The “hole” story

How to be more productive drilling holes in concrete

by Kay Falk

Size, geometry, design, bit composition, concrete mix, proper tools and drilling technique all play a role in successful concrete drilling, according to bit and anchor manufacturers. They say misconceptions about these crucial elements exist among contractors and end-users, which affects their hole-making productivity.

Most agree carbide-tipped bits are best for concrete drilling, but users’ actions can greatly affect their life.

“One common misconception is that some carbide bits are suited for drilling through rebar. Although it would be ideal to find one bit that could drill masonry and steel effectively, current carbide, diamond and such products are optimized for one or the other,” says Erich Herman, DeWalt masonry drilling product manager.

“If you encounter rebar, be prepared to relocate the hole or use a product specifically designed for drilling through rebar,” he says. 

Mike Dill, ITW Buildex market development manager, says bit life is greatly overestimated on jobsites. “Most users expect carbide bits to last indefinitely when drilling concrete, block or brick. This may be true when drilling hollow block, but it’s not true for concrete.

“If the local aggregate in the concrete mix is very hard, drilling is difficult and slow. This causes carbide to wear faster.”

Tom Carroll, president of CS Unitec Inc., agrees that carbide bits shouldn’t be used where hard aggregate, rebar or steel reinforcing wire are part of the concrete. “Diamond bits and wet or dry drilling machines can handle these materials,” he says. “There is a wide range of lightweight handheld diamond core drilling equipment for hole diameters from 3/8" to 12". When cutting blind anchor or through-holes, diamond bits and their machines are more efficient because they are lightweight and save crews time. They drill faster, the bits last longer and the safety slip clutch helps protect the user from jams.”

Harder not better
Ian Murphy, product manager for Simpson Strong-Tie Anchor Systems, points out that users often believe that harder carbide is better. “Concrete bits make their holes by chiseling the concrete, not cutting it,” he says. “If the carbide is too hard, it can shatter upon impact.”

Some contractors also believe that sharp-ground carbide is better. “This isn’t necessarily true,” Murphy says. “For hammer drilling, a sharp edge can be easily chipped, causing the whole piece of carbide to fail under impact stresses.”

When it comes to bit size, the larger the piece of carbide on the drill bit, the longer that bit will last, says Herman. “Most users don’t understand that carbide tip geometry is as important as its size.”

DeWalt recommends selecting a bit with a flatter carbide tip. “A flatter tip has bigger shoulders and places more carbide in the wear area — the circumference of the hole being drilled — and can grind away more material over the life of the bit,” Herman says, “Compared with a pointed tip, as a flatter tip wears, it will drill a full size hole longer. Pointed tips wear away quickly, drilling smaller and smaller holes until an anchor will no longer fit in the hole.”

Lenny Colasuonno, vice president of sales and marketing at Powers Fasteners, says a larger headed drill bit will drill faster This is a common misconception among users.

“We have found that some of the new Tri Cutter bits and Y cutter bits actually drill faster because the head does not take up the circumference of the hole,” he says. “The concrete dust extracts faster and increases the drilling speed.”

Still other end-users think you can drill faster by leaning or putting pressure on the drill. “This actually slows drilling because you are not letting the drill motor do the work,” Colasuonno says. “All hammer drills strike the back of the drill bit while turning clockwise. Putting pressure on the drill slows drill speed and reduces bit and tool life.”

Use tips
The manufacturers suggest several ways contractors can lower the cost per hole and obtain greater productivity when drilling in concrete:

• Make sure the impact energy of the hammer transfers all the way to the tip of the bit. “Use a rotary hammer that is rated larger than the bit being used. Avoid using chuck adapters and always use the shortest bit possible,” Herman says.

• Let the tool do the work. Don’t push harder on a rotary hammer to increase drilling speed. “Pushing harder overworks the drill motor and can cause premature failure of the rotary hammer,” Herman says. “It can cause the bit to bind, create excess dust in the hole and may even overheat the carbide tip.”

Dill adds, “Apply only enough pressure to advance the bit until you see material moving out of the hole. It may seem like it’s taking more time, but you’re not saving any time or money by constantly changing (and buying) bits.”

• Use proper tools and techniques. You want a power tool that chips and rotates simultaneously, such as hammer drills or rotary hammer drills. “A standard drill only rotates and the user’s muscle is what advances the bit,” Dill says. “In this case, the carbide and the brazing that holds it to the shank will overheat and the bit tip will fail prematurely.”

With the proper tool, the bit tip chips the concrete while rotating, requiring less force. “This keeps the tip cool and extends its life,” Dill says. “It’s a good practice to back the bit out of hole periodically while drilling to remove dust in the hole.”

• Check your bits. After drilling 80 to 100 holes, check the bits for wear. “Worn bits drill slower and make anchor installation more difficult,” Colasuonno says.

• Put a bit of grease on the bit’s shank before starting a job.  “Lubricating the shank will reduce the amount of wear on it and the chuck,” Murphy says.

• Select bits for the job at hand.

Manufacturers agree that bit quality affects bit life. The trouble is it’s not always easy to determine quality by simply looking at a bit. Here are some tips:

• Check the surface quality of the flute. It should not have any marks, chips, notches or defects.

• High surface roughness means low steel hardness. Good bits have a smooth surface.

• The brazing joint should not have pores, holes or cracks and should have a uniform color.

• Look for continuous brazing around the tip (no voids).

• The carbide should be on-center to the shank of the bit.

• Good-quality hammer bits do not have sharpened cutting edges.

• Price is usually a good indicator of quality. The processes necessary to manufacturer a quality bit are expensive. “Shop around for price, though. Some companies increase the price of the drill bit based on brand recognition,” says Colasuonno.

Other things to remember:
• Drilling speed is a function of the flutes. Generally, more free space (air) in the flutes means the bit can pull dust from the hole faster, which means faster drilling. You can choose between two-fluted and four-fluted bits. Two-fluted bits will generally be less expensive, but four-fluted bits are capable of removing more material and drilling faster.

• Hole quality depends on the number of carbide cutters on the tip. “When drilling smaller diameter holes (less than 5/8") or when hole roundness is not a major concern, a single piece of carbide across the bit tip is usually enough,” Herman says. “These are usually referred to as ‘two cutter' bits.”

• When quality counts on larger-diameter holes and when some sort of anchor will be placed in the hole, multiple pieces of carbide will help improve the concentricity of the hole.  These “four-cutter” or “multi-cutter” bits cause less vibration, too.

• If there’s a possibility that you may hit rebar, a quad or four-cutter head is better. “A bit with less than four cutters can bind on the rebar and break or jolt the drill from the user’s hands,” Murphy says.

Take care of your bits
To ensure your concrete bits give you long life, clean concrete dust out of the flutes. “This can help maintain faster drilling speeds and prolong bit life. Use a dry rag or a wire brush.

“Some bits, particularly SDS Max and spline, come in a plastic tube. Replacing the drill bit in the tube after use helps protect the bit from harmful elements and prevents rust,” Herman says.

Above all, Dill says it pays to avoid chipping the carbide. “This is the heart and soul of the bit. Once chipped, the bit degrades rapidly.”

“Do not throw or drop the bit,” Colasuonno adds, “It’s not too difficult to get a piece of carbide to break off when used incorrectly. If you take care of the bit, you can expect to get about 100 holes per bit in average grade concrete.”

Published in the March/April 2006 issue of Contractor Tools and Supplies magazine.

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