Choosing the Right Drill Bits

A contractor who purchased a Terrier with rotary capabilities for site investigation work has been in touch with us recently.

He was concerned that the rig and/or tooling did not appear to be performing properly on a contract he was currently employed on. This contract involved taking core samples with a conventional T2101mm water flush core barrel in hard rock (granite) to a depth of 10m below ground level. At the time the contractor was using a mid series impregnated core bit.

Progress with the coring had come to a virtual stop and affected the contractor’s ability to make a viable income from his plant and equipment. There was also the concern that this lack of production would endanger the contractor’s future employment on the contract. Production had been to everyone’s satisfaction until a pink granite had been encountered in the boreholes.

The contractor was asked to make certain checks on the rig and to confirm that there was no damage to the in-hole equipment, that it was set to the correct parameters and that the impregnated bit was not worn out.

We ascertained that the rig was operating in accordance with its specification, the in-hole equipment was undamaged and performing correctly and there was plenty of life left in the impregnated drill bit. It was considered that if everything was ok with the rig and the equipment that the impregnated bit was not well suited to the formation it was being used to drill and advised him to switch to a softer matrix bit which would strip more easily, which he did not have.

In the absence of a sand blaster to sharpen the bit he was advised to “dress” the bit with a file to open up the cutting face of the crown. By chipping away at the matrix it allows the impregnated diamonds to become exposed and thereby provide a keener edge to the bit. While awaiting the delivery of a bit that is better suited to the formation he has employed this method of sharpening the core bit.

The contractor has now been able to increase production by using this method to a level acceptable to himself and to the client.

Production will suffer if you do not use a bit that is suited to the formation to be drilled, no matter how good your plant and equipment.