A Greenlee knockout punch is a tool used to punch a precisely sized hole through metal. Usually, that hole is going into an electrical panel, but sometimes it's through sheet metal or steel studs. It is designed to use minimal force to complete that job. A basic set costs around $30. When you need a hole to mount conduit or cables in an electrical panel, this product will help you do the job.
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What a Set Includes
A basic set includes a draw stud, a die and a matching punch. A combination set may have four or more dies and matching punches along with one or two draw studs. And all combination sets come in a carrying case to keep the pieces organized and together. Depending on the set, it may include a ratchet. The more expensive set includes a hydraulic pump to draw the die into the punch. Unless you’re an electrician, you’ll probably be using a wrench (or a ratchet) on a single set that you purchased for the job.
Laying Out the Hole
Most often you’ll be just adding a conduit or cable to the panel and you’ll only be drilling one hole. But whether it’s one or five, you still need to lay out the holes so they are properly located. When organizing the holes, be sure that they aren’t running into each other or located too close to the edge of the box. For a half-inch punch, place the center of the hole a half-inch from anything else.
Using The Punch
You will need to drill a pilot hole in the center of where you want the hole. The size depends on the draw stud. You need to drill a hole large enough for the draw stud to go through it. Put the draw stud through the die and push it into the drilled hole. On the other side of the metal, thread the matching punch onto the draw stud and tighten it by hand until the metal is tight between the punch and die. Use a wrench or ratchet to further tighten the draw stud. As you turn the stud tighter and tighter, the die will cut through the metal until it breaks out and leaves a nice neat hole behind. If you’re working on an electrical panel you can put in a ROMEX® connector for your cables or a conduit connector to attach your conduit to. And if you’re making holes in metal studs, you’ll have no problem running the circuit from one end to the other.