Core bits that don’t cut: How to sharpen them
Sharpening a bit is also known as stripping. There is no easy, one-size-fits-all solution to sharpening a bit. It takes experience and knowledge. It is also something that technicians are often asked about, so in this blog I provide a few suggestions and basic guidelines.
Diamond Impregnated core bits are made to be self -sharpening, meaning they are designed to stay sharp throughout the drilling process. In ideal conditions, as the matrix wears away, new sharp diamonds are exposed at a constant rate, and dull or worn diamonds are released. However, if the drilling parameters are not properly set or if the core bit is not suited to the type of ground conditions, the diamonds on the face of the bit can become worn without the matrix abrading away. The core bit becomes polished and it will stop cutting.
Possible causes of a polished bit
This situation can arise for a couple of reasons:
The drilling parameters or settings that you have chosen do not suit the ground conditions. You should refer to the recommended parameters found in published documentation or check here.
The core bit may not suitable for the type of rock formations in your ground. Core bits are developed for particular hardness and level of abrasiveness. Also, if the ground is variable, the core bit you started drilling with may not be suitable once you have reached a certain depth. Make sure you have the right type of core bit.
What you can do
Once this happens, the core bit will need to be sharpened or stripped. The procedure involves removing a thin layer of the matrix in order to expose the diamonds so that the bit can start cutting again. It is not necessary to remove the core bit from the bore hole as this can be done while drilling continues, by changing or adjusting some of the drilling parameters. It is important to note that this technique will consume your matrix, the amount of which will depend on the driller’s experience, ability and reflexes.
Momentarily increase the weight on bit (WOB) by15 to 20 percent,
At the same time, reduce the water flow to near the minimum recommended by the core bit manufacturer.
When you see a slight increase in the WOB and a spike in the rotation torque, these are indications of stripping and wearing away of the matrix to expose new sharp diamonds. The bit should begin to cut.
Immediately restore the water flow to the original volume and lower the WOB.
Try adjusting your parameters to different drill settings than the ones you used previously, to avoid having the problem repeat itself.