In motor cars brake drums are very rare, thus the turning off of the drums on a lathe is hardly ever done any more. Considering the spare-part prices and the hourly rates charged in the workshops, it is hardly worthwhile. This is different as far as trucks and busses are concerned, although, also in this section drum-brakes have become more seldom. In addition, from the diameter, 2 mm of material, instead of the 1 mm allowed for motor car drums, may be turned off. However, when changing the brake linings, this tolerance is not exploited completely, because, when the vehicle goes into operation again, the permitted thickness of the brake drum could be undercut. The linings are available in various thicknesses. To achieve a good fit between lining and drum, as soon as the measurements of the turned drum are known (see figure 3), the linings are shaved. Because drum and wheel remain together throughout the turning off process, they represent a heavy unit. Therefore, either the machine must have a lifting mechanism (see figure 2) or the workshop must have a chain-lift to be able to re-mount the wheels safely.
If the lathe operates unattended, one should begin with the turning off on the inside of the drum. Otherwise, one would have to, very precisely, determine the stopping point to avoid the risk of a possible crash. When returning to it's starting point, the machine always switches itself off at the right place. For economic reasons, only the absolutely necessary amount of material is turned off. The open side of the drum can often have a larger diameter than the closed side. Therefore, as a rule, several turning operations are necessary. With each pass only a little more material is taken off. This is not the case if there are deeper grooves in the surface. Only after the entire surface has been processed, can one be satisfied with the result. It is important that one the starts turning off on the inside of the drum, because normally, after a great deal of braking, a ledge can develop. The drum can still fit over the linings, but snags when tightening the central nut. It takes a bit of experience, to remove just enough material, that the the wheel can rotate freely, without having to remove the wheel and drum from the lathe, to try it out on the axle. One can easily imagine, what would happen, if at this point, too much material is removed … 11/10