provides maximum strength with some range of adjustment of the position of support block 20 toward and away from workpiece 16 an arm 24 is mounted on each support block 20 by a shoulder screw 26 that permits the arm to pivot about an axis centered on screw 26 each arm 24 has a cam surface 28 around each axis with a continuous varying diameter when an arm 24 is moved in the direction indicated by arrow 30 the cam diameter increases increasing clamping pressure on the edge of workpiece 16 in three of the arms 24 shown an extended handle 32 is provide with which the arm is manually moved between clamped and unclamped positions the other arm 24 seen to the lower right in fig1 shows an alternate embodiment where in place of handle 32 a gear segment 34 is provided that engages a gear in a power drive 36 typically a small electrical gear motor to automatically move the upper portion of arm 24 between clamped and unclamped positions as seen in fig1 and 5 grooves 37 similar to screw threads are provided along cam surface 28 preferably the cam surface 28 and power drive surface 34 are suitably hardened or heat treated for strength and durability grooves preferably have sharp apexes and are slanted so that as the grooves engage and move along the edge of workpiece 16 the grooves force the workpiece tightly down against upper plate 14 as well as horizontally in two additional directions against opposing index pins bars and the like and other opposing clamps as shown in fig1 and 2 since clamp assemblies 18 do not extend above the upper surface of workpiece 16 machining tools such as mills can traverse the entire upper surface without interference from or impact.