flat depending on surface texture end 80 of the finger may yieldingly contact the presented terry material i e be shifted from the dash line position shown in fig1 to the lower full line operating position indicated this finger is then ready to sense the first work 34 corner 34 as it approaches the operating instrumentalities the piston rod 74 is vertically operable against spring return in an air cylinder 82 held by a clamping block 84 secured to the plate 66 a tension spring 81 connected to a pin projecting from the plate 66 yieldingly maintains the arm 64 in contact with the lower end of a vertically adjustable stop screw 83 threaded through the block 84 thereby enabling the detecting mechanism to be suitably adjusted for different thicknesses of pile material it will be understood from the foregoing that when the work contacting end 80 is in normal work contacting position i e engaging the work surface at its effective level an opposite end 86 of the finger 76 is disposed just beneath the beam axis x x as soon as the finger end 80 under influence of the spring 78 drops off the edge of the terry material t or more precisely the terry pile moves away from the end 80 the change is detected by the photo coupler 60 by reason of the mentioned opposite end 86 of the finger rising to block the beam x as a consequence of this beam interception an output signal from the photo coupler 60 is sent to an amplifier 88 fig1 and also fed as amplified to an adjustable turn start timer 90 and a test circuit 92 this circuit 92 checks to determine at the start and end of each corner.