Surface Roughness Measurement Questions and Answers

Answer: A

The centre line average roughness is 5 μm.

Ra is the mean of the absolute deviations from the centre line. For a symmetric triangular profile of peak-to-valley height h, the centre line lies at h/2 and each deviation varies linearly from 0 to h/2, so its average is h/4:

\[ R_{a} = \frac{h}{4} = \frac{20}{4} = 5\ \mu\text{m} \]

10 μm is the amplitude h/2, the largest deviation rather than the average one, and 20 μm is the full peak-to-valley height. 6.67 μm would be h/3.

Answer: A

The ten-point height is about 4.57 μm.

The ten-point height \(R_{z}\) is the average of the five highest peaks minus the average of the five lowest valleys, measured from a line parallel to the mean line.

\[ \begin{aligned} \text{peaks} &: 8.3 + 6.9 + 6.7 + 6.3 + 6.2 = 34.4 \\ \text{valleys} &: 1.92 + 1.97 + 2.01 + 2.81 + 2.82 = 11.53 \\ R_{z} &= (34.4 - 11.53)/5 = 4.57 \end{aligned} \]

The distractors come from using all ten of each instead of the extreme five: the mean of all ten peaks is 5.99, the mean of all ten valleys is 3.09, and their difference is 2.90.

Answer: C

Roughness grade N6 corresponds to 0.8 µm R_a.

The ISO roughness grade numbers double the R_a value at each step: N5 is 0.4 µm, N6 is 0.8 µm, N7 is 1.6 µm and N8 is 3.2 µm. N6 is a fine ground or reamed finish.

25 µm is grade N11, a rough sawn or flame-cut surface. 0.1 µm is N3, a lapped or polished finish. 8 µm is not one of the grade values at all; it lies between N9 (6.3 µm) and N10 (12.5 µm).

Answer: C

The Ra value refers to the roughness of surface.

Ra is the arithmetical mean roughness: the average of the absolute heights of the surface profile above and below its mean line, measured over a sampling length, usually in micrometres. A smaller Ra means a smoother surface; a fine ground surface might be around 0.4 µm, a rough turned one several micrometres.

Ra does not describe the coating on a surface, the heat treatment it has had, or its hardness. Those are specified separately on a drawing.