illustrated at 18a alignment of the side blocks 20 with the central block 10 is accomplished by means of a generally horizontal channel 24 on each side face 26 of the central block a longitudinal projection 30 extending from an inside face 32 the one facing the central block 10 of each side block 20 engages into the channel 24 in one face of the central block 10 the longitudinal projection 30 and the channel 24 act as a tongue and groove while there is sufficient clearance between the projection 30 and the channel 24 to permit the projection to slide in the channel the fit between the projection and channel is snug enough to hold the side block 20 in alignment with the central block 10 the clamp 9 is held together by a manually operated threaded fastener 34 the fastener 34 used in the invention is a threaded rod which enters the clamp 9 through a smooth center bore 36a in one of the side blocks 20 passes through a longitudinally extending horizontal slot 38 extending transversely through the central block 10 and engages into a center threaded hole 36b in the other side block 20 the slot 38 is used in the central block 10 because it allows the fastener 34 and side blocks 20 to slide along the central block 10 thereby making various clamp opening sizes possible the range.