Get to the core

Take these steps to core-drill concrete efficiently, accurately and safely

For holes over 1-1/2" in diameter, core bits offer advantages over solid-body bits. They drill faster because they drill only the circumference of the hole instead of removing all of the material.

There are two types of core bits for concrete drilling: carbide-tipped and diamond-tipped. How you do the job will depend on the type of core bit you are using. Based on the type, here is what core bit manufacturers suggest you do to get accurate holes fast.

Carbide-tipped core bits
“Carbide core bit systems have sharp, durable carbide teeth brazed to a core body that either screws or wedges onto a shank that gets inserted into the drill’s nosepiece. They drill concrete without water for lubrication or cooling. Any water put on the carbide-tipped core bit after beginning drilling can shock and shatter the inserts because of the heat produced during core bit use,” says Patrick Kearl of Relton Corporation. If you must use water to reduce dust, apply it when you start drilling.

Carbide-tipped core bits are available in threaded and taper fit systems. “Threaded shank systems work fine in rotary-only systems because the threads are less likely to break than they are in a hammer application. A taper-fit shank on carbide-tipped core bit systems gets locked into the core head by the hammering action of tool,” he says.

A challenge with any core bit system is to get the hole started accurately. That’s why they have a starter point that centers the bit for the cut. For most efficient cutting, this pilot bit should be removed once the hole is established. But few workers take them out to finish the hole, says Kearl. “Instead, they continue to cut with the starter bit in place. This takes extra energy that could be used to cut the core.”

The hammering action used with carbide-tipped core bits effectively chips away at concrete, but is no match for rebar in concrete. For longest core bit life, you should use a rebar cutter, suggests Kearl.

“It must be the same diameter as your core drill to cut through rebar. Use a 1/2" 3-jaw chuck drill motor in the rotary-only mode,” says Kearl. Once you cut through the rebar, use the carbide-tipped core bit to finish the hole.

Carbide-tipped core bit teeth can be re-sharpened. “It pays to keep them sharp. A dull cutting edge is not as efficient and creates more heat, which can lead to premature failure,” says Kearl.

Finally, Kearl suggests using the drill’s depth guide. You’ll spend less time on the job, less wear on tools and operator, and use less epoxy if you are setting chemical anchors in the hole.

Diamond-tipped cores for drilling rigs
“Selecting the proper core bit for core drilling can save money, time and aggravation. There are a wide variety of sizes and quality grades on the market. To select the best one for your job, start by asking some basic questions,” says Thom Fisher of Diamond Products.

First, identify the material to be cut. “The aggregate of the concrete will determine the bond specification needed for the core bit. The bond is the material that holds the synthetic diamonds on the segment,” Fisher says.

A good bond specification rule of thumb to remember: Use a soft bond type to cut hard concrete and hard bond type to cut softer concrete and asphalt. The amount of steel in the concrete also affects bond choice, says Fisher.

Second, consider the price. “Core bits are typically priced based on their diamond concentration,” says Fisher. “For jobs requiring a small or moderate amount of drilling, you don’t need a core bit with a high diamond concentration. Use core drills with higher diamond concentrations when time or the volume of work mandates greater cutting speed.”

Then, decide on the core bit type. Diamond core bits come in segmented, turbo and dry-diamond or dry-vacuum types.

“A turbo bit will cut faster and smoother than a conventional segmented bit because of its fluted segment design. It channels slurry better and reduces vibration. However, turbo bits do not last as long as segmented bits. For block or brick cutting, use a dry-diamond hole saw or dry-vacuum bit,” he says.

Operating performance factors
Proper bit selection is only one facet of diamond core drilling. How you use the bit can greatly affect performance.

“Many variables come into play when core drilling. Understanding how the bit cuts and what you can do to tweak performance can help you get the most holes from the bit in the least amount of time,” says Fisher. He offers these tips:

• Check the concrete aggregate and match the bond of the core bit to it. A bond specification that is too hard prolongs cutting time; a bond that is too soft wears too fast and drives up costs.

• Use the correct feed pressure. With practice, workers who use core-drill rigs learn to sense the right amount of pressure needed to make the cut.

• Check for re-rod in the concrete, then plan for it before making the cut. Maintaining drill speed is very important when cutting through rebar or other concrete-embedded steel.

• Use the right speed (rpm). If the core bit’s diamond abrasive polishes, speed is too high. If the job takes too long, it is running too slow or is the wrong bond specification for the job. The drill must have the power to maintain cutting speed.

• Align properly. The rig must be properly anchored with concrete anchors, a vacuum or a post jack for accurate cutting and longest possible bit life. “Standing on the drill rig is not a safe or effective way to anchor a core bit system,” says Fisher.

Published in the November/December 2001 issue of Contractor Tools and Supplies magazine.

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