When it comes to drawing a part to be manufactured for real, it is not necessary to add an SF specification to each and every feature. The vast majority of features do not need them since the common manufacturing processes achieve the SF required and more often than not, the SF is unimportant. It is only in a few instances, where a surface is functionally important, that it is necessary to define a SF. Indeed, specifying a SF is the exception rather than the rule and I have seen many drawings that do not have any SF specifications on them at all!
Note that the vice assembly drawing in Figure 3.1 has no SF specification. This should not be surprising since it is an assembly drawing with no manufacturing information. The movable jaw drawing in Figure 3.2 has just one SF specification. This is for the two bottom surfaces of the jaw where it contacts the body. In this case a fine SF (Rz < 0,2um) is required to minimise friction and ease movement. Such a fine SF can be easily achieved by polishing. Although not shown, there would be a complementary SF specification on the body detail drawing. There are no SF requirements on the hardened insert drawing in Figure 3.3 simply because they are not needed for the correct functioning of the vice.
With regard to the SF parameter values produced by common manufacturing processes, it is unfortunate that few SF parameter values have been published but many have been published in research papers. Books that give details of some SF parameter values are those of Dagnall (1998) and Mummery (1990). Griffiths (2001) gives the results of an investigation linking 2D and 3D SF parameters to common manufacturing processes. The graph in Figure 6.18 compares surface heights and lengths in the form of the 2D parameters Rz and RSm. These parameters are the average height and average length and therefore represent the average 'unit event' dimensions. The ratio of length to height varies from less than 10:1 to greater than 100:1, with an average in the region of
T Surface Gnncltig
O CyftXbiCfll Grinding
O Hen MUHng
X OrfHkig ir Shaping
Average Peak Spacing (Rsn) um
Figure 6.18 Surface finish parameters Rz and RSm for a range of common manufacturing processes
10:1. On the diagram, best-fit least-squares lines are drawn for each of the individual processes. They show that the unit event dimensions or the height to length ratio varies between processes. This can be represented by the equation:
where A and B are constants. As a first order approximation, one can say from the figure that the 'B' values are fairly constant whereas the 'A' value varies for each process. The largest Rz/RSm ratios correspond to the abrasive unit event processes like grinding and lapping and the smallest ratios correspond to cutting processes like turning and milling. Furthermore, the former processes tend to produce lower surface roughnesses than the latter.
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