Straight edge
Straight rulers and squares are gauges of metalworking from hardened steel . They are used to check surfaces for evenness ( flatness ) and straightness . The hair angle is also used to check squareness.
The shape is basically a straight ruler ( straight edge ) or a right angle, each with a defined measuring edge. The measuring edge is in the cross section is not flat, but as a cutting tapered ground , very fine lapped and glare frosted or blackened in order to avoid reflections. The measuring edge is provided with a small radius to maintain the dimensional accuracy even under poor conditions.
application
The light-gap method is used for testing . The measuring edge of the instrument is placed on the surface to be tested. In the backlight, the light gap shows whether the surface is straight or uneven. With good lighting, deviations from 0.002 millimeters, two thousandths of a millimeter, can be seen.
Naming
With the straightedge, “hair” -fine deviations should be recognized intuitively. A human hair , however, is much coarser and has a diameter of 0.04 mm to 0.12 mm. The straight edge is therefore 20 to 60 times more accurate.
