to control the flow of air to the up down actuating cylinder 178 and at the same time to control the stroke of the carriage cylinder 174 a bolt 184 is secured to a threaded passage in the carriage assembly 172 and is in alignment with the limit switch 180 when the head of the bolt 184 contacts the limit switch 180 the operating pressure within the cylinder 174 spikes upward and this spike triggers a control circuit shown schematically at 412 fig2 to stop the motion of cylinder 174 at the same time contact between bolt 184 fig6 and limit switch 180 opens this normally closed limit switch and shuts off air pressure to cylinder 178 the spring within cylinder 178 then lifts the wiper assembly 176 from the silk screen a similar bolt 186 cooperates with the limit switch 182 to define the end of movement of the carriage assembly 172 in the opposite direction toward the upper right in fig6 and to lift the wiper assembly 176 from the silk screen at the end of the stroke in that direction the bolts 184 and 186 may be adjusted lengthwise to vary the stroke of the carriage cylinder 174.