Machinery's Handbook, 31st Edition
Parallel Bars 751 tolerances defined in Table 3. Squareness and parallelism tolerances are Total Indicated Readings (TIR) over the full length of the bar. The following table shows some common sizes that are readily available from most manufacturers of precision parallel bars. These are representative of the sizes that may be available in the general market, and almost any size or combination may be specially made for a specific application. Table 3. Tolerance Limits for Type II - Ribbed Parallel Bars Size Tolerance Limits Width/Height Variance Matched Pairs Straightness Parallelism Squareness Inch Sizes (inch) (inch) (inch) (inch) (inch) 1 1 ⁄ 2 × 3 × 24 0.0002 0.0005 0.0005 0.0005 2 × 4 × 24 0.0002 0.0005 0.0005 0.0005 2 1 ⁄ 2 × 5 × 24 0.0002 0.0005 0.0005 0.0005 3 × 6 × 36 0.0002 0.0005 0.0005 0.0005 4 × 8 × 36 0.0002 0.0005 0.0005 0.0005 Millimeter Sizes (mm) (mm) (mm) (mm) (mm) 35 × 75 × 600 0.005 0.013 0.013 0.013 50 × 100 × 600 0.005 0.013 0.013 0.013 50 × 152 × 600 0.005 0.013 0.013 0.013 75 × 150 × 1000 0.005 0.013 0.013 0.013 100 × 200 × 1000 0.005 0.013 0.013 0.013 Type III, Box Parallel.— The Type III “Box” Parallel is designed to provide a wide work- ing surface. The Type III can either be square or rectangular in cross section. The rect angular design will have a rib running through the middle of the block the entire length. This provides support to the two larger surfaces. The surfaces of all six sides will be finish ground to an average of 16 µ inch Ra and all sharp edges will be removed. The ground sur faces will all be free from grind chatter marks and grind burn marks.
Fig. 3. Type III, Box Parallel The Type III parallel bar, after casting and rough grinding, will be subject to a seasoning or aging process. This process can either be artificial or natural, and insures the stability of the material after the grinding process. The finished Type III parallel bars will have a material hardness of 180 BHN (Brinell Hardness Number) checked with a 10mm ball and a 3000 Kg load. Each of the six working surfaces will not vary from a true plane (straightness) by more than 0.0002 inch per foot. The opposite sides and ends will be parallel to each other within 0.0005 inch. All adjacent sides will be square to each other within 0.0005 inch as well. The
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