Control mechanism for thread rolling attachment

In a thread rolling attachment have a reciprocable operating element for selectively swinging a pair of pattern rolls into engagement with a rotating workpiece, and a fluid pressure cylinder having a piston rod reciprocable at one end in an axial bore in said cylinder between retracted and advanced positions, and projecting at its oposite end slidably out of one end of said cylinder to engage and inpart reciprocation to said operating element, an improved mechanism for controlling the flow of fluid under pressure to said cylinder, comprising

• reversible valve means connected to a first plurality of ports in said cylinder and operable selectively to supply fluid under pressure from a supply thereof alternately to opposite ends of said cylinder, thereby to effect reciprocation of said piston rod,
• said cylinder having in the opposite end thereof a further port opening at one end on the bore in said cylinder and connected at its opposite end to said valve means to effect reversal thereof when fluid under pressure is supplied to said further port from the bore in said cylinder,
• a valve stem movable mounted in said opposite end of said cylinder and having thereon a valve seat normally closing said one end of said further port when said piston rod is in its retracted position, and
• means connecting said valve stem to said piston rod and operative, when said piston rod has moved to its advanced position, momentarily to shift said valve stem and said valve seat in a direction to open said one end of said further port, thereby to admit fluid under pressure from said cylinder bore to said valve means to reverse the latter.