The symbol for the location tolerance symmetry is used to describe a tolerance zone on technical drawings.
This is bounded by two planes symmetrical to the specified reference axis.
The distance between these planes is indicated together with the symbol on the drawing. The same applies to the reference axis.
Measured values on the toleranced center plane must lie between the parallel planes.

Symmetry in Injection Molding

Quality Assurance: The measured values of the center plane of the groove must be within two surfaces 0.07mm apart

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This glossary of plastic industry is provided by PLEXPERT Canada Inc.