Core drilling systems and recommendations for their productive use

Diamond core drilling is a high-speed, high-production method of drilling in concrete, stone, asphalt or masonry structures. It is fast, safe, quiet and does not cause impact or vibration damage to the immediate surrounding structure. Diamond core drilling minimizes spalling, eliminates fractures and doesn’t transfer vibrations to the surrounding structure. The clean, straight openings require no patching or other cosmetic repair and allow the next operation to be installed or carried out without further delay. Core drills cut easily through rebar, allowing the holes to be placed wherever it is most suitable. Holes of almost any diameter up to 60 inches are easily drilled to make openings for plumbing, electrical and HVAC installations.

Core drill systems
There are many types of drills on the market today that can be categorized into three basic types. The lightweight drilling machine can be hand-held or feature a short-column designed for drilling holes up to 3 inches in diameter. It usually has a trigger on the handle to turn the motor on and off and can look like an oversized shop drill. Generally, it is used as a hand-held drill, but most come with an auxiliary lightweight column and base. The lightweight drill would find common use with general contractors for the occasional requirement to drill small-diameter holes.

The medium- and heavy-duty drilling machines are often standard equipment for the professional sawing and drilling contractor. The medium-duty drill is equipped with a 15 to 18 amp electric drill motor with a 2-1/2-inch column. This drill is designed to handle bit diameters ranging from one-half to 8 inches. Under normal conditions, vacuum attachments or bases can be used to support the drill and column assembly. The components of these types of core drill systems consist of a column or post, motor and carriage, base with anchor slot and leveling screws, and a water feed system. (Figure 1)

The larger and more rugged version is usually equipped with an 18 to 20 amp electric drill motor or a hydraulic motor (sized to the bit diameter being used), a 2-7/8- to 3-inch column and a base. This drill is used on jobs requiring deep or large-diameter holes, drilling in structures that are heavily reinforced or for jobs requiring higher production rates. More specialty accessories are available for this model than for other models.

Pre-drilling operation
It is critical to first determine the type of drill that will be needed on the job. In order to do this, you will need to know information such as the ultimate use for the hole, the precision required, material to be drilled, water control required on the job and whether anything is imbedded in the structure.

Before setting up for core drilling, the operator should first ensure that the following accessories are available:

  • Roto-hammer or hammer drill
  • Anchors and anchor setting tools
  • Threaded studs, bolts, nuts and washers
  • Wrenches for bits, drill spindle, extensions, leveling screws and anchor nuts or bolts
  • Level and tape measure
  • Bit extensions and adapters
  • Water hoses, electric cords, ground fault interrupt and assorted electrical plug adapters
  • Power supply (if the drill motor is other than standard electric)
  • Pump and water supply
  • Wet pick-up vacuum cleaner

After determining the proper drill system and accessories required, the first task is to anchor the drill. Measure the distance from the center of the base anchor bolt slot to the center of the drill spindle. This measurement is used to locate the anchor bolt hole from the center of the hole to be drilled. Mark the center of the hole to be drilled and measure from this mark the distance determined previously. This is the spot where the anchor-bolt hole will be drilled. After drilling and setting a half-inch anchor, the drilling machine is placed over the anchor and secured with a bolt or nut. (Figure 2.) Then adjust the four leveling screws to stabilize the base and plumb the column to the hole location. Finally, securely tighten the anchor bolt or nut.

The drilling machine must be properly secured to the work surface. Movement of the column or base during drilling will cause binding, chattering and premature wear to the bit and the core drill rig. If the work surface is a wall, special precautions are necessary. The measuring and anchor setting method described above also applies to drilling on a wall, whether it is concrete, brick or masonry. However, the mounting procedures and considerations require special provisions. Anchors and backing plates are needed to distribute the forces of securing a core drill. A vacuum base should never be used on a wall to secure a drill as the loss of power may result in injury or death.

Vacuum base
The vacuum base is recommended for holes that are 6 inches in diameter or less. However, a vacuum base is not advised for use on rough or cracked surfaces because the vacuum would not hold. A vacuum base should also not be used when drilling deeper than the bit length or into a slab with rebar larger than #4 on 12-inch centers. Care must be used to keep down-feed pressure light when using a vacuum base on a tiled floor. In this instance, high down-feed pressure can cause the vacuum base to pull tile from the floor, causing the core drill to detach.

