Machinery's Handbook, 31st Edition
762 Right-Angle Plates out of squareness or flatness is introducing error in a machining operation or inspection process. There are several calibration procedures and methods for verifying the accuracy of the right-angle plate. Flatness Calibration: The first feature that should be calibrated is the flatness of the working surfaces. The tolerances for the calibration of any individual right-angle plate is defined by the manufacturer and in their absence the GGG-P-441A shall provide an acceptable calibration tolerance. For Type I, II and VI angle plates, the setup for the flat ness calibration will appear similar to Fig. 7.
TI Measuring Planes Detail A TI
Electronic Height Gage
Surface Plate
Fig. 7. Calibration of Surface Flatness Place the angle plate (the Test Instrument , or TI ) on the leveling plate, as shown, and use a jack stand or similar device to support the TI in position. A bubble level can assist in lev eling the plate. When the plate has been leveled, use the electronic indicator to measure the surface following the pattern shown in the Detail A of Fig. 7. This pattern is generally called the Union Jack in that it resembles the stripes on the British flag. Observe the readings and record the maximum plus and the maximum minus readings overall. Subtract the maximum minus reading from the maximum plus reading; the result is the overall flatness of the surface. This value must meet the manufacturer’s tolerance limits for the model under test or the tolerances specified in the GGG-P-441A. The same process is performed for the side opposite and the sides of the TI. All readings must meet the tolerances stated by the manufacturer or the GGG-P-441A standard. Flatness is only checked on TIs that require external hardware for support and leveling on the surface plate. On units not requiring external support, such as Type III plates, flatness is checked as a function of the parallelism calibration. Parallelism Calibration (Type III Right-Angle) The Type III universal right-angle plate has a parallelism tolerance due to the unique design of this unit. As discussed previously, the Type III is finished on all sides and is square and parallel to all working surfaces and sides opposite. General Parallelism Tolerances Size Range Total Indicator Reading > 2.5 to 24 inches 0.0004 inch TIR > 24 to 36 inches 0.0008 inch TIR > 36 to 48 inches 0.0012 inch TIR > 48 to 60 inches 0.0016 inch TIR Before beginning the calibration process, attention must be paid to the surface condition to be certain that no burrs or damage to the edges or surface is present that might affect the outcome of the measurements. A super fine-grained “hard Arkansas stone” may be used
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