a U-shaped member having a guide leg and a clamp leg parallel to each other and joined together at a bottom end of each by a transverse base leg;

a guide channel formed in said guide leg at an angle to said guide leg and having an entrance for a drill bit on an exterior part and an exit at an interior surface which opposes the workpiece working face;

a plurality of elongated ribs formed on the interior of said guide leg below said guide channel exit, said ribs being parallel, spaced apart and transverse to the vertical axis of said guide leg, and a slot through said guide leg in the area of said plurality of guide leg ribs that is transverse to said plurality of ribs;

a shelf on which a second face of the workpiece that is transverse to the workpiece working face is to rest, said shelf having an edge with a plurality of parallel and vertically spaced ribs to inter-engage at a selected place on said guide leg with said guide leg ribs; and

a screw that extends outwardly from said shelf edge to extend through said slot, wherein the part of the guide leg interior having said ribs is recessed relative to the portion having said guide channel exit and said shelf includes a wall extending above one side of said shelf and whose inner surface is aligned with the part of said guide leg inner surface having said guide channel exit.