Keyhole Basics: Coring 101

By E. Marshall Pollock | February 2010 Vol. 65 No. 2
Truck-mounted coring unit with stabilizers

Both of these issues can be resolved by the use of hydraulic stabilizers, fixed to the deck or frame of the vehicle, on either side of the core drill. When adjusted, these stabilizers will take the spring action out of the suspension and concentrate the weight of the vehicle down through the core drum for an accurate and precise cut. At the same time the stabilizers can be adjusted to position the core drill to be perpendicular to the horizon in the plane that parallels the width of road.

The core drill must also be capable of adjustment so that it is also perpendicular to the horizon in the plane that parallels the length of the road. This is usually done by adjusting the hydraulic cylinder that elevates the core drill to the upright position from the bed of the vehicle. When both of these adjustments have been made, the resulting core will be true and plumb to the horizon.

Once you have the core drill properly adjusted, coring operations can begin. The working end of a good quality coring bit has cutting teeth or segments made of a composite matrix in which layers of diamond crystals are embedded. Unlike carbide, high speed steel and other types of drills that cut material with a sharp cutting edge, diamond drills work by grinding away material on a micro level. As they wear their way through the material, diamond crystals in the outer layer are worn down and replaced by those in the next layer and so on. The rate at which this wear takes place depends on the hardness of the material to be cut (concrete or aggregate in asphalt or re-bar in reinforced concrete), the rotational speed and down pressure on the drum, as well as the matrix in which the diamonds are embedded. A properly segmented coring drum can be expected to cut between 70 and 100 cores before needing to be re-tipped. But optimum performance and life can only be achieved when the correct segments are chosen and the drum operated at the proper speed (RPMs) and pressure.

Faster drill speed or greater pressure may appear to increase production efficiency, but the trade-off will be a significant increase in friction and heat which considerably reduces the life of the bit. A drum that spins too quickly will glaze the segments and result in slowed penetration and longer cutting times. Conversely a drum that spins too slowly cannot properly expose the diamond layer and will slow the penetration rate. As a rule of thumb, an 18-inch diameter coring drum should rotate between 180-200 rpm.