A device measuring variations in ball cups comprises a ball (6) mounted in a small depression in a flate plate (2). The components are held together by a weld (9) and a straight slot is cut in both the plate and ball. The slot passes through the widest point of the ball dividing it into two equal halves. In operation a master ball cup (16) is centred by being pressed against the ball by means of a spring (18). The point of a measuring probe (20) is urged against the cup and a standard measurement obtained. Subsequently, further ball cups are centred and measured in a similar manner.
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