Straight edges and testing straightness

A straight edge is a long, narrow, rigid bar with at least one very accurately straight working edge. It is used as a reference for checking the straightness and flatness of surfaces, including machine slideways.

Types

1. Plain (toolmaker's) bevel-edge straight edge: a steel bar with a bevelled working edge, up to about 300 to 500 mm long; the narrow edge touches the work along a line.
2. Wide-edge straight edge: a flat bar with a wide working face, used with a surface plate for checking larger surfaces by the light gap.
3. Cast iron I-section / box-section straight edge: for lengths of 1 to 3 m, ribbed to give high rigidity and low weight. It is supported at the Bessel points (about 0.22 L from each end) so that its deflection under its own weight is a minimum.
4. Angle (square-edge) straight edge: an L-section edge used for checking the straightness of inside corners and the squareness of two faces.

Testing

• Light gap method: The straight edge is held with its edge on the surface and viewed against a bright background. If the surface is straight, no light passes; even a gap of a few micrometres shows as a line of light. The size of the gap can be estimated from the colour of the light: very narrow gaps show coloured (bluish) light, while wider gaps show white light.
• Feeler or blueing method: Prussian blue is smeared on the edge and moved over the surface; the high spots are marked and removed by scraping. A feeler gauge may be pushed into any gap to measure it.
• Flatness: The test is repeated in several directions on the surface (along the sides and the diagonals).

Straight edges themselves are checked for accuracy by reversal against a surface plate or by the three-straight-edge method.