## 222 Surface roughness chart

Figure 2.8 shows the range of surface roughness values likely for various manufacturing processes. The ranges determined are bounded within the risk index, A, in the same way as the process capability maps, because a similar cost-surface finish relationship exists, as suggested for tolerance and cost. This is shown in Figure 2.14 for several machining processes. The finer the surface finish required, the longer the manufacturing time, thereby increasing the cost (Kalpakjian, 1995).

Select manufacturing process and material combination to be investigated

Select component design with ideal processing geometry

Find out the working dimensional limit of the process, 0mâ€ž

Sel three characteristic dimensions at:

D, - 0.1Dma,: D2 - 0-5Dma, Dj - Q.9D, and using standard company practice, manufacture 3 batches of 30 components fi,. 81 and 03 with dimensions D2 and respectively

 Measure (to two decimal places) the actual values of the 30 characteristic dimensions in Si, labelling them X, ^ > r Calculate the starcfard deviation a-, for batch 8, using the formula: 30 1 r Calculate the tolerances for use on the process capability map: T1A = 4(7] and 7,0 â€” So-,

Repeat the above three steps to find TM and for Ba and Tw and rS[i for fla respectively

Label the graph noting the manufacturing process and material processed

Figure 2.13 Process capability map construction flow chart

22 20 18

I 16

22 20 18

I 16

 Cylindrical grinding -\ Surface grinding T End milling "Y \ ^ Reaming \ \Turning\\/ \ \ i \ \ \ \ * \ \ \ \ V \\ Peripheral milling / Shaping and planing - vv^ Drilling i i ;

0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 Surface finish (^m t

0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 Surface finish (^m t

Figure 2.14 Relative manufacturing time as a function of surface roughness for several machining processes (BS 1134, 1990)

For a given manufacturing process, shown on the vertical axis in Figure 2.8, and design surface roughness, shown on the horizontal axis, the risk index 'A' on the shaded band to the right is taken as representative of the risk of obtaining the desired surface roughness. At A = 1, this corresponds to a surface roughness that is easily achieved by the manufacturing process. The intermediate values of 'A' (A = 1.3, A = 1.7, A = 3) correspond to increasing technical difficulty/cost to achieve the design surface roughness. A = 9 corresponds to a surface roughness that is technically unattainable using the manufacturing process. A secondary (and possibly a tertiary) process may be required to achieve the design surface roughness. All surface roughness values are in ^ Ra, which is the most common format.

0 0