A vacuum pump is not designed to pump water and is equipped with a protective water filter shut-off device. The seal is the sponge rubber gasket that lines the bottom of the base. (Figure 3.) It is important to make sure the base and rig are secure before starting to drill. The base seal is removable and should be kept off of the base when not in use.

Accurate hole drilling
Starting the drill bit accurately is the most important part of a successful job. Locate the center of the hole to be drilled and lower the drill spindle close to the surface being drilled, leaving about 2 inches between the spindle and the surface. If water control at the work site is not too critical, turn on the water to the spindle and see if the water jet impacts the surface at the center point of the hole to be drilled. If water control is critical, carefully insert a short length of a sharpened wood pencil into the water through the hole in the spindle, then lower the drill until the end of the pencil just clears the surface. If the drill is aligned to the hole, the pencil point should line up with the center point of the hole being drilled.

Always start a small hole using the shortest possible bit length, coupling it to the motor as closely as possible. Do not use a bit extension when starting a hole. When using spacer plates for the first time, never trust them to be preset. Carriages, although they look the same, are often machined to different tolerances and will change the true center of the bit. Always check to see if the bit aligns with the center mark of the hole being drilled.

When starting a hole, the biggest problem is keeping the bit from wandering or “chattering.” To avoid this problem, hold a block of wood against the barrel of the core bit while applying light down pressure to the feed control. Once the bit has penetrated to a depth equal to at least the depth of a segment, you can add additional down force on the bit using the down-feed control.

Large and Deep-Hole Drilling Considerations
Drilling holes larger than 16 inches in diameter, or smaller holes deeper than 6 feet requires special considerations for the operator. First, set two anchors as widely spaced in the drill base anchor slot as possible. When drilling holes up to 20 inches in diameter in good concrete, you can safely use 1/2-inch anchors. When drilling holes above 20 inches in diameter, use ¾-inch anchors. When drilling into brick or material that won’t hold an expansive type anchor, use an epoxy anchoring system or a through bolt with backing plates.

To properly secure the base, snug the anchor bolts evenly, then use the leveling screws to tighten the drill base against the anchor bolt or stud. The use of lead wedge-type anchors is not acceptable as they can loosen or strip out when subjected to drilling forces and vibrations.

It is highly advisable for operators to use a back brace to support the column for large-diameter bits. Some drill columns come from the manufacturer already set up to accommodate a back brace, but if a drill rig is not so equipped, a back brace can be fabricated out of metal or wood. The important aspects of a back brace are that the brace should be at a 45 degree angle to the column and the base or structure, and should be securely fastened to both the column and the base or structure.

Poorly-aligned bits
If a 4-foot-deep hole is started at an off-center angle of 1/16 inch, it will be 1/2 inch off center by the time the desired depth is reached. This may or may not be acceptable to the customer. However, the time required to drill such a hole will be longer, because the barrel will rub against the sides of the hole, causing drag and less power supply to the diamonds. This drag can become great enough to absorb all the available power from the drill motor, leaving nothing to power the diamonds through the material. The untrained operator in this case would assume that either the bit has closed up or that he has hit heavy steel. Any corrective action taken by the operator short of realigning the drill will have little or no positive effect.

Core removal
When drilling in floors or other horizontal surfaces using drilling cores larger than 10 inches in diameter, set a ½-inch anchor in the center of the core. This anchor will be used when the operator removes the core with an eyebolt and lifting device.

When drilling large-diameter holes, it is very important to be aware of what will happen to the drill bit when the core is cut free. When drilling in walls or other vertical surfaces, stop a 1/4 to 3/8 inch short of the opposite side of the wall or structure. Remove the drill bit, break the remaining material free by inserting a wedge or pry bar in the kerf left by the bit, and apply a side load on the core. If you are not sure of the wall thickness, roto-hammer a small probe hole in the core area, and measure the structure thickness. Plug the probe hole prior to core drilling to keep water and slurry from escaping. After the core is broken free, screw an eyebolt into the anchor in the core and use the appropriate lifting device and slings for the weight of the core.

