plate 18 and scroll plate 21 because of the lost motion connection of the cam follower block 79 and slot 77 and the direct positive angular connection between the cam follower block 78 and its slot 76 this relative rotation produces quick opening of the dies 19 to thereafter permit continued reverse rotation of the motor to axially retract the spindle 24 and cutter head 12 to their starting position a second limit switch not shown detects arrival at the starting position and discontinues operation of the motor 16 until the workpiece 11 is removed and replaced with a new workpiece the cutting head 12 is selectively adjusted to thread bars 11 of different diameters by changing the position of the shoulder bolt 52 and cam follower block 79 on the scroll guide 46 as previously disclosed several threaded holes 51 with a radial orientation with respect to the axis 13 are angularly spaced on the arcuate outer surface of the scroll guide 46 for this purpose the constant rise angle of the scroll plate cam grooves 41 allows the simple change in position of the bolt 52 and block 79 on the scroll guide 46 to be essentially all that is necessary in the way of adjustment to accommodate changes in bar diameter fig5 through 8 illustrate details of one form of a preferred die 19 for use in the apparatus 10 in forming tapered threads the die 19 is provided in the form of a generally.