The hole truth

Make big holes in thin metal accurately and neatly with the right cutting accessory

by Clair D. Urbain

Cutting holes in thin sheet metal can give you fits if you don’t know the facts or your options.

Hole-cutting experts say there are a variety of ways you can achieve fast, accurate and neat holes in metal, and choosing the best hole-making method depends on the number of holes you must make, the time in which you have to do it, your need for accurate, clean holes and safety issues.

“It comes down to cost per hole and whether metal chips are an issue,” says John Nethery, senior product manager at Greenlee.

In this article, four experts provide you with tips and options.

Hole saws
Hole saws are relatively inexpensive and can cut holes up to 6 inches in diameter. They can efficiently cut accurate holes in many material types and thicknesses, says Dave Byrley, director of product development and quality at the M.K. Morse Company.

“If you are hole-sawing metal less than 1/16" thick, clamp a board behind the metal so it doesn’t flex,” says Byrley, who also suggests running the tool at a speed and pressure to continually make chips. “But don’t put too much pressure that the drill stalls or catches on the work.”

To keep chips from filling in the gullet of the hole saw teeth, Byrley suggests drilling a hole just inside the hole saw’s cutting path to allow chips to drop out of the cut as the teeth cut into the metal.

Cutting holes in stainless steel is a different ball game. Here, speed and pressure are important. The experts suggest using level, even pressure while cutting.

“Provide enough pressure to continue cutting the metal to form a chip. If you don’t form a chip, the blade will create work-hardening,” says Byrley.

Adds Peter Chiello, a product manager at Milwaukee Electric Tool Corporation, “You’ll also typically want to cut at a slower rpm. You think of stainless steel being a tougher metal, but it’s really more of a gummy material. Stainless steel gets extremely hard near the drilling area as heat builds up during drilling. Slower rpm reduces work-hardening effect caused by drilling.”

For large projects, use a carbide-tipped hole saw cutter, which is an industrial-grade hole saw with five to eight high-performance carbide cutting teeth, and a slower speed. A cutting fluid helps, but isn’t necessary.

The variety of hole saw tooth configurations allow you to match the hole saw to the job.

“Carbide-tipped hole saws offer longer life in abrasive materials; tungsten carbide grit hole saws can be used on materials that are too hard or abrasive for standard bi-metal hole saws,” says Byrley.

Hole cutters
Hole cutters have hit the market within the last two years and changed how electricians, maintenance workers and others who make holes in metal do their jobs.

If you took an annular cutter and crossed it with a hole saw, you’d end up with a hole cutter. Not all tool manufacturers carry them in their product lines, but those who don’t are looking at them closely.

“We have developed a tungsten carbide-tipped hole saw that has a 13-millimeter shank that fits portable electric drills,” says Lowell Frey, vice president of Champion Cutting Tool Corporation. “It can drill up to 13/64"-thick material in diameters from 9/16" to 3". It can be used on steel, stainless steel, enameled steel, aluminum, plastic, PVC and fiberglass-reinforced plastic.”

Hougen developed a similar product it calls the Holcutter, which is made with precision-ground, heat-treated teeth. Hougen reports these cutters can make a hole three times faster and much cleaner than hole saws in sheet metal and have a much longer use life.

Some companies label them as high-speed steel cutters, but Nethery says some clarification is needed.

“These tools can’t be run at high speeds,” he says. “They are made out of high-speed steel and should be operated at 350 rpm on smaller sizes and 120 rpm for 11/₂" diameter cutters.”

The steel hole cutters are about twice as expensive as a bi-metal hole saw, but Nethery says they will last as much as 10 times longer and make a higher-quality hole.

Greenlee’s carbide hole cutters are equipped with a quick-change arbor that allows the user to precisely complete the pilot hole, then allows the user to attach the cutter to the arbor to make the cut.

“It eliminates the cutter from crashing into the workpiece as the bit for the pilot hole goes through the work piece,” says Nethery. “It also offers greater visibility to assure the hole is on the right location.”

The quick-change arbor also allows users to easily change from one hole size to another; an ejector spring makes slug removal easy.

Step bits
A step bit is a viable option for making holes from 1/2" to 1 3/8" in diameter, says Frank Maletske, technical services manager at Gardner Bender. The drill point is self-centering, so it’s easy to start a hole accurately.

“However, the hole diameter isn’t perfectly sized because the step bit will remove slightly more material than needed before the bottom of the hole reaches the 3/4" size,” he says.

To address this issue, Greenlee developed step bits that have longer lands at popular size intervals so users can stop drilling when it reaches the correct hole size.

Knockout punches
If extremely accurate holes are needed and chip production is a concern, then a knockout punch may be the tool of choice, says Maletske. The downside to them is that they take time.

“First, a stud hole must be drilled. Then, depending on the size of the hole needed, one or more punches must be pulled through the metal,” he says.

While most knockout punches are driven by a ratchet wrench that slowly pulls the punch teeth through the metal, more sophisticated units use hand-operated hydraulic pumps to drive the units.

“Knockout punches make high-quality holes,” says Nethery. “However, they do wear over time, and when the punch needs to be replaced, it’s also a good idea to replace the draw stud. The high forces exerted over time can affect the stud’s threads and could result in less life for the punch.”

In stainless steel, Maletske says a more aggressive knockout punch is needed.

“Punches for stainless steel are made with high-alloy steel,” he says. “Because stainless steel has uneven hardness, the punch may pull through one part of the metal before the others. That means one part of the punch will hit the back side of the knockout while the other teeth continue to cut through the harder steel. That puts great pressure on the knockout punch.”

Typically, punches made for stainless steel have shorter lives. Maletske says you can get longer life by selecting a punch that’s one grade heavier than what you are cutting. s

Clair D. Urbain is the editor of Contractor Tools and Supplies magazine, a sister publication of MRO Today. To learn more, visit www.contractortoolsandsupplies.com.

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