Machinery's Handbook, 31st Edition
756
Parallel Bars
Top View of Surface C
Meter
Surface C
Electronic Height Gage
A B
Surface Plate
Surface D
Fig. 6. Parallel Bar Matched Set, Height and Width Calibration
Size limitations for granite parallel bars are not as rigid as those for individual bars, acceptable limits for size being ± 1 ⁄ 32 inch. However, the tolerance for size of matched sets is the same as that for steel and cast iron. The calibration procedure for granite parallel bars is the same as that for steel and cast iron. As a final note, it should be mentioned that a Pratt & Whitney Supermicrometer can also be used to calibrate the height and width of a parallel bar or a matched set of parallel bars. The Supermicrometer is set up using a gage block or gage block stack to the precise dimen sion of either the height or width of the parallel bar and zeroed at that point. The gage block is then removed, and the parallel bar is set on the elevating table between the anvils of the Supermicrometer and supported by two flatted rounds. The parallel bar is then measured at the first calibration point and the reading compared to the tolerance limits to assure compli ance. The bar is then measured at each remaining calibration point, verifying at each point the compliance to the tolerance limits. This same process is then completed for the “B” parallel bar in the matched set, and the values obtained in the measurement of the “A” bar are compared to assure that deviations in the matched sizes are within the tolerance limits. This method is very good for smaller parallel bar matched sets if a Supermicrometer is available. Right-Angle Plates Right-angle plates or knees, as they are more generally referred to, are divided into six different types as defined in Federal Specification GGG-P-441A. Right-angle plates are used in both machining applications and the setup and inspection of machined parts and assemblies. The plate material is usually high grade, fine-grained, controlled-process iron or iron alloy castings. These castings are always of uniform quality, free from blow holes, porosity or other material inconsistencies, and defects. All castings after being rough cut are stress-relieved to assure dimensional stability and become more stable as they age. With proper care and surface maintenance, angle plates can perform for many years. Fine angle plates over 50 years old are still in service. Right-Angle Plate Grades.— Most right-angle plates are manufactured to meet the guidelines and tolerances laid out in the GGG-P-441A or IS 2554:1971 (India Standards Bureau). While these two governing documents may differ slightly, they both achieve the goal of establishing dimensional guidelines and tolerances that enable the man- ufacturer to produce a dependably consistent product with values and accuracies that customers require to maintain a high degree of measurement confidence in their quality systems. Angle plates come in grades according to the surface finish that is applied and the dimensional accuracies that they are manufactured to. The assigned grade is in direct
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online