Bit and core problems
Stuck bits
There are many reasons why bits become stuck in a wall or slab. Sometimes a small sliver of rebar becomes wedged between the barrel and the wall of the hole. Other times, loose rocks or chips of concrete or masonry are the culprit. One way to prevent this from happening is to constantly drill into the material. Removing and then reinserting the bit into the hole increases the chances that a bit will become wedged or stuck in a hole. Once the operator has started a hole, he or she should never stop drilling unless there is a problem.

The easiest method to dislodge a stuck bit is to rotate the bit backward with your core drill wrench. This reverse action will usually release the wedge enough for you to pull the bit from the hole. Once the bit has been removed, the operator should break the core free and remove it and any loose material from the hole before resuming drilling.

If all attempts to free the bit have failed, the only recourse is to overdrill with a larger-diameter bit. This works only if the operator is using a standard or closed-back bit. For proper overdrilling, the operator should use a bit that is 1 inch larger in diameter than the stuck bit.

Stuck core
It is always possible that a core can become stuck. The first step to take when this happens is to determine the reason. The most common reasons include:

  • Segment relief between the barrel and the segment is too small, allowing the core to rub the inside of the barrel.
  • A piece of rebar has become wedged between the core and the barrel.
  • A fragmented or broken core where a piece has wedged between the remaining pieces and the barrel.
  • When drilling on grade slabs, some sub-base aggregate may become wedged between the tube and the segments.

When a core becomes stuck inside a barrel, the first method of removal is to turn up the water pressure from the water supply and gently tap on the side of the barrel with the flat of a wrench or the handle of a hammer. The combination of water pressure and vibration from tapping will cause the core to eject from the barrel. Do not tap hard enough to bend or dent the barrel.

Severely stuck cores may have to be driven out by passing a punch or metal rod through the center of the threaded connector and hitting it with a hammer. Take care not to strike the core bit barrel or the segments.

The proper use of a diamond core drill machine requires more than just good equipment. It requires the skills of a professional cutting operator with the knowledge to effectively operate the tools in conjunction with the analytical skills learned either on the job or in training programs such as those offered by the Concrete Sawing and Drilling Association.

Any discussion of operating diamond cutting equipment should include safety precautions for the operator.

  • Operators must read and understand the manufacturer’s operating manual and safety requirements for each piece of equipment they operate. When core drilling, always wear personal protective equipment required by local, state or federal standards, including, but not limited to, hard hat, hearing protection, safety glasses and safety footwear. A high-visibility vest is required for roadwork.
  • Do not wear loose-fitting clothing and jewelry while operating equipment. Always keep hands and clothing away from all moving parts.
  • When electric drilling, make sure all electrical equipment, such as cords and generators, is in good operating condition.
  • When operating hydraulic drills, make sure all hoses and fittings are in good condition.
  • Utilities are always a concern. Make sure no electrical or gas lines will be cut and that there are no other obstructions. If you cannot inspect the backside of the work area, you need assurances that no hazards are present.
  • When drilling floors of suspended slabs, make sure the area below is properly secure. Keep everyone away from the area of falling cores. If possible, take measures to catch the core directly beneath the floor.
  • When drilling large diameter holes through walls, an additional drop-in anchor is recommended to give more strength in holding the drill in place. Extra bracing for the mast is also recommended to secure the drill. Make sure the concrete has sufficient strength to support the drill in areas where the anchors are placed.
  • When using suction vacuum pads to secure the drill to the floor, make sure of proper suction to the floor before starting the drill motor.
  • Never remove the core bit from the drill motor by putting a wrench on the bit and starting the drill motor.
  • When lifting the core drill on the wall for mounting, first mount the stand to the wall and then slide the carriage onto the stand. This helps eliminate lower back strain.
  • When anchoring the core drill to a wall, anchor directly above or below the hole whenever possible.
  • Operators should never stand on the drill stand to secure it in place.

Patrick O’Brien is executive director of the Concrete Sawing and Drilling Association.

This article was prepared with information from the CSDA Cutting Edge, CSDA Sawing & Drilling 101 and CSDA Operator Certification training manuals. These manuals are the course textbooks for CSDA training programs. For more information about CSDA training programs, contact CSDA at (727) 577-5004 or visit