Machinery's Handbook, 31st Edition
754 Parallel Bars Example: For a 12.00" UUT, the gage blocks should be placed 0.554 × 12.00 = 6.650 inches apart and located (12.00 – 6.650) ⁄ 2 = 2.675 inches from each end, as shown in Fig. 5. This distance should be very close to the calculated distance, but it does not have to be exact. Establishing the distance using a machinists’ rule within 0.100" is sufficient.
Top View of Surface A
Meter
Surface A
Electronic Height Gage
Gage Block
Surface Plate Surface B
Fig. 5. Parallel Bar Flatness Setup The UUT will be marked at five points equally spaced along the distance of each surface. The marks should be done with a felt-tipped marker or other suitable method. By general convention, the largest parallel surfaces are designated A and B. The two smaller surfaces are C and D. The markings A1 through D1 and A2 through D2 should be aligned with one another. A datasheet similar to the one shown in Table 7 should be created to collect the data and aid in the calculations of the deviations. With the UUT placed on the gage blocks as shown, configure the electronic indicator to measure “over” and set the indicator amplifier range to ±0.0002 with a resolution of 0.00001". It is recommended, however, to begin the readings at a slightly higher range and dial the scale in to a more sensitive resolution to determine the magnitude of the error. This will keep the readings on scale. Take the final readings in the highest sensitivity scale. Place the indicator contact point on the UUT at the A1 position and zero the amplifier. All readings will have the x1 position as the zero point (A1, B1, C1, D1). All deviations will be from this point. Sweep the surface of the UUT, recording on the datasheet the deviation as indicated at each of the calibration points. Ensure that each point does not exceed the documented tol erance for the type, style, grade, and size of the UUT. When Side A has been scanned, adjust the electronic indicator contact point for “Under” measurements, place the contact point at the B1 calibration point and zero the indicator amplifier. This measurement is taken from the underside of the UUT, and the data collected will be used to calculate the parallelism of the two sides A to B. Once again, sweep the surface and record the deviations of the surface at each of the designated calibration points. Verify that all readings for sides A and B are within the tolerance limits assigned to the UUT. Parallelism will be calculated from the sum of the deviations of each side. Calculate the sum of the deviations at each calibration point for surfaces A and B. The sum of the devia tions will reveal the parallelism of the two surfaces. Verify that the values calculated are within the assigned tolerances for the UUT. Carefully move the UUT to the adjacent sides (C and D) with side C up and repeat the sequence to determine the flatness and parallelism for sides C and D. Once complete and the values have been determined to be within the assigned tolerance limits, remove the UUT from the gage blocks and place on the surface pate. If this is a matched set of parallel bars, place the second member on the gage blocks as for the first member and complete the sequence for the second member.
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