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FOUR TIPS ON PUNCHING THE PERFECT HOLE
Drilling holes may seem easy in theory, but as a long-term veteran of the extractive industry explains, there are several factors that determine both productivity and quality control. The following is a summary of those rules as contained in six tips – plus a bonus piece of advice for drilling managers.
Sharp bits
How quickly a bit will dull varies on its application. In hard abrasive rock. And how do you know when to change out a bit?
When the diameter of the flat area of a worn button on a bit exceeds one-third of the base diameter of the button, you should change it.
We compare a flattened button to a pair of flat-soled shoes, which spread the weight of the wearer across the bottom of the shoe. A rounded button as like a pair of stiletto high heels, which transfer the weight to the surface in a very small area. The smaller the button area touching the surface, the greater the psi and rock-breaking stress brought to bear upon the rock surface. The sharper the bit, the less energy reflection you will get, and the faster the bit will cut the rock.
Balance feed, rotation, percussion
These forces are the key variables, and optimizing them can be aided by watching for such aspects as the size of cuttings coming out of the hole. The chips should be as large as possible, thumbnail-sized ideally. The goal is to minimize the amount of mechanical kinetic energy and heat bouncing back to the drill, while transferring as much energy as possible into the rock to spall and break it.
Drill smart
Set up a rig precisely. If it is supposed to be vertical to the surface, the mast dial should read 90 degrees, not 85. “Close enough” isn’t good enough.
When the drill bit is lowered to the rock surface, ease into the boring. When collaring in, grind out a pocket for the bit to sit in before powering up. Otherwise, the drill head is apt to skew off, producing lateral stress on the string. If loose, unstable material is left over from a prior shooting, proceed carefully, because the unstable rock can easily fall into the hole atop the drill head and become wedged there. If possible, clear away unconsolidated surface material before beginning to drill, because if you can’t hold that hole open, you are in trouble.
A larger diameter drill string
Compared to the diameter of the bit, a large diameter drill string is stiffer and minimizes hole deviation while it maximizes air flushing capacity. This is especially important when drilling in porous material that bleeds off the air.
In using a larger diameter pipe or drill rod, check to ensure adequate clearance is being maintained for cuttings to flow to the surface. With that provision met, the velocity of the flushing airstream is increased or better maintained, it helps the operator maintain a hole to hole cycle time.
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