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 registered on the gauge or recorder attached to the press is too small the wheel is taken off again, and another substituted. In the case of driving and coupled wheels, additional security is provided by driving keys into keyways cut into both wheel and axle, but in the case of other wheels no keys are used. .

Tyres. The tyres, which come from the steel manufacturers, are rolled without weld. They are bored inside to an internal diameter slightly less than the outside diameter of the wheel centre, on to which they have to be shrunk, the allowance being about ¹⁄₁₀₀₀ of the diameter of the wheel centre. Formerly tyres were bored in a wheel lathe, but it is found much more convenient and expeditious to bore them on a boring and turning mill of the type shown in Fig. 26. The tyres are placed horizontally on a revolving table as shown in Fig. 27. The tool holders