# Chapter

51. True or false? All answers can be found in the text or in the figures in Chapter 6.

■ All surfaces contain short and long wavelength components.

■ The evaluation length is five times the sample length.

■ The parameter 'Rq3' is the RMS value of the third sample length.

■ The parameter 'Ra' is a spacing parameter.

■ The parameter 'Ra' is the most important parameter.

■ The 16% rule says the surface is considered acceptable if more than 16% of the measured values are less than the value specified.

■ Only the 'Ra' value can be shown at position 'X' on the tick symbol.

■ The 'point' of the tick symbol should be placed on the surface outline or an extension to it.

■ It is important to specify the surface finish of each and every surface of a part.

■ The surface lay symbol 'C' means the surface must be cut.

52. Using a surface finish parameter of your choice, explain the formula 'TnN' (Section 6.3).

53. Obtain a set of random numbers. Assume them to be in microns. Assume that they are the sample length data points. Using this set:

■ calculate the equivalents of Rz, Ra, Rp and Rv values;

■ check that the skew parameter (Rsk) value is zero for the random set.

54. Explain the meaning of the following terms: 'sample length'' and 'evaluation length' (Section 6.2).

55. If the evaluation length peak to valley height of the schematic profile in Figure 6.9 is 10 microns, use scaled measurements to calculate:

■ the five sample length Rz values;

■ the five sample length Rv values;

■ the five sample length Rp values.

56. Explain why it is necessary to use a filter on a set of raw surface finish data before calculating any of the roughness parameters (Section 6.2).

57. Explain the meaning of the following terms: BAC, ADF (or HDF), PnN, ms, PC filter, Rmr(c).

58. Draw the ISO 1302 (or BS 308) 'tick' symbol for the following conditions (Section 6.5):

■ The specified surface is to be polished such that, when it is measured using a profilometer set for a sampling length of 0,25 mm, the Rz value must be less than l,0|im.

■ The specified surface is to be lapped such that the surface finish is between 0,2 and 0,4um Ra when the sampling length is 0,25 mm.

■ The specified surface is to have a maximum surface finish of Rz = 3,0um using the 16% rule. The surface is to be ground such that the lay is perpendicular to the plane of the drawing. All other values are to be the default ones.

■ The specified surface is to be cast and no machining is permitted after casting. The surface finish must be no greater than Rq = l,0um.

59. Sometimes it is beneficial to use average SF parameters like Ra in preference to extreme parameters like Rz. As a first order approximation, one can say that 9Ra = Rz. Using this value, convert the various Rz values given in Figure 6.18 to Ra values and create a new process capability table of Ra against manufacturing process similar to the one in Figure 5.6.

60. Figure 6.16 shows the lay classes according to ISO 1302:2001. Research the lay produced by manufacturing processes and add another column to the table in Figure 5.6 stating the lay class.