In addition to dimensions, technical drawings often contain symbols (symbols) with values.
By this, the tolerance of important geometric properties or relationships is represented. This is used to describe permissible deviations.

The straightness tolerance specifies a maximum deviation of the contour at a certain point of a molded part.
Tolerated elements can be straight lines or axes.
The measured deviation at any point of the real component must not exceed the Tolearnz value displayed together with the symbol.

For a measuring point on a surface toleranced with straightness, it applies that this point must lie within a distance of two straight lines.
The distance between the two straight lines is determined by the specified tolerance value.
In this case, surfaces can be straight in the X-axis even though they are curved along their Y-axis.

The axis of a cylinder may deviate from its position by a specified tolerance value.
This deviation may only be cylindrical and is called ‘straightness tolerance of an axis’.

This drawing entry is regulated in the ISO 1101 standard or, for North America, in the Geometric Product Specification standard ASME Y 14.5.

Straightness in Injection Molding

Quality assurance: Example of the tolerance specification on a cylinder axis (incl. meaning)

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