Core drilling is one of the most common methods used for exploration of underground minerals. The core drills are machineries used to retrieve a core mineral sample of any location under the ground. Some layers like hard rock or concrete are easily drilled and tested with the help of such drilling. Unlike other kinds of drilling which is used to dig holes, core drilling is aimed mainly in exploration of the composition of rock or types of minerals present anywhere under the ground in a trouble-free way.
The core drill or core barrel which is used in collecting samples of underground samples and mineral deposits consist of three main components- an outer tube to which the core bit is attached, an inner tube and a barrel head. The outer tube rotates to dig a hole in the ground. The cutting effect of core bit usually makes a way through hard rock and concrete to collect samples. The inner tube does not rotate as to keep the sample in an undisturbed form. Once the desired amount of sample is collected in the core retainer attached to the inner tube, it is pulled out safely and is sent for further testing by geologists.
The core bits play an important role in digging the hole in a fast and easy manner. According to the requirement of the area to be dig, different core bits are used. The bits are infused in the matrix that wears constantly to sharpen the blades. This aids in effective drilling of the hole and collection of sample without much hassle.
According to the type of core drilling required, different core bits are used. For example for digging hard rock, concrete and stones, diamond bits infused to a soft matrix is used. This enables the driller to pass easily through hardened rock and substances. For softer rock, less expensive material is used with a much harder matrix for making it long lasting. For such purpose bits made of tungsten carbide chips are commonly used.
Polycrystalline bits are also used which are made of bonded tungsten carbide and synthetic diamond particles. It may be used to cut both hard and soft rocks which much efficacy. The hardness of matrix controls the rate at which bits are worn away.
During core drilling, a flushing medium is constantly supplied to the space between the outer and the inner tube. It helps in removing the cutting of drill bits and cooling the bits. For this purpose; water, compressed air or foam is pumped into the barrel in between the tubes, which passes through the drill bits. Water is generally considered best flushing medium for core drilling but is required in large amounts, making it sometimes harder to reach water sources. Compressed air may be passed through the tubes to solve the purpose but it is quite expensive than using water, though it works best for soft clay and damp chalk. In some cases like unconsolidated sand and soft sandstone, mist or foam may be used as a flushing medium.