A knockout punch is a tool or set of tools used to create a hole in electrical boxes or panels. These tools also cut through heavy sheet metal, plastic and fiberglass. The two-part knockout punch includes a cutting head or die and a threaded draw stud. The tool sells separately or in sets that include various sizes standard to electrical, plumbing and other types of industry.
Most types of electrical panels today are manufactured with pre-stamped knockout plugs, which eliminate the need for a knockout tool. The pre-stamped knockout plugs remove easily using a pair of electrical pliers or similar tool by punching the plug and twisting it loose. Conduit connects to these holes to accommodate additional electrical circuits. In certain situations, creating additional holes or enlarging existing ones is required to accommodate the conduits.
Operation of a knockout punch is performed either manually, or with a hydraulic pump attachment for easier handling. Both types of knockout punches require drilling a hole or using an existing hole in the electrical box or panel. The threaded draw stud inserts through the hole. Then, the cutting head or die threads onto the draw stud until it is snug against the electrical panel.
Manual knockout punches have a large bolt head located on the draw stud. An adjustable wrench, standard wrench or socket tightens the bolt head. This tightening then pulls the cutting head through the material, creating a suitably sized hole.
Hydraulic versions of the knockout punch use a hydraulic hand pump for easier operation. The handheld hydraulic pump resembles a common grease gun. It attaches to the draw stud and is manually pumped to draw the cutting head or die through the material to create the hole.
Common knockout punch sets include a case, dies — or cutting tools — and draw studs. These are designed to create standard conduit size punches ranging from 0.5 inch (about 1 cm) through 2 inches (about 5 cm) and from 2.5 inches (about 6.3 cm) through 4 inches (about 10 cm). Hydraulic versions of knockout punch sets also include a hydraulic hand pump. The pieces in the set are typically manufactured out of high carbon steel, allowing the knockout punches to cut through lighter steel with a thickness of up to 10 gauge (about 25 cm).