A knockout punch is an electrician's favorite tool for making new holes in an electrical box or panel. A knockout punch set gives you the choice of many different sizes of knockouts. Classic manual knockout punches are operated with a socket wrench. There are also heavy-duty knockout punches that use a hydraulic pump, but these are overkill for most residential wiring. Most punches will cut through mild steel up to 10 gauge, plus stainless steel, plastic, and fiberglass.
The alternative to a knockout punch is to use a hole saw and drill, but a punch makes a cleaner hole with less effort and noise.
How to Use a Knockout Punch Set
Knockout punches include three basic parts: a draw stud (basically a large threaded bolt), a die (a metal cylinder) and a punch, which does the cutting. To make a knockout hole:
- Drill a hole through the material, using a drill and metal-cutting bit. The hole must be sized for the draw stud. Small draw studs are usually 3/8" in diameter, while large studs may be 3/4".
- Fit the appropriate size of die onto the draw stud, then insert the draw stud into the pilot hole.
- Thread the desired size of punch onto the draw stud and hand-tighten it to the backside of the material.
- Turn the draw bolt with a ratchet wrench; this pulls the punch toward the die. Keep turning until the punch cuts all the way through the material.
- Separate the draw stud and punch and remove the little ring of waste material lodged inside the die. This is called a slug and usually is split in half by the punch, giving this kind of punch the nickname "slug-buster."
Tips for Using a Knockout Punch Set
The best tool for drilling the pilot hole for the draw stud is a step drill bit. This cone-shaped bit drill holes of a variety of sizes and are made for sheet metal and other thin materials. Self-tapping or self-drilling step bits don't need a pilot hole to get started.
If you've ever drilled a hole in metal you know it can be impossible to drill a large hole without using a pilot hole or making progressively larger holes.
Some knockout punch sets include a ratchet wrench sized for the draw studs in the set. This is handy to have because the wrench and punches are always in the same place. No searching around for a standard ratchet wrench and socket or having to resort to an adjustable wrench in a pinch.
A good standard set of knockout punches comes with punches for 1/2", 3/4", 1", and 1 1/4", sometimes up to 2", and includes a small and large draw stud. Larger sets cut holes like 2", 3", 4" and larger but may require a different punch for each sized hole.