Machinery's Handbook, 31st Edition
Parallel Bars 753 parallels. The configuration is basically the same as the Type I solid parallel; the only dif ference is in the material and the tolerance limits. The granite parallel is supplied in either individual members or matched sets. Granite parallels must meet the tolerances defined in the table below and will appear similar to Fig. 1. Table 6. Tolerance Limits - Granite Parallel
Grade AA Grade B Flatness and Parallelism Flatness and Parallelism Flatness and Parallelism Grade A
Size (in.)
0.000025 0.00003 0.00004 0.00006 0.00015 0.00020 0.00025
0.00005 0.00006 0.00008 0.00010 0.00030 0.00040 0.00040
0.00010
0.5 × 1 × 6 0.750 × 1 × 6 0.750 × 1.5 × 9 1 × 2 × 12 1.5 × 3 × 18 2 × 4 × 24 1.5 × 4 × 30
N/A
0.00010
N/A N/A N/A
N/A Calibration, Precision Parallel Bars, Naval Air Systems and Air Force Metrology.— It is commonly understood that wear and naturally occurring damage will affect the per formance of all precision measuring equipment. Identifying an out of tolerance condition before it can impact a critical measurement is the primary goal of calibration. Parallel bars are no exception to this rule. A precision-ground steel or cast-iron parallel bar may become worn or even deformed from daily wear or excessive forces applied. A granite parallel can also show evidence of wear, and, due to the extremely close tolerances applied to granite parallel bars, the monitoring of these instruments is even more critical. It is not extremely difficult to perform the calibration of the parallel bar, and it can be achieved in a relatively short period of time in a temperature-controlled environment on a clean surface plate with an electronic indicator, a height transfer standard, and a few gage blocks. The features that will be observed are flatness, parallelism, and height and width of matched pairs. The exact height and width of individual parallel bars is not a critical feature, but in this status they can only be used independently. As an additional note, a Pratt & Whitney Supermicrometer can be used to measure the exact height and width of a set of matched parallel bars, but, in the absence of this instru ment, the same results can be achieved with the instruments listed above. The uncertainty of the measurement is effectively the same, and the confidence in the results is just as high with either method. Flatness (Straightness) Calibration Method.— In any calibration, the first steps are to ensure a clean working surface and a proper setup of the standards and measuring instru ments. To that end, the first step is to clean the surface plate thoroughly with an approved surface plate cleaner and place the electronic indicator next to the working surface. The indicator should be powered up and allowed to warm up. The surface plate should be of an accuracy grade to provide a surface flatness deviation no greater than 0.00005" within the working surface that will be used. The UUT or Unit Under Test should be cleaned and, as a preliminary step to calibration, deburred with a clean ultra-fine hard Arkansas stone or gage block deburring stone. Only the edges should be stoned as this is where most burrs will occur. However, all edges, corners and surfaces should be observed to ensure that no damage or burring is present before proceeding. For precision cast-iron or steel parallel bars, the UUT will be set up on two gage blocks of the same size and accuracy grade. A 2.000" gage block is recommended as a minimum. This will allow access to both sides of the parallel bar when taking data to determine the flatness of each side. It is important that the gage blocks are placed at the correct points to support the UUT without any sagging effect on the bar. These are called Airy points and are calculated by a simple formula: Airy point separation distance = 0.554 L ,where L is the length of the UUT.
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