The knock-out tool is an example of how tools evolve to perform tasks easier, faster, and more efficiently, helping electricians get more done in less time.
Knock-out tools—also called knock-out punches—are a single-purpose tool designed to make clean, round, conduit-size holes in electrical boxes and panels.
Once, this task was performed by drilling a small hole and reaming it to the required size. Electricians used step bits, metal-cutting hole saws, and chisels.
The beginning of those days came in 1928 when Greenlee introduced a line of knock-out tools.
So, knock-out tools have been available long before today’s electricians were even apprentices.
Knock-out Punch Set
A knock-out punch is an electrician’s favorite tool for making new holes in an electrical box or panel. A good knock-out punch set gives you the choice of many different sizes of knock-outs. Classic manual knock-out punches are operated with a socket wrench.
There are also heavy-duty knock-out 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 knock-out punch is to use a hole saw and drill, but a punch makes a cleaner hole with less effort and noise.
First, a pilot hole is drilled slightly larger than the screw of the knock-out punch. Then the die is placed on the screw, and the screw is inserted into the pilot hole. The screw is then threaded into the punch, and the screw is tightened until the punch is drawn entirely through the sheet metal.
The manual system uses a screw with a standard hex head or square head and is driven using an Allen key or wrench. A manual knock-out punch can handle holes from 0.5 to 1.25 in (13 to 32 mm). The ratchet system has a custom ratcheting wrench that uses a ball screw to make the process faster and easier.
This system has a mechanical advantage of approximately 220:1 and can punch holes up to 3 in (76 mm) in diameter in 10 gauge mild steel. A hydraulic system is much bulkier and heavier than the other systems, but it is the easiest to use and can make holes up to 6 in (150 mm) in diameter. It is a two-piece system where the dies are attached to the ram, connected to the hydraulic unit via a flexible hose.
Size families and shapes
There are several sizing systems for these punches. The two most common are those sized for standard electrical knock-out sizes and those for true dimensional holes. A 3/4 inch conduit size punch punches a hole that is approximately 1.1 inches in diameter for a 3/4 nominal size conduit.
A dimensional size punch makes a hole very close to the indicated size. Punch sets are available in both imperial and metric sizes. Chassis punches are available in several shapes, round being the most common.
Other shapes include square, hexagonal, and unique shapes for holes with key tabs and D-sub connectors. Unique shapes often use square or keyed bolts and a separate nut on the punch end to ensure the punch and die alignment.
How to Use a Knock-out Punch Set
Knock-out punches include three essential parts: a draw stud (basically a large threaded bolt), a die (a metal cylinder), and a punch, which does the cutting. To make a knock-out 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 maybe 3/4″.
- Fit the appropriate size of the 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 material’s backside.
- Turn the draw bolt with a ratchet wrench; this pulls the punch toward the die. Keep turning until the punch cuts 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 Knock-out Punch Set
The best tool for drilling the pilot hole for the draw stud is a step drill bit. This cone-shaped bit drills holes of various sizes and is 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 knock-out 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 knock-out 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.
A knock-out tool’s essential components include a punch and die, draw stud, and device to pull the stud, ranging from hand-operated ratchet wrenches to hand-powered hydraulic pumps battery-powered hydraulics.
Tools are rated for mild steel or stainless steel, and many may also be used to punch holes in aluminum, fiberglass, and plastic. Standard knock-out punch hole sizes range from ½ to 6 inches in diameter.
Improvements through evolution can be a slow process, but several of the most significant advances in knock-out tools are recent developments.
In May 2017, Milwaukee Tool introduced Exact Rapid Reset Draw Studs for use with its established lithium-ion-powered M18 Force Logic knock-out system.