Surface Measurement

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 straightness and flatness of surfaces, and for testing the straightness of 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; the narrow edge touches the work along a line.
2. Wide-edge straight edge: a flat bar with a wide working face for use with a surface plate; for checking larger surfaces by 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. They are supported at the Bessel points (about 0.22 L from each end) so that deflection due to its own weight is the 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 there is no gap; a gap of 0.002 mm or more shows as light. The size of the gap is estimated from the colour of the light: a gap of about 0.002 mm shows blue, 0.0075 mm shows white.
• Feeler or blueing method: Prussian blue is smeared on the edge and moved over the surface; high spots are marked and are removed by scraping. A feeler gauge may be pushed into any gap.
• Flatness: The test is repeated in several directions (along the sides and diagonals) of the surface.

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

A surface plate is a heavy, rigid plate with a flat upper face which gives a reference plane for marking out, measurement and inspection of flat surfaces and of heights (with a height gauge or dial indicator).

Materials

• Cast iron: fine-grained, stress-relieved, ribbed underneath to be stiff; the usual choice, can be scraped and is self-lubricating. Machine tool workshops use it widely.
• Granite: hard, nearly no burrs when scratched (dust does not raise a burr), non-magnetic, rust free, little thermal expansion and good wear resistance; is preferred in inspection rooms.
• Glass or ceramic: for small plates in lightweight use.

Grades

Plates are made in grades 00, 0, 1 and 2 (IS 2285): 00 and 0 for laboratory and inspection, 1 for inspection room, and 2 for workshop use. The permissible flatness error increases with size and with grade number.

Manufacture

The plate is cast, aged, rough machined and finished by grinding and then by hand scraping. The flat is produced by the three plate method: three plates are scraped in turn against each other (A against B, B against C, C against A) until all three mate perfectly; only a true plane can fit the other two in every position.

Uses

Marking out, checking flatness using a straight edge, checking squareness and parallelism using a dial indicator and angle plate, as a base for sine bars and slip gauges, and for measuring height with a vernier height gauge. It is kept clean, covered, supported at three points and checked regularly.