INTRODUCTION:
Lately, I’ve been getting questions about drilling in really hard rock. Really hard rock in drilling is around 7-8 on Mohs scale. Mohs hardness scale is a chart showing the relative hardness of various minerals. The scale consists of numbers 1 through 10; 1 being the softest and 10 being the hardest. Very hard rock includes gneiss and quartz, and extremely hard includes rock like jasperite, quartzite and taconite.
How hard is your rock? The simplest and most reliable way to determine rock hardness is to perform a scratch test using a hardness kit and compare the results with Mohs scale. If you do not have such tools, you can still determine the hardness using a pocket knife or a metal saw, although results may not be as precise.
Pick the right core bit
Each type of rock will demand a certain type of matrix and each matrix is made of metallic powder, alloying elements and diamonds. How many diamonds are used and the hardness of the matrix are related to the hardness of the rock. Drill matrices are developed to perform ideally within a certain range of the scale so you need to pick a bit suitable to the hardness of the ground to be drilled.
Harder ground needs a bit with a higher number. It seems counterintuitive, but hard rock needs a softer matrix. A bit with a higher number has a softer matrix that will allow diamonds to be exposed efficiently. The exposed diamonds will cut the rock better.
Watch your diamonds
When drilling in hard ground you want to avoid overexposing your diamonds and risk having them pop off or break. To make sure you avoid this, you need to pay attention to your drilling parameters. You will have to play around with them to find the sweet spot, which is the ratio where you are advancing and getting the best penetration rate possible with the least amount of feed pressure. In general, not enough water flow risks burning the bit and too much can keep the bit from sharpening. Too low a rotation speed can cause excessive wear on the matrix while too high a rotation speed can polish the diamonds.
It could happen that you end up polishing the diamonds, but if this occurs you have the option of sharpening them (see our blog on this subject). However, if the diamonds break off, you will have lost matrix height and the life of your bit will be greatly reduced.
Check your core lifters often
Core lifters can take a beating in hard ground formations. If the ground is hard and silicified the core lifter springs can end up being worn down more quickly. This is because the spring has to work harder to break the core. It is recommended that you check them every single run. You can do this by wetting a piece of core and putting it into the end of the core case to make sure that it is functioning properly. Make sure the wet piece of core does not slip!!!!
The single most important decision when drilling in hard ground always comes down to picking the right core bit. But keep in mind that you should always be looking at other variables as well, such as abrasiveness and variability. Check out our guide on choosing the right core bit for related information or contact our technical team for advice.