12.1 Cylindricity

The natural extension to roundness and its measurement is sphericity, but in fact easily the most important solid extension is cylindricity. The fact is that the need to generate two rotations is small. A cylinder, on the other hand, allows the possibilities of rotation and/or translation. This is the combination of the circle with a straight line generator.

For many years after roundness instruments had been developed by Reason, the measurement of cylindricity seemed unnecessary. The reason given now sounds naive: 'If the shaft is circular in three places and the surface finish is acceptable the shaft is bound to be a good cylinder' (Figure 12.1).

Figure 12.1: Roundness checks

The assumption was that only a really good machine tool would produce a circular and smooth shaft and by inference a good cylinder. Now cylindricity should be taken more seriously. The problem is that it is complicated.

One look at a crankshaft, for example, is enough to be convinced of the need to measure and to correlate profiles in different planes. A typical situation is shown in Figure 12.2.

Figure 12.2: Possible errors in crankshaft geometry

There are some basic form errors and positional errors that occur regularly in cylindricity. Some are shown in Figure 12.3.

Figure 12.3: Errors of form that can be checked

Eccentricity and concentricity are two of the most important parameters. The assessment is shown in theory and practice in Figure 12.4.

Figure 12.4: Eccentricity assessment (as in Figure 11.65)

Eccentricity assessment = (M...