Machinery's Handbook, 31st Edition
Parallel Bars
749
Parallel Bars Parallel bars are used for workpiece support during layout, machining, and inspection operations. Parallel bars are made from either steel, cast iron, or granite, come in a wide range of sizes and are either used alone or in matched pairs. In general, there are four types of parallel bars: Type I, Solid; Type II, Ribbed; Type III, Box; Type IV, Adjust - able. Although the parallel bar is a relatively simple tool, it is still considered a precision instrument and must be handled with the same attention to care and handling as other precision ground and finished supporting gages. Most precision parallels are made from heat-treated steel and hardened to 55 to 60 on the C Rockwell scale. Generally, they have a finish of 8 µ in Ra with a fine finish free from all grind marks, chatter, or cracks. Granite parallel bars are also very desirable in an environment where thermal expansion is a con- sideration or where steel or cast iron would not be acceptable. Granite parallel bars are made to a very high degree of accuracy and can be used in a precision inspection setup. Type I, Solid.— Type I parallel bars are designed to be used independently as an individ ual bar to aid in inspection setups. They can be used as a reference surface or to establish a vertical plane perpendicular to a base plane. They can also be used as an extended support plane, when used in matched pairs, to establish an elevated plane that is parallel to the base plane. Type I parallel bars have four finished sides that shall not vary from a true plane by more than 0.0002 inch per foot (0.005mm/300mm). The adjacent sides will be square to each other within 0.0005 inch (0.012mm). Type I parallel bars will appear similar to Fig. 1 and will meet the tolerance specifications stated in Table 1.
Fig. 1. Parallel Bars, Type 1, Solid Table 1. Steel and Cast-Iron Parallel Bar Tolerances, Inch Sizes
Width/Height Variance Matched Pairs (in.)
Tolerance Limits
Straightness (in.)
Parallelism (in.) Inch Sizes
Width/Height (in.)
Size a
(in.)
(in.) 0.002 0.0005 0.0002 0.0002 0.0002 (mm) 0.0500 0.0130 0.0050 0.0050 0.0050
(in.)
(in.)
(in.)
0.0001 0.0001 0.0001 0.00015 0.0002 (mm) 0.0025 0.0025 0.0025 0.0040 0.0050
0.0002 0.0002 0.0002 0.0003 0.0004 (mm) 0.0050 0.0050 0.0050 0.0080 0.0100
0.0002 0.0002 0.0002 0.0003 0.0004 (mm) 0.0050 0.0050 0.0050 0.0080
8 to
1 ⁄
3 ⁄
16
4 to
1 ⁄
3 ⁄
4
4 to 1
3 ⁄
1 ⁄
8
8 to 1
1 1 ⁄
1 ⁄
2
2 to 3
1 1 ⁄
Millimeter Sizes
(mm) 3 to 5 6 to 20 20 to 25 25 to 35 35 to 75
0.0100 a Size as applicable to either width or height dimension specified by the manufacturer.
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