Machinery's Handbook, 31st Edition
728 Surface Plates flatness is calculated from the recorded data. This method was developed by JC Moody while working for Sandia Corporation, and the resulting plot of the data points is referred to as a Moody Plot. There are many software packages available that simplify the process of calculating this plot and provide a detailed printout of the data in a graphical format.
TI MFG Label or Certification Label (North Reference)
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Fig. 1. Union Jack Pattern Repeat-o-Meter Method (Fed. Spec. GGG-P-463c) The easiest and quickest method of monitoring for surface plate wear and tolerance adherence is the Repeat-o-Meter method. A repeat reading gage similar in design to the one shown in Fig. 2 is used to establish the variation in flatness of the surface plate.
Indicating System
Handle
Two Fixed Feet
Floating Contact Flexure Spring 3/8 ´´
Fixed Foot
3/8 ´´
5 ´´ 3/8 ´´
5 ´´ Fig. 2. Repeat Reading Gage from Federal Specification GGG-P-463c
The gage is placed at the center of the surface plate, and the indicator is set to zero. The eight-line Union Jack pattern, Fig. 1 , is then scanned in the sequence defined in the Federal Specification GGG-P-463c. The result of the maximum reading minus the minimum reading shall not exceed the flatness tolerance expressed in Table 4a and Table 4b for inch- and millimeter-sized plates, respectively. Table 4a. Tolerance for Repeat Reading of Measurement (microinches) Diagonal / Diameter Range Grade AA Grade A Grade B Obtained Over Thru Full Indicator Movement (FIM), microinches … 30 35 60 110
30 60 90
60 90
45 60 75 90
70 80
120 160 200 240 280 100
When not specified
120 150 …
100 120 140 50
120 150
100 25
All Sizes
When specified
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