Surface Plate Tolerances
(All tolerances are taken from GGG-P-463c Tables II, III, and IV)
Flatness
| Size (inches) | Diagonal (inches) | Area (Sq. Ft.) | Grade AA / Laboratory (inches) | Grade A / Inspection (inches) | Grade B / Toolroom (inches) |
|---|---|---|---|---|---|
| 6 x 6 | 8.5 | 0.25 | 0.000050 | 0.000100 | 0.000200 |
| 6 x 12 | 13.4 | 0.5 | 0.000050 | 0.000100 | 0.000200 |
| 8 x 12 | 14.4 | 0.7 | 0.000050 | 0.000100 | 0.000200 |
| 9 x 12 | 15 | 0.75 | 0.000050 | 0.000100 | 0.000200 |
| 12 x 12 | 17 | 1 |
0.000050 |
0.000100 | 0.000200 |
| 12 x 18 | 21.6 | 1.5 | 0.000050 | 0.000100 | 0.000200 |
| 18 x 18 | 25.5 | 2.25 | 0.000075 | 0.000150 | 0.000300 |
| 18 x 24 | 30 | 3 | 0.000075 | 0.000150 | 0.000300 |
| 20 x 30 | 36.1 | 4.1 | 0.000100 | 0.000200 | 0.000400 |
| 24 x 24 | 33.9 | 4 | 0.000075 | 0.000150 | 0.000300 |
| 24 x 30 | 38.4 | 5 | 0.000100 | 0.000200 | 0.000400 |
| 24 x 36 | 43.3 | 6 | 0.000100 | 0.000200 | 0.000400 |
| 24 x 48 | 53.7 | 8 | 0.000150 | 0.000300 | 0.000600 |
| 30 x 30 | 42.5 | 6.3 | 0.000100 | 0.000200 | 0.000400 |
| 30 x 48 | 56.6 | 10 | 0.000175 | 0.000350 | 0.000700 |
| 36 x 36 | 50.9 | 9 | 0.000150 | 0.000300 | 0.000600 |
| 36 x 48 | 60 | 12 | 0.000200 | 0.000400 | 0.000800 |
| 36 x 60 | 70 | 15 | 0.000250 | 0.000500 | 0.001000 |
| 36 x 72 | 80.5 | 18 | 0.000300 | 0.000600 | 0.001200 |
| 48 x 48 | 67.9 | 16 | 0.000200 | 0.000400 | 0.000800 |
| 48 x 60 | 76.8 | 20 | 0.000300 | 0.000600 | 0.001200 |
| 48 x 72 | 86.5 | 24 | 0.000350 | 0.000700 | 0.001400 |
| 48 x 96 | 107.3 | 32 | 0.000500 | 0.001000 | 0.002000 |
| 48 x 120 | 129.2 | 40 | 0.000700 | 0.001400 | 0.002800 |
| 60 x 60 | 84.9 | 25 | 0.000325 | 0.000650 | 0.001300 |
| 60 x 72 | 93.7 | 30 | 0.000400 | 0.000800 | 0.001600 |
| 60 x 96 | 113.2 | 40 | 0.000550 | 0.001100 | 0.002200 |
| 60 x 120 | 134.2 | 50 | 0.000750 | 0.001500 | 0.003000 |
| 72 x 72 | 101.8 | 36 | 0.000450 | 0.000900 | 0.001800 |
| 72 x 96 | 120 | 48 | 0.000600 | 0.001200 | 0.002400 |
| 72 x 120 | 139.9 | 60 | 0.000825 | 0.001650 | 0.003300 |
| 72 x 144 | 161 | 72 | 0.001100 | 0.002200 | 0.004400 |
| 96 x 96 | 135.8 | 64 | 0.000775 | 0.001550 | 0.003100 |
| 96 x 120 | 153.7 | 80 | 0.000975 | 0.001950 | 0.003900 |
| Size (inches) | Diagonal (inches) | Area (Sq. Ft.) | Grade AA / Laboratory (inches) | Grade A / Inspection (inches) | Grade B / Toolroom (inches) |
To determine the flatness tolerance of a surface plate not included in the above chart, use the following formula:
Total Flatness Tolerance (Grade AA) in microinches = 40 + (D²/25), where D = Diagonal of the plate in inches. Round the result to the nearest 25 microinches.
Total Flatness Tolerance (Grade A) in microinches = Grade AA Tolerance * 2
Total Flatness Tolerance (Grade B) in microinches = Grade AA Tolerance * 4
The flatness tolerance does not include the surface along the edge of each size plate as follows:
Diagonal of Plate Distance in from Edge (All Grades) 12 through 48 1 inch over 48 1.5 inches
Repeatability
| Diagonal Range (inches) | Grade AA / Laboratory (inches) | Grade A / Inspection (inches) | Grade B / Toolroom (inches) |
|---|---|---|---|
| through 30 | 0.000035 | 0.000060 | 0.000110 |
| over 30 through 60 | 0.000045 | 0.000070 | 0.000120 |
| over 60 through 90 | 0.000060 | 0.000080 | 0.000160 |
| over 90 through 120 | 0.000075 | 0.000100 | 0.000200 |
| over 120 through 150 | 0.000090 | 0.000120 | 0.000240 |
| over 150 | 0.000100 | 0.000140 | 0.000280 |