Apparatus for indicating and/or controlling the extent to which a rotary member is rotated

946,516. Lathes. ROLLS-ROYCE Ltd. May 4, 1962, No. 17341/62. Heading B3T. [Also in Division G4] In a lathe control system, Fig. 1, as the cut approaches the desired depth a green light 31 comes on and when the desired depth is reached a red light 30 comes on and at the same time a unit 29 locks the tool carrier against further inward movement as described in Specification 936,716 which is referred to. Overall control is effected by a unit 26 (no details given) which receives signals from a shaft digitizer 24 (see Division G4 and below) indicative of the actual depth of cut and signals from record cards on tape indicative of the desired depth of cut. The shaft position digitizer comprises coarse 57 and fine 56 monophylic scales, Figs. 2 and 3, coupled by continuous gearing to a shaft 23 and is characterised in that transitional errors are reduced by an arrangement whereby the sensing units for the coarse scale are normally clamped to the scale so as to move with it, the clamping only being released, momentarily, each time the fine scale completes a revolution, at which instant the coarse sensing units are retarded with respect to the scale, by one increment of the scale. Thus although the fine and coarse scales are continuously coupled, because of the motion of the coarse sensing units the arrangement is equivalent to an intermittent coupling between the scales. In the particular arrangement described, Fig. 3, photo-electric sensing is used and light transmitted through the coarse scale 57 passes via light guides 73 to fixed cells 75. In operation the member 74, 81 carrying the light guides is normally frictionally clamped to the coarse scale. Each time the fine scale 56 completes a revolution a magnet 46 attached to it passes in close proximity to a second magnet 87. This causes firstly the clamping to be released and secondly, as the fine scale continues to rotate, the retardation of the light guide carrying member 74 by one increment of the coarse scale, this increment being set by stops 93.