A rotary-to-linear motion conversion device comprises a rotatable shaft (32) and an annular cam surface member (36) encircling the shaft (32) and pivotally mounted on a bracket (44) secured to the shaft (32). The member (36) is engaged by an arm (27) of a pivoted cam follower (26) biased towards the member (36). The portion of the member (36) diametrally opposite the pivot (40) engages a slider (48) rotatable with the shaft (32). The slider (48) engages a flange (56) and a collar (60). Between the collar (60) and a fixed stop (64) are four selectively withdrawable interposers (70, 72, 74 and 76) of increasing incremental widths. Two such devices are included in a typewriter character selection system to control tilt and rotation of a typehead. The tilt and rotate cam members are mounted so that they pivot to vary the cam rise. The rise of the cam is controlled by the interposers under the influence of the keyboard.