The actual core drilling operation is usually completed when either a so-called breakthrough or a certain borehole depth in a material to be worked has been reached. The material to be worked is usually a mineral material such as, for example, concrete, brickwork, or similar materials.
A breakthrough means the material to be worked is cut or drilled through completely by a drilling tool designed as a drill bit. In this case, the material which has been cut away or cut out is located in the interior of the drill bit, in the form of a so-called drill core, and may be removed together with the drill bit from the borehole. When a certain borehole depth is to be reached, a cylindrical ring is merely cut into the material to be worked. In this case, there is no formation of a drill core which has been detached from the material and which may be removed together with the drill bit from the borehole. After the drilling operation, the drill core remains intact as a column in the borehole and must be separated from the rest of the material and removed from the borehole in a separate operation after removal of the drill bit.
After the core drilling operation for producing a cylindrical borehole has been completed, the water supply of the core drilling system which is necessary for cooling and flushing is usually shut off, the drive of the core drill is stopped, and the drill bit is withdrawn from the borehole which was produced.