Machinery's Handbook, 31st Edition
Right-Angle Plates 757 correlation to the quality and accuracy of the angle plate. Federal Specification GGG-P- 441A specifies three Grades of surface finish applied to right-angle plates. In addition, there are six distinct Types of angle plates that have unique properties according to their specifically designed target uses in the manufacturing process. Grade A angle plates are the highest grade and are used in precise applications where the smallest deviation from square (90 ° ) is required. These plates are used in various industries for clamping and holding work in a vertical position. The scraped surface is ben eficial for supporting work in a very precise vertical plane and facilitating the dispersion of lubricants and coolants during the manufacturing process. Grade A plates are required to have working surfaces that are fine precision-scraped with relief spots to prevent sticking of gage blocks or precision-lapped instruments. These relief spots allow the surface to be controlled to a high degree of flatness. Each square inch of surface on a Grade A plate has 15 to 18 spotting cavities, from 0.0002" to 0.0005" deep, and a bearing surface of 20 to 40 percent. The bearing area does not deviate from the mean plane by more than 0.0001 inch in over 24 inches, and no more than 0.0002 inch in up to 60 inches, as per Table 1. No square inch may vary from the adjacent area by more than 0.0001" per square foot. The adjacent working surfaces on any Grade A angle plate shall not vary from square by more than 0.0001" in every 8". This equates to 2.5 arc seconds. These angle plates make excellent fixturing for high precision shaping, milling, grinding, drilling, or boring operations. Table 1. Maximum Permissible Deviation of Bearing Area from Main Plane ( GGG-P-441A )
Maximum dimension of working surface, inches
Maximum permissible deviation of bearing areas from mean plane, inch
Above
To and including
Grades A and B
Grade C
12 24 36 48
2 1 ⁄ 2
0.0001
0.0002 0.0004 0.0006
12 24 36 48
0.0002
60 0.0008 Grade B plates are made to meet requirements of precision inspection and calibration operations. Although the style of the plate can accommodate uses in any of the manufactur ing practices, the precision-ground surface finish lends itself to inspection applications on the surface plate and use in calibration laboratories, high precision inspection stations, or situations where a fine precision-ground finish is desired and the highest degree of accu racy necessary. The Grade B angle plate has a working surface that is precision ground to a roughness average value of not more that 16 microinches for plates less than 64 square inches and 32 microinches for larger plates. The precision-ground surface of the Grade B plate is well suited to the applications involved in the inspection process of high accuracy, tight tolerance, precision-made parts. The fine-ground finish offers a precise surface that does not deviate from the mean plane by more than 0.0001 inch in over 24 inches, and no more than 0.0002 inch in up to 60 inches, as per Table 1. No adjacent areas shall vary with each other by more than 0.0001 inch per square foot. The aspect that is most critical about the angle plate is the control of squareness and the ability to pass on the accuracy to the parts being machined or inspected. All adjacent working surfaces will not vary from square by more than 0.0001 inch in every 8 inches. This high degree of accuracy in the Grade A and Grade B angle plates provides the measurement assurance necessary to inspect precision parts and calibrate precision measuring instruments. Grade C angle plates are the workhorses of the plate grades. They are designed to have a smooth machined finish and controlled to not exceed a surface roughness of 32 r.m.s. The bearing area of the working surface shall not exceed a deviation from the mean plane any greater than between 0.0002 to 0.0008 inch depending on the size of the bearing area, as per Table 1. Furthermore, no adjacent square foot may deviate from another by more than
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