Machinery's Handbook, 31st Edition
Surface Plates 727 Table 3a. Total Flatness Tolerance, Rectangular Surface Plates GGG-P-463c Rectangular Plates Inch Sizes, Tolerances in Microinches Millimeter Sizes, Tolerances in Micrometers
Width (inch)
Length (inch)
Grade AA
Grade A
Grade B
Width (mm)
Length (mm)
Grade AA
Grade A
Grade B
12 18 18 24 24 36 48 36 48 60 72 48 60 72 96
50 50 50 75
100 100 100 150 150 200 300 300 400 500 600 400
200 200 200 300 300 400 600 600
300 450 450 600
1.3 1.5 1.6 1.9 2.2 2.9 3.9 3.6 4.6 5.9 7.5 5.6 6.9 8.5
2.6 2.9 3.3 3.8 4.3 5.7 7.8 7.2
5.2 5.9 6.6 7.6
12
300
18
450
75 100 150 150 200 250 300 200 300 350 500 700
600 900 1200 900 1200 1500 1800 1200 1500 1800 2400 3000 3000 2400 3600
8.6 11.5 15.5 14.4 18.4 23.6 29.9 22.4 17.6 33.9 50.0 70.8
24
600
800 1000 1200 800 1200 1400 2000 2800 3000 2400 4400
9.2 11.8 15.0 11.2 13.8 17.0 25.0 35.4 36.9 30.8 53.8
36
900
600 700 1000 1400 1500 1200 2200
48
1200
12.5 17.7 18.5 15.4 26.9
120 120 96 144
60 72
750 600 1100
1500 1800
73.9 61.6 107.7
Table 3b. Total Flatness Tolerance, Round Surface Plates GGG-P-463c Round Plates Inch Sizes, Tolerances in Microinches Millimeter Sizes, Tolerances in Micrometers Diameter (inch) Grade AA Grade A Grade B Diameter (mm) Grade AA Grade A Grade B 12 50 100 200 300 1.1 2.3 4.6 18 50 100 200 450 1.3 2.5 5.3 24 75 150 300 600 1.6 3.2 6.3 36 100 200 400 900 2.3 4.6 9.2 48 125 250 500 1200 3.3 6.6 13.2 Calibration of Surface Plates.— Surface plates, like other precision instruments, will drift out of tolerance in time and need to be periodically checked and even adjusted to maintain accuracy. For a surface plate, this adjustment involves lapping the surface and physically removing material until the entire surface is once again flat to within the limits of the grade to which it was made. This is a labor-intensive adjustment that should only be attempted by a trained technician with the appropriate tools. An untrained technician, even with the proper lapping tools and compounds, may cause more harm than good when attempting such an adjustment. However, with proper care and cleaning, a surface plate may not need lapping at every calibration. Completing the calibration process will give the quality assurance program the data needed to identify wear patterns on a surface plate and have the proper maintenance completed before an out of tolerance condition occurs, thereby avoiding a costly failure impact analysis and reinspection of parts, or even a recall of finished parts from a customer. The calibration process is a cost-effective alternative to these undesirable effects. Fig. 1 illustrates the “Union Jack” or eight line pattern that is used to analyze the overall flatness of the surface plate working area. Each line is measured independently, and the positive and negative elevations are recorded at predetermined intervals. The peak to valley
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