Mag drills make the cut

Magnetic-based drills take machining ability to the jobsite for accurate and productive holes.

You can’t beat a machining center to make accurate, productive holes in steel. The only drawback is that you must take the work to the machining center.

But with a magnetic-based (mag) drill outfitted with a twist drill bit or annular cutter, you can take the power, cutting accuracy and productivity of a machining center to a jobsite.

The powerful electromagnetic bases on today’s mag drills provide the holding power needed to cut holes through thick steel.

“The key to these tools is portability. They may not cut as fast as a machine in the shop, but they allow you to make holes on the jobsite, not in the shop. We even offer hydraulic tools that can be used down to 450' underwater to work on oil rigs. Plus, today’s 25 lb. mag drills can do what older 70 lb. mag drills did in 1974,” says Bill Hildebrand, director of market development at Jancy Engineering.

Mag drills have been around for about 100 years and were first used with twist drill bits. However, in the last 30 years, they have been used with annular cutters used in machining processes. This has allowed smaller mag drills to make larger holes than what is possible with a twist drill. “Twist drills can only drill a 1 1/4" maximum diameter hole in a single pass. An annular cutter can do a hole up to 4" in diameter,” Hildebrand says.

Mag drill selection
There are several mag drill models available and selecting the right model for the job is as important as using it correctly, says Ernie Leopold, product manager of metal working products at Fein Power Tools. “Mag drill selection is driven by the width and depth of the hole and the space in which the tool can work. Some models also have a reverse feature, adding versatility for tapping through and blind holes. Heavier mag drills are needed for larger holes. Most models can be used with an annular cutter, which can cut up to 40 percent faster than a twist drill bit. They are good for larger diameter holes; less material is removed using an annular cutter vs. a twist drill bit.

“The annular cutting process also requires much less machine power because it cuts a plug or slug rather than cutting material out of the entire hole. You don’t remove all of the material with an annular cutter,” he says.

The magnetic base affects the tool’s ability to cut, says Leopold. “A smaller footprint means less stability and sometimes less holding power. The holding power also varies from brand to brand,” he says.

Selection of a mag drill depends on the application at hand. Every mag drill is a bit different and some are designed for specific uses.

“Eighty percent of the holes cut with a mag drill are 1 1/16" or less. That’s why smaller units are coming on the market and are very popular,” says Jim Kaiser, national sales and marketing manger at Hougen Manufacturing. “They are built for tight locations. They were designed to be much shorter so they could meet their original task and market of making holes in truck frames without pulling the wheels. When fabricators and bridge builders saw the HMD 115 and 150 models, they liked them and adapted the units for their jobs. The units weigh only 20 lbs. We find that most of our products evolve into other uses than the original one for which it was designed.”

“These are very straightforward machines. Some are for multi-use, offering the ability to use annular cutters, be fitted with a Jacobs chuck for twist drill use or used with taps to thread holes that have just been drilled. These units may not be as productive, but they are more versatile,” says Kaiser.

“When choosing a mag drill, consider the power-to-weight ratio. If workers must use the tool for awkward upside down or horizontal work, they will appreciate a lighter weight tool. There is a tremendous difference between positioning a 30 lb. tool vs. a 50 lb. tool,” says Leopold. “Most mag drills are built to be used in awkward positions, so they have a secondary means of support for the drill in case the magnet loses its bond. Use that strap or clamp. It will help protect the tool and the operator in case the base breaks free from the workpiece,” he says. Also, use the guard that comes with the machine to protect the operator from the annular cutter and any metal chips being removed.

The annular cutter is a smaller machine and has a faster speed of cut, which helps reduce operator fatigue. However, they are limited only to being through-hole machines. “Twist drills are only a third as fast as annular cutters and can take as much as 10 times the thrust of an annular cutter and a twist drill needs more through-hole stroke. A twist drill may need 1/2" additional travel to start and complete the cut due to the lead angle of the bit’s flutes,” adds Hildebrand.

Get a grip
Getting a good magnetic grip is key to safety and productivity. For that, you need adequate metal thickness and a good, clean surface. “The metal to which the base will attach should be at least 3/8" thick. Any thinner material requires additional metal backing or additional clamping. The holding power exponentially decreases as metal thickness decreases. You can provide additional thickness by backing up the material with a metal plate, but it’s not the same as having a continuous thickness of metal. Another option is to use a clamp or locking pliers on material less than 3/8" thick,” says Kaiser.

