Machinery's Handbook, 31st Edition
764 Right-Angle Plates Then, beginning with a 0.1000-inch gage block, gently insert the gage block between the TI and the granite angle block at the top edge of the TI. Continue placing increasingly larger gage blocks between the TI and the granite angle block until the next larger gage block cannot be inserted without moving the TI. A gentle force should be applied to make sure the gage block is in direct contact with the granite surface, and as the gage block slides between the TI and the granite there should be no resistance until reaching the gage block size that is too large to go between without moving the granite. Record the size of this block as Block A. Rotate the TI and place the side opposite against the granite angle block as shown in Fig. 9b. Repeat the gage block sequence as above and record the size of the gage block as Block B. To calculate the squareness of the working faces of the rotated TI follow the formula: A B 2 Squareness = − Example: Block A = 0.1001 inch and Block B = 0.1009 inch. Solution: The calculation would be 0.1001 minus 0.1009 divided by 2. The result being (–)0.0004 in overall squareness of the working surfaces to a perfect perpendicular plane. Repeat the sequence for all remaining sides opposite. All results must meet the tolerances specified by the manufacturer or the GGG-P-441A standard. If for any reason the results do not meet the specified tolerances, it is possible to have the right-angle plates refurbished at a fraction of the cost of purchasing a new one. It is also possible that the deviations may be acceptable to the quality system, and, if the results are discussed with the Quality Engineer, the decision may be made to accept the unit “As Is” and annotate the deviation for future reference. The unit may also be downgraded to a less critical operation and the previous application be taken over by a new unit of acceptable condition. These decisions must be made by the quality engineers or quality managers and follow the guidelines of the quality system in place. Measurements Using Light Measuring by Light-wave Interference Bands.— Surface variations as small as two millionths (0.000002) inch can be detected by light-wave interference methods, using an optical flat. An optical flat is a transparent block, usually of plate glass, clear fused quartz, or borosilicate glass, the faces of which are finished to extremely fine limits (of the order of 1 to 8 millionths [0.000001 to 0.000008] inch, depending on the application) for flat - ness. When an optical flat is placed on a “flat” surface, as shown in Fig. 1, any small depar- ture from flatness will result in formation of a wedge-shaped layer of air between the work surface and the underside of the flat. Light rays reflected from the work surface and underside of the flat either interfere with or reinforce each other. Interference of two reflections results when the air gap measures exactly half a wavelength of the light used and produces a dark band across the work sur face when viewed perpendicularly, under monochromatic helium light. A light band is produced halfway between the dark bands when the rays reinforce each other. With the 0.0000232-inch-wavelength helium light used, the dark bands occur where the optical flat and work surface are separated by 11.6 millionths (0.0000116) inch, or multiples thereof.
7 fringes × .0000116 = .0000812 ″
.0000812 ″
.0000116 ″
Fig. 1.
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