Diamond core drill

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Diamond core drill bits are used to bore large holes in brick, concrete and stone. They are not generally used in other materials. The bit consists of a metal cylinder, usually relatively soft steel mounted on an arbor. Industrial diamonds are embedded at the open end of the cylinder. In the image, the diamonds are on the metal segments attached to the end. The segments are thicker than the cylinder wall, so most of the bit does not rub in the hole being bored. The sloping slots in the cylinder wall help carry the dust out.

Diamond core drills can be used with or without water lubrication. The drill shown can cut a 115 mm diameter hole through a single-thickness brick wall in less than a minute, running at about 300 RPM. The resultant hole is very cleanly cut. Another obvious advantage over conventional masonry drills is the core bit's efficiency — since very little of the removed core must actually be abraded, the power and operator effort required to drill a very large hole is reduced enormously, and the capacity of a given portable drill greatly increased.

This form of core drill wanders hopelessly when presented to a flat surface, and needs a centering mechanism. The arbor can carry a masonry bit to bore a centering hole. (The version shown has a plain 10 mm rod; a 10 mm masonry twist drill must first be used to drill the centering hole for the rod.) A wooden or stone template, a close fit for the cylinder, can also be used to guide the bit at the start of the cut. After the first few millimeters of cut, the centering mechanism may no longer be needed, although it will help the bit to bore without wandering in a deep hole.

Diamond core drill bits for use with portable drills are commonly available in diameters from 20 mm to 130 mm. The only limit on length of the cylinder, and thus depth of the hole, is the need to remove the bit from the hole to clear dust. 300 mm cylinder length is not uncommon, although shorter bits are usual. By breaking the core off from time to time and using a shank extension, a diamond core drill can drill to depths many times its length.

Some disadvantages quickly become apparent when using these bits:

• The portable drill used must be capable of producing considerable torque at low speed
• They tend to bind if choked with dust, or if allowed to wander away from the central axis of the planned hole
• The kick-back from a powerful drill may be severe under some conditions, and long side-handles should be used, preferably with two operators for very large holes. Careful consideration should be given to this when using such equipment at height or in confined spaces, for obvious reasons.