Machinery's Handbook, 31st Edition
752 Parallel Bars size as specified will also be within 0.0005 inch. The Type III box parallel will appear similar to Fig. 3. Tolerances are given in Table 4. Table 4. Tolerance Limits for Type III Box Parallel
Size (in.)
Straightness (in.)
Squareness (in.)
Parallelism (in.)
0.0001 0.0001 0.0002 0.0001
0.0005 0.0005 0.0005 0.0005
0.0005 0.0005 0.0005
4 × 4 × 6 4 × 6 × 6 5 × 8 × 12 10 × 10 × 10
0.0005 Type IV, Adjustable Parallel.— The adjustable parallel is a precision parallel that is ad- justable to any height within a specified range. This design allows a flexibility in the use of the parallel that the other designs do not possess. The adjustable parallel is made of close- grained, seasoned cast iron. After initial casting and rough machining, the members of the parallel are subject to an aging or seasoning process that can either be natural or artificial. When complete, the member will have a hardness value of no less than 87 RB on the Rock well B scale. The final machining on the adjustable parallel shall have a finish of 16 µ in Ra on the working surfaces and a 32 µ in Ra on the sides. The sides of the adjustable parallel are not designed to be working surfaces.
Fig. 4. Type IV, Adjustable Parallel The adjustable parallel is made of two pieces. One piece is a fixed member machined with a dovetail slot, and the other a sliding member machined with a dovetail that fits into the slot and slides smoothly. It is this feature that gives the adjustable parallel the ability to be adjusted to any height within the range specified. The width of the bearing surface between the two members will be 1 ⁄ 2 the total thickness of the parallel. The two members are held in place and fixed by a locking screw arrangement that secures the two members and prevents them from moving or slipping after they have been set. Adjustable parallels are supplied in either individual pieces or in sets that cover a wide range. The adjustable parallel will conform to the tolerances defined in Table 5 and will appear similar to Fig. 4. A bilateral tolerance will be applied to the parallelism and flatness Table 5. Tolerance Limits for Type IV - Adjustable Parallel Size (range) (in.) Length (in.) Thickness (in.) Parallelism (in.) Straightness (flatness) (in.) Lock Screws 0.375–0.500 1.750 9 ⁄ 32 0.0005 0.00005 1 0.500–0.6875 2.125 9 ⁄ 32 0.0005 0.00005 1 0.6875–0.9375 2.6875 9 ⁄ 32 0.0007 0.00007 1 0.9375–1.3125 3.5625 9 ⁄ 32 0.0007 0.00009 2 1.3125–1.750 4.1875 9 ⁄ 32 0.0009 0.00010 2 1.750–2.250 5.0625 9 ⁄ 32 0.0009 0.00012 2 Granite Parallel, High-Precision.— The granite parallel is designed for the very high precision applications where thermal or magnetic properties must be taken into consider ation. The granite parallel is made with a much tighter tolerance than the cast-iron or steel
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