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A drilling cheat sheet: Six tips on punching the perfect hole

In daily drilling production, the mission of the drill is to punch holes. It’s that simple.” That is the view of Bill Hissem, a 40-year veteran of the extractive industry and a senior mining engineer at Sandvik Mining and Rock Technology.

Hissem has consulted and worked in every state of the United States, plus in Canada and on several other continents, including South America, western Europe and Australia.

From this broad experience, he has developed fundamental rules to guide drill rig operators in their quest to efficiently punch the perfect hole. The following is a summary of those rules as contained in six tips – plus a bonus piece of advice for drilling managers.

Read the operator's manual

“You really don’t want to do stuff with your drill that it shouldn’t do,” Hissem says. “Some try to just operate a new drill like they did the old one, but that only works if the new one is like the old one.”

Chances are, it isn’t, with the nature of technology being to evolve and advance. So read the manual, Hissem insists, adding most of today’s operator’s manuals are clearly written and illustrated.

Sharp bits

How quickly a bit will dull varies on its application. In hard, abrasive rock, an operator can get as little as 61m out of a bit – compared to the experience of a top-hammer drill operator in Oklahoma who told Hissem he once got more than 15,000m out of one.

How do you know when to change out a bit?

{{quote-A:R-W:300-Q:"Quality control should vie with productivity in determining the success of an operation"}}Hissem advises doing so when the diameter of the flat area of a worn button on a bit exceeds one-third of the base diameter of the button.

He compares a flattened button to a pair of flat-soled shoes, which spread the weight of the wearer across the bottom of the shoe. He describes 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.

“You never get 100 per cent of the energy applied to the bit into the rock,” he says. “The best you can get is 95 per cent, with five per cent bouncing back.”

His point is, 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 optimising 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 minimise 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.

“The challenge has never been about generating enough power,” Hissem says. “Rather, it is about improving the durability, longevity and efficient energy transfer of the drill string and cutting tools.”

He believes metallurgical advances and new drill technologies have together produced “a modern physics miracle”. Effective operators can tap into this “miracle” by tuning and balancing the forces being applied.

Drill smart

{{image2-a:r-w:300}}This tip is a collection of several fundamental techniques:

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, Hissem says.

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 minimises hole deviation while it maximises 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, which, Hissem says, helps the operator maintain a hole to hole cycle time.

A sixth sense

Watching only gauges can be self-defeating, because by the time they register an anomaly, something significant already has happened in the ground. Hissem talks of the old days when drill operators were exposed to the heat and cold and dust and noise. “Then we got cabs and old-timers didn’t like it, even though they were more comfortable, because it cut them off from sensory information.”

So-called “smart drills” are being developed with on-board intelligence and systems that will separate operators even more from the “feel” of operation. Until such machines dominate drill sites, operators should keep sensing the pulse of their machines. They should actively monitor such aspects as the flushing action, any pitch changes in the drill string, subtle movements that indicate a change in rock pressure, the size of cuttings, and groundwater.

Hissem believes a master driller can fully develop this “sixth sense” in a couple of years.

Motivating operators

For drilling managers, quality control should vie with productivity in determining the success of an operation. If the only directive to those in the field is to get it done fast, operators will take costly shortcuts to gain metres per hour and please the boss.

But holes will angle off, safety will be compromised and, ultimately, more expense will be incurred. Hissem recalls the advice of his grandparents to “make haste slowly”. Or, to use another maxim, look before you leap. “Rarely can one rush quality,” he says.

Source: Sandvik Mining and Rock Technology

Article courtesy of Aggregates Manager (US). Visit:

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