Core drilling is a technique used in construction to obtain large diameter holes in masonry walls so that plumbing, HVAC and electrical conduit can pass through the wall. Most masonry drilling is accomplished by hammer drill where a hammering motion breaks down the wall, and the turning of the bit evacuates the dust. Core drilling is drilling in the more conventional sense, where the bit sharpness and turning does the work. There are two basic methods, dry core and wet core Dry core is simply drilling without water as a coolant.

A core drill bit differs from a standard drill bit in that the bit is cunstructed like a hollow cylinder with one end open. Bits spin onto a threaded shaft. At the end a flat section that encloses the female part of the drill bit has two flat sections to fit a wrench that is used to tighten or remove the bit. Dry core bits differ from wet core bits in that the cylinder has long, narrow gaps in it for dust evacuation and cooling. Wet core bits have passages for water, but appear solid.

The actual cutting is done by a set of teeth, often tipped with diamond. These teeth are rectangular, giving the business end of a bit an appearance not unlike the crenellation found on a medieval castle,

Dry core drilling is suitable for openings up to about three inches (10 cm), in soft masonry where metal structural elements are not anticipated. Iron rebar or other metal obstacles are death to dry core bits. The drill is usually a standard angle drill with a threaded chuck. The key in starting a dry core drill is to get a section of the bit started then straighten up in the direction you have to go.

Wet core allows for much larger holes, is not affected by iron work, and the machines are often attached to the wall and floor, allowing for much more precise drilling. They are also a lot heavier, without counting the attached 3 gallon water tank and pump. Wet core drills often weigh over 100 pounds (46 kg).

Wet core drills are usually anchored to the floor either mechanically using anchors, through suction created by a pump and sometimes both. A lever controls the bit, and water is conveyed from tank to drill by a small hose. The tank is manually pumped to maintain a steady, but not particularly large flow. The drill operator must take care to apply the correct pressure. A Torque gauge helps keep appropriate pressure. The operator must also remember to maintain water flow.

When used properly a wet core can cut smoothly through poured concrete reinforced by rebar and then a steel decking without difficulty. The bits stay cool and sharp, and with proper use last much longer than cheaper dry core bits. If drilling through the floor, a helper or some other means of access control must be used to keep innocents from wandering underneath the drill site, as the drill cuts around a center core, which may fall and conk someone. Wet core drilling produces little or no dust, but water and concrete slush will drip from the hole for a time.

Use a dry core if the conduits are small (under 2 1/2" i.d.) and you expect soft walls, such as recently laid concrete blocks. Use a wet core if the wall is thicker, grouted, has lots of rebar or larger holes are needed. For holes under two inches a Rotohammer may be used, which is really nothing more than a drill big enough to snap your arms if you don't take care. If many holes are needed in one location, the wet core may hold the advantage because the bits stay cool. Either way core drilling is a major asset in modern construction.

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