Specification guide · Procurement and specification
Drawings and inspection features for FDM parts
A fully dimensioned drawing is not automatically a good manufacturing document. For FDM, a few function-critical features should be unambiguous while non-critical geometry is not constrained unnecessarily.
In brief
State what the part must do: datum, functional feature, allowed deviation and inspection method. This turns geometry into a testable requirement.
Datums first
Dimensions are only clear when the originating face or axis is defined.
- Use the mounting face as the primary datum.
- Dimension hole patterns from functional datums.
- Do not use distortion-sensitive outer contours as the sole datum basis.
Separate critical and non-critical dimensions
Tight tolerances increase orientation, inspection and finishing effort.
- Critical
- Fits, sealing faces, centre distances, mounting heights
- Functional
- Clearances, cable paths, stops
- Non-critical
- Free outer contour without contact function
Specify finishing
A finished feature needs stock allowance, final size and machining access.
- Mark precision holes for drilling or reaming.
- Specify threaded insert type and installation direction.
- Identify cosmetic faces and permitted support marks.