The magnet is one of the most critical parts of the drill, says Hildebrand. “The drill point’s breakaway force, which is the force of how much pressure can be put on the magnet, diminishes as the machine’s design positions the bit holder farther away from the center, and can affect its leverage. That’s why a machine running a twist drill must have a bigger magnet than one running an annular cutter because the total energy of the twist drill requires more downward pressure than an annular cutter.

Also, look at magnet protection. Most tools have a relay that will not allow the drill to start unless the magnet is energized. Plus, Hougen offers a patented safety switch that automatically shuts the tool off if the magnet loses its grip on the material being cut.

If you must make holes into stainless steel or other nonferrous materials, clamps can be used to hold the base to the material. Or, a vacuum base attachment, available on several mag drill models, can be used to mount the base to the material being cut.

Use coolant!
No matter what size of hole is being drilled, the experts advise that using coolant is a good idea. Most models are equipped with a reservoir that can meter lubricant into the center of the cut for annular cutters. This is very important for annular cutters.

“Coolants are typically water-soluble/biodegradable oils that are easy to clean up and paint over. Coolants take heat out of the cut.

“They are generally applied with a gravity drip from the reservoir on the machine or with an oil can. Holes that are more than 1/2" in depth need internal coolant. Using internal cooling can increase tool life by as much as 30 percent,” says Hildebrand.

In horizontal or overhead applications, use a beeswax-based or paste-type lubricant.

When applied to the cutter, it melts as the cutter and material heat up and provides lubricity.

“It is not good as a liquid coolant, but works well where gravity is working against you,” says Kaiser.

Use the right speed
Full-sized and oversized mag drills may offer two or even four speeds. Use slower speeds for larger diameter cuts or harder materials. Look at the annular cutter box or the drill’s operating manual, or even on the machine for speed guidelines for hole size and material type, says Hildebrand.

Jim Mullin, division manager at Evolution Power Tools, concurs. “A good rule of thumb is to run a high-speed steel cutter at 385 rpm divided by the cutter diameter. Also, 550 rpm is okay on holes up to 1 1/2" but on any larger holes, the higher speed will cause chattering. Most small mag drills are single speed and medium-sized machines often have a high-low setting and work well for cutting up to 9/16" holes. However, some drills with the correct baseline speed can cut up to 2" diameter holes,” he says.

To cut stainless steel, Mullin suggests dropping cutter speed to 190 rpm and using cutters designed specifically for this task.

Feed rates
Making holes with twist drill bits takes tremendous pressure to shave the metal away. The drill bit has only two cutting edges and it must remove all of the metal in the hole; an annular cutter has anywhere from four to 12 cutting edges and removes only enough metal to create the hole and leave a slug.

Both cutting tools require appropriate feed rates for top productivity, however, an annular cutter will generally beat a twist drill in speed.

It’s a matter of touch and feel to get the proper feed rate, says Kaiser. “Feeding too slowly will result in no cutting and heat buildup, but feeding too fast doesn’t work, either. The lower rpm from the unit dogging down greatly reduces the metal removal rate. You can feel the cutting tool work and the appropriate feed rate should produce a long, stringy, stiff chip.”

Feed rate can be automated on some models, reports Kaiser. “A self-feed model can take operator variability out of the cutting equation. The operator simply sets the unit up and lets the unit make the cut. It monitors feed pressure against rpm to get greater productivity and greater hole uniformity. It also allows workers to multi-task while cutting the holes.”

Stay sharp
No matter how well the cutter is made, it will get dull over time and affect its hole-making ability. “A good sign that it’s time to sharpen the cutter is when you can’t pull a good chip and the cutter chatters in the hole, even at slower speeds,” says Mullin.

When you look at mag drills, consider it as a system, says Hildebrand. “It is all about cost per hole. Even though the mag drill and cutters are more expensive than a drill and a twist bit, they offer a lower cost per hole. For example, a 1" x 1" annular cutter may cost about $50, but it can be resharpened about six times at less than $10 per sharpening. That really reduces the cost per hole. If you are looking at longevity, the best measure is number of inches cut, not number of holes,” he suggests.

Published in the September/October 2007 issue of Contractor Tools and Supplies magazine.

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