Machinery's Handbook, 31st Edition
750 Parallel Bars Some common size configurations available from most manufacturers of Type I preci sion parallel bars appear in Table 2. This is a representation of sizes that are available; however, it is possible to have parallel bars custom made to a special order for a particular application. In that case, either the tolerance specifications of the engineering drawing or the specifications defined in the Federal Specification GGG-P-61a shall apply. It is at the discretion of the designing engineer which tolerances will apply in this case. Table 2. Common Sizes of Parallel Bars Width and Height (in.) Length ± 0.002 (in.) Width and Height (in.) Length ± 0.002 (in.) Width and Height (in.) Length ± 0.002 (in.)
6 6 6 6 6 6 6 6 6 6 6 6 6
6 6 6 6 6 9 9 9 9 9 9 9
12 12 12 12 12 12 12 12 12 12 12 12
8 – 1
16 – 2 – 2 –
2 – 1
1 ⁄
7 ⁄
7 ⁄
1 ⁄
1 ⁄
8
2
8 – 1
2 – 2
1 ⁄
3 ⁄
1 ⁄ 1 ⁄
5 ⁄ 3 ⁄
1 ⁄
16
8 4
16 –
16 – 1 4 – 1 4 – 2 3 ⁄ 3 ⁄
3 ⁄
7 ⁄
11 ⁄
1 ⁄
8
4
16 – 1
2 –
3 ⁄
1 ⁄
1 ⁄
13 ⁄
8
16
4 – 4 – 4 – 4 – 8 – 8 – 8 –
2 – 1
1 ⁄ 1 ⁄ 1 ⁄ 1 ⁄
3 ⁄ 1 ⁄ 5 ⁄ 3 ⁄ 1 ⁄ 3 ⁄ 7 ⁄
1 ⁄
8 2 8 4 2 4 8
4 – 2 –
1 – 1
1 ⁄ 1 ⁄
3 ⁄ 5 ⁄
1 ⁄
8 8
2
1 – 2 1 – 3 4 – 1
2 – 1
1 ⁄
4 – 1
2 – 1 2 – 1
1 1
1 ⁄
1 ⁄ 1 ⁄
1 ⁄ 1 ⁄
1 ⁄ 1 ⁄
3 ⁄ 1 ⁄
4 2
4 2
4 – 2
3 ⁄
4 – 1
1 1
2 – 2 2 – 3
3 ⁄
3 ⁄
1 ⁄ 1 ⁄
4 – 1
3 ⁄
3 ⁄
1 ⁄
2
3 ⁄ 12 Type II, Ribbed.— The Type II parallel bars are rectangular in cross section and made from a cast gray iron. The casting is ribbed to provide lightness while maintaining strength and rigidity. The ribbing on the Type II parallel bar extends to the outer surfaces of the bar and becomes an in- tegral part of the working surface. The working surfaces of the Type II have a fine ground surface that will not exceed a value of 16 µ in Ra. All the sharp edges are removed, and the surface is free from all machining marks such as grind chatter or burn. The Type II parallel bar is finished on the four sides of its length. The ends not being considered working surfaces are not finished for work applications. The Type II parallel bar is a much more rugged design that the Type I and is designed to support workpieces of larger dimensions. Type II parallel bars are commonly available in sizes up to 4 by 8 inches with a length of 36 inches. Although larger in overall size, the Type II still maintains a very close tolerance in straightness, parallelism, and squareness. 8 – 1 3 ⁄ 8 – 1
Fig. 2. Parallel Bars, Type II, Ribbed After each bar is cast and rough ground, it is subject to a seasoning or aging process. This process can either be natural or artificial. The aging process is necessary to provide long term stability after finish grinding. Once the bar is aged and seasoned, the final dimen sions will remain stable and not warp or twist for many years. Well-made and seasoned castings have been known to be in service for nearly 100 years and still hold their original geometry. Type II parallel bars will appear similar to Fig. 2 and will conform to the
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