Machinery's Handbook, 31st Edition
Surface Plates 729 Table 4b. Tolerance for Repeat Reading of Measurement (micrometers) Diagonal / Diameter Range (mm) Grade AA Grade A Grade B Obtained Over Thru Full Indicator Movement (FIM), micrometers … 800 35 60 110
800 1500 2200 3000 3800
1500 2200 3000 3800
45 60 75 90
70 80
120 160 200 240 280 2.5
When not specified
100 120 140 1.3
…
100 0.6
All Sizes When specified Note for Table 4a and Table 4b : If it is intended that small objects be measured on large surface plates, it should be noted that a larger tolerance in flatness over small areas is permitted on larger plates. All points on the work surface shall be contained between two parallel planes, the roof plane and the base plane. The distance between these planes shall be no greater than that specified in the tolerance table for the respective grades. Autocollimator Calibration (Ref NAVAIR 17-20MD-14, 1 MAY 1995).— Calibration of a surface plate by autocollimator is one of the most accurate and relatively quickest methods in use today. The method has been in use since the advent of the autocollimator in the 1940s but was refined by JC Moody with the development of the Moody Plot analysis method that utilized the data collected on perimeters, diagonals and bisectors. The result ing pattern resembles the “Union Jack” and is referred to as such. Autocollimators today have been enhanced with the inclusion of CCD devices, digital readouts, computer interfacing, and automatic data collection that calculates the devia tions and analyzes the results in a fraction of the time previously required, producing a full color graphical diagram of the surface plate variances. Fig. 3 and Fig. 4 show the positioning of the turning mirrors used when performing the surface plate calibration with an autocollimator.
North Side
Standard mirror positions lines 1, 3, 6
MFG nameplate
NW
Line 3
Note: Example uses mirror base feed with 5-inch length
NE
Movable mirror positions for lines 1, 3, 6
0
Arrow heads on lines indicate direction of movement of mirrors
5
10
15
20
Line 8
Line 8
25
30
Numbers indicate lines that are read with autocollimator in this position
35
Autocollimator position 1
40
I-3-6
45
50
SW
SE
Line 4
55
First reading must be with mirror nearest autocollimator
Fig. 3. Placement of the First Turning Mirror and Retroreflector: TO 33K6-4-137-1
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