of a housing surrounding the rotatable cam members primary and secondary rotatable cams or barrel cams 32 and 34 are generally cylindrical in shape each having suitably formed helical c1m grooves extending around the periphery thereof primary cam 32 has a single helical cam groove 46 provided on the surface thereof the pitch of cam groove 46 varies over the axial length of primary cam 32 generally symmetrically outwardly from the longitudinal center thereof the pitch of cam groove 46 is substantially zero at the longitudinal center of primary cam 32 so as to provide a period of dwell and has an increasing pitch as the groove progresses toward each of the opposite ends 47 and 49 thereof so as to provide acceleration deceleration portions on opposite sides of the dwell portion secondary cam 34 has a diameter greater than primary cam 32 and as shown includes three helical cam grooves 48 50 and 52 on the periphery thereof each of which has a constant pitch over the axial length thereof and a transverse width greater than that of groove 46 as shown in fig3 cam groove 48 is substantially deeper than either of the other two cam grooves 50 and 52 provided on secondary cam 34 and is positioned in circumferentially aligned relationship with cam groove 46 provided on primary cam 32 while it would be possible to provide only a single cam groove such as groove 48 on secondary cam 34 the provision of additional grooves enables the axial length thereof and hence the cost of fabrication to.