Application Notes AP-1118 Squareness, Parallelism and Straightness Measurement Intruduction In determining machine geometry, a laser Beam is used to produce an ultra precision Equipment to the straight-edge. An optical Square is used to bend the laser beam Precisely at a 90 angle, and a quad-detector Is used to determine the centroid of the laser Beam. A straightness measurement is the Measurement of perpendicular motion along a Travel path as shown in Fig. 1. The ideal travel path is a straight line. Any Deviation from the straight line in the horizontal Direction is called horizontal straightness. In The vertical direction it is called vertical Straightness. Straightness measurement on the device tool spindle

ACTUAL TRAVEL PATH WITH PERPENDICULAR MOTIONX A B IDEAL TRAVEL PATH, STRAIGHT LINE

FIG. 1 DEFINITION OF STRAIGHTNESS

Attach a target with a cross in the center to the travel Mechanism and align the cross to the center of the laser beam. Assume the laser beam travels from A to B and follows the ideal path. The center of the cross should follow the actual travel path. By measuring how far the cross is deviated from the center of the laser beam along the travel path, we can determine the straightness. For more accurate measurement, the centroid of the laser beam can be measured by using a quad-detector which is a large area photodetector cut into four quadrants, as shown in Fig. 2.

Cross Lines Target

2 1 3 4

Quad Detector Fig. 2 Target