Hard-core consistency

Diamond-bit core drills gives quick, perfectly round results. Here are tips to get the most from these tools.

by Kay Falk

Need to cut consistent-diameter holes in reinforced concrete, asphalt, stone, masonry or similar hard material? Diamond-bit core drilling is your method of choice. With it, you can achieve high-speed, high-production, safe, fast and quiet results without the impact or vibration that jackhammers or rotary hammers produce that could affect the surrounding structure.

Core drilling provides you with a perfectly round hole and easy debris removal. Diamond-bit core drills also cut through reinforcing rod without having to use a torch.

Getting started
Even Superman didn’t leap over tall buildings without checking out the situation. When core drilling, you should do the same.

Start by assessing the cutting job. Determine the purpose of the cut, its size and the material you’ll be cutting through. This will help you decide what kind of core drill and bit to use.

For one or two small holes up to 6" in diameter, you could use a lightweight model powered by an electric motor. These models are either hand-held or mounted to a lightweight post. Because of their lighter weight and mounting ease, they can be used to core-drill holes where it would be difficult to set up a bigger core drill.

Hand-held core drills have built-in levels to help the user make the hole plumb and square.

These drills have three speeds which allows the user to match the drill speed to the hole size. Normally, 1" to 2" holes can be drilled in high speed; 3" to 4" holes at medium speed and 5" to 6" holes at low speed.

For cores up to 8" in diameter, a medium-duty drill may be the ticket. They are powered by 15- to 18-amp electric motors, and have a 2.5" column. With motor spacers, some can cut up to 14"-diameter holes. They usually have two speeds.

For larger, deeper holes and going through heavy reinforced materials, a heavy-duty drill powered by a 20-amp electric motor or hydraulics may be the answer. They work best for high-production jobs and can cut holes more than 24" in diameter when equipped with motor spacer adapters. Variable speeds are common.

Setup and operation
Once you’ve matched the drill to the job, be familiar with its setup and operation. Decide if you’ll cut dry or with water. If you cut dry, make sure you have a dry bit and consider the dust, which is hard to control. You’ll need to wear proper respiratory protection, too.

While wet cutting looks messier, it’s actually easier to control than dust and that’s why contractors generally prefer wet drilling.

When cutting wet, decide what water control measures you need to use. A wet vac is handy for cleanup.

Organize your tools so you can work efficiently. Think through the entire job before starting it.

Depending on the job, you’ll need a rotary hammer or hammer drill; anchors and anchor-setting tools; threaded studs, bolts, nuts and washers; wrenches for bits; the drill spindle; extensions; leveling screws and anchors, nuts or bolts; a level and tape measure; bit extensions, reducer couplers and adapters; water pump, hoses and connectors; and a water containment ring.

Don’t forget to consider what happens when the core drill breaks through the concrete. If the core you’re removing has the chance of falling through to surfaces below, you’ll want to clear that area.

If the core will be heavy, you’ll need to make arrangements for its safe removal. This can be accomplished with eye bolts and anchors that support the core from below.

If you must core-drill large-diameter holes overhead, you may want to secure the core from above.

Securing the drill
Mark the hole’s outside diameter and its center, then decide which anchoring system to use to keep the post and base rigid. If the drill isn’t anchored properly, the core bit may chatter whenever it hits reinforcing rod, which this could damage the bit and/or drill. Poor base anchoring may also affect how the the bit penetrates the hole.

The three most common anchoring methods are drop-in anchors, a vacuum pad to secure the drill rig to the floor or jacking screws.

Drop-in anchors are best for mounting a core drill to a concrete or masonry wall. They’re also good for larger bits or on floor surfaces where you can’t use a vacuum pad.

Vacuum pads are great for smooth concrete floors and when drilling holes less than 6" to 10" in diameter. They shouldn’t be used when the concrete has larger than No. 4 rebar on 12" centers. Don’t use them near the edge of the floor opening or building or for wall-mounting.

Jacking screws can be used where you can’t use a vacuum pad, such as motels, hotels or apartments built from precast concrete which has too rough a surface. To use jacking screws, you need to have a rigid ceiling above the location you’re drilling.

Once you’ve secured the drill according to manufacturer’s recommendations, check drill alignment by lowering the spindle close to the surface and making sure the center of the spindle is aligned with the center of the hole.

Use a pen or pencil in the spindle as a pointer. Make sure the water that cools the bit is directed at this center point.

Troubleshooting the job
Like any construction job, conditions are sometimes not exactly as you expected, which can bring on problems. Here are some common problems and ways to solve them:

• If you see smoke or fire, you have a water problem. Check the water supply. The material may be harder than you thought and you need a more heavy-duty drill and bit. It could also mean you’re feeding the bit into the concrete too fast.

• The bit of your hand-held drill is “running” and damaging the surrounding area. Try tilting the bit slightly as you start the cut. Once the bit has cut a slight groove, straighten the drill so you will be drilling a straight and level hole.

• If the bit chatters or wanders at the beginning of the cut, keep the downfeed pressure light. Don’t increase to full pressure until the bit has penetrated the material. Too much or too little pressure can damage the bit.

• When slurry turns clear, you’re cutting through rebar. It’s time to ease up on the pressure until the bit is through the rebar. If possible, stop, break out the core and remove any loose pieces of steel before proceeding.

• If you’re experiencing damaged bits, try stopping the drill 1/4" to 3/8" short of going all the way through. Remove the bit, stabilize the core if necessary and use wedges or a pry bar to break the core free. Smaller cores can be broken free by prying them with a screwdriver. Once it’s free, finish the hole.

• If the bit binds, it could be poor bit alignment. Disconnect the power and unscrew the bit. Move the drill carriage to the far end of the column and add bit extensions to make up the distance. Then resume drilling, starting at a reduced rpm. If this doesn’t solve the problem, back the bit out until it no longer binds, retighten all leveling screws and the anchor bolt. Resume drilling using a slow downfeed rate until you get the core past the binding area.

• Bits showing an unusual amount of wear, even though the machine is aligned, can indicate the reinforcing rod or hard aggregate has polished the cutting segment. Use abrasive material or turn off the water and cut 1/2" or less, then let the bit run without pressure for about a minute. This will expose new diamonds. If this doesn’t help, you may need a bit with a harder matrix.

• Bits get stuck. The problem may be small slivers of rebar or loose bits of concrete or masonry have become wedged between the wall of the hole and the bit. If so, disconnect the power, turn the water pressure up and rotate the bit back and forth with your core drill wrench. If this doesn’t loosen the bit, disconnect it from the spindle and continue to work it back and forth with the wrench. If this works, back the bit out of the hole, break the core free and remove the debris from the hole before resuming drilling the hole.

• If the core is stuck in the bit, first turn up the water pressure. It may help to gently tap the side of the barrel with a hammer handle. If it’s still stuck, you can insert a metal rod through the center of the threaded connector to push the core out of the bit.

Ways to learn more
These core-drilling tips are based on extensive information from the Concrete Sawing and Drilling Association (CSDA) in St. Petersburg, Florida, and Ohio Concrete Sawing and Drilling, Inc., Sylvania, Ohio. To get a copy of the CSDA’s instructional  DVD, “Flat Sawing, Wall Sawing and Core Drilling,” contact the CSDA at (727) 577-5004 or visit www.csda.org. To get copies of two videos, “Basics of the Hand Held Core Drill” and “The Basics of Core Drilling,” contact Skip Aston at (419) 841-5656 or e-mail him at skipaston@ohioconcrete.com. 

Both sources contain more excellent core drilling advice and good core drilling techniques.

Published in the July/August 2004 issue of Contractor Tools and Supplies magazine.

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