensional one.
The practical control is to finish in more than one stage with a stress-relief step or a natural stabilisation period between roughing and finishing on plates where flatness is critical. State on the drawing both the allowance at the final operation and the flatness requirement after finishing — those two together determine whether the process chain will work.
FAQ
How much machining allowance should a mold base plate have?
It varies by operation and plate size. The figure that matters is the one before finishing, set by the distortion of the preceding roughing step.
Why do plates move after roughing?
Rough machining releases internal stress, so the plate bows between operations. The finishing allowance has to absorb that movement as well as tool marks.
Which plates need the most allowance?
Thin plates and large plates. Thin plates have little stiffness; large plates move more between operations.
How do I improve flatness on a large plate?
Finish in more than one stage, with stress relief or stabilisation between roughing and finishing. State allowance and post-finish flatness together on the drawing.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.