The finest screw clamps available for assembling and repairing sensitive antique instruments as well as assembling new instruments. The clamping pads apply pressure directly over the instrument's ribs to prevent damage to the edges of the top and back. The clamps can be set with an accuracy which significantly reduces clamping pressure compared to conventional clamps. The toxin-free plastic repels glue, is resistant to wear and flexible to prevent marking the surfaces of the instrument. The pads slide along grooves in the length of the spindles to allow quick adjustment of the clamp. The reversible upper pads allow the clamps to be also used for gluing the ribs to the back of the instrument.