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Tips on Punching the Right-Sized Hole

RW contributor Buc Fitch had an e-mail discussion with an engineer who wanted to make holes in some electrical project boxes. Despite searching the Web and various hardware and supply stores like Lowes, TrueValue and Tractor Supply, he came up empty-handed.

Here’s yet another method to hang on to your copper cables. Cable theft has included cutting and removing cables from conduits, so Copper Keeper is a method of locking wires or cables in a conduit, discouraging their removal.

The cables are run through four slots or pockets in a rubber stopper. This is inserted into the conduit; once in place, a compression bolt is tightened. As the proper torque is approached, the rubber stopper is compressed against the conduit and wires.

This action locks the wires in the conduit, preventing them from being cut and pulled through the conduit easily. The Copper Keeper also keeps the conduit dry, as it forms an almost waterproof seal, which also deters dirt and debris.

Here’s the link to the Web site:

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(click thumbnail)Fig. 1: Mount cue speakers overhead so everyone can hear.Fig. 1 shows a neat method of mounting round cue speakers, as used by Bill Ryall, engineering manager for the Nassau Maine cluster.

Centering the speakers above the talent, over their head, ensures that everyone in the studio will hear the cue audio clearly.

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Equipment construction projects are not as bountiful now in stations as they were in the distant past, when as much as half of the station might be custom-built by the engineers.

However, nowadays we need to at least drill holes for passing wires or fiber, and when a really big hole is needed for mic connectors and the like, a hole punch is the usual solution.

RW contributor Buc Fitch had an e-mail discussion with an engineer who wanted to make holes in some electrical project boxes. Despite searching the Web and various hardware and supply stores like Lowes, TrueValue and Tractor Supply, he came up empty-handed. Buc pointed the engineer toward Greenlee chassis punches.

With the high cost of precision tools overall, you do not buy a punch that costs $50 for a one-time use, so when Buc found a need recently for an odd diameter foreign connector, he went looking for a punch that he owned that was “close enough.”

The classic Greenlee chassis punches are marked with the diameter of the hole these actually make, but at least two other pedestrian hole punch series are not marked directly with the diameter they punch.

“Knock out” punches as used in the electrical industry come in “trade sizes” that match the diameter of the hole needed for the terminating fixture. For example, a 1/2-inch electric punch makes a 0.886 inch diameter hole (close to 7/8-inch) for the terminating end diameter.

“Pipe punches” from the plumbing industry apparently are designed to punch a hole to clear the outside diameter of the pipe. In small pipe size, pipe is made from the inside out with varying diameters; so a pipe with an inside diameter of, say, standard 1/2-inch is going to have a totally different outside diameter.

While Buc was searching for a suitable punch, he decided to make a chart of the actual hole made by the “knock out” and “pipe” punches, as near as he can determine. We hope this chart will simplify your selection process in the future.

Electrical (Trade size)Actual Hole DiameterInchesInches 1/2 0.886 (a nominal 7/8) 3/4 1.125 1 1.362 (a nominal 1-3/8) 1-1/2 1.953 2 2.421 2-1/2 2-15/16 Pipe SizeActual Hole Diameter 1/2 7/8 3/4 1-1/16 1 1-3/8 1-1/4 1-3/4
Some of the decimal values were taken from the “ridge” markings of the punches and are the manufacturer’s statement of how big a hole they think the punch actually will make.

The most common regular chassis punches are manufactured by Greenlee and are sized in real dimensions. A 3/4-inch punch will make a 3/4-inch diameter hole. These punches are designed for punching thin-wall aluminum panels and chassis.

Greenlee punches and kits aren’t cheap. You can sometimes find punches at a Hamfest, but you’ll learn to guard them with your life.

Not only are the punches costly, once you’ve used one, you’ll wonder why you ever put up with the drill bit-and-file method of creating holes in panels and chassis.

Buc furnishes a link to the Mouser online catalog, a source of Greenlee punches and kits are sold. Select the “page” view in the column next to the item description for the entire punch inventory and pictures. Go to, enter “Greenlee punches” and click Search.

(click thumbnail)Fig. 3: Put your thinking cap on and come up with a use for this cable.You’ll note that there are a few specialty punches that can make life much easier and produce a more professional product. These include the DB series of punches for D-sub connectors, and the K series of punches for relay sockets. Some punches are available off the shelf at Home Depot. Check the electrical department, but note these will be in electrical trade sizes. Chicago Freight offers punches in pipe sizes.

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Ed Histed, formerly programming and operations director of Route 81 Radio, was clearing through some things at the transmitter site over the summer. While cleaning out a box of “stuff” he came across the cable shown in Fig. 3.

He asks whether any readers might have an idea what the cable could have been used for. I have a thought, but will wait until we hear from you.

By the way, which end do you suppose should be plugged in first?