Different Bits Can Aid in Drilling in Rock

Manufacturers give their insights into what type of drill bits contractors should be looking at for the tough digs.
Different Bits Can Aid in Drilling in Rock
Vermeer introduced the Lance Pro bit at the 2017 CONEXPO-CON/AGG, shown here on the D40x55 directional drill. The bit was first designed for the company’s plate family of bits, but it was reinvented as a scoop bit for cobbled rock.

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Rock is not so tough after all when matched up with some of today’s drill bits. The bits are taking horizontal directional drilling probes to places previously untried and at gratifying speeds.
A tricone is probably the most universal bit employed to cut through rock. The design of the bit dates to Howard Hughes’ oilfield tool company and features three angling discs with teeth that interact to crumble rock. The toothy discs revolve on bearings and range in size from 3 inches to 3 feet.

“A driller can always use a tricone,” says Steve Quisenberry, sales manager at Drillhead. He contributed to the design of the company’s latest tricone model. As of four years ago, the company produces its own products but has always done a lot of private-label manufacturing. “A tricone is field-proven and can take a beating.”

Jeff Davis, product manager of HDD tooling at Ditch Witch, also calls a rotary disc bit an industry staple — but it may not be for long. “The rotary disc still is very, very important, but one thing that has come out of late is the PDC bit. It came from oilfield technology and has crossed over into HDD drilling. We are seeing multiple, multiple users being very successful using PDCs.”

A PDC, or polycrystalline diamond compact, bit is a more conventional working end on a drill string. It features three or more raised cutting teeth with diamond-hard buttons that maximize rates of penetration and drilling intervals. It indeed is growing in popularity among drillers. “PDC bits,” echoes Quisenberry, “are hot on the tail of tricones.”

Another traditional choice is the plate bit, a flat piece of tempered steel cut  in different shapes and edged in a variety of ways. “It is the most universal for most rock,” says Jason Zylstra, Vermeer product manager. “There are hundreds of plate designs, outfitted in a number of ways, with hard-facing or carbide inserts or rotary teeth. People have come up with a lot of ideas on how to alter the plate.” An imaginative example is Drillhead’s new Rok-Klaw Xtreme, which looks like three sharp fingers emerging from a furry claw to scratch at medium-hard rock.


Different types of rock call for different bits, of course. “Are you going to find soft or medium-soft, medium-hard or semihard, or hard or very hard rock?” Davis asks. “The bit you choose is going to be about hardness — and solidness. Is the rock a solid formation or a layered formation or cobblestone? You choose your bits accordingly.” One example: Vermeer introduced at the 2017 CONEXPO-CON/AGG its Lance Pro bit, which was first designed for the company’s plate family of bits. It was reinvented as a scoop bit for cobbled rock.

But even the best bits won’t get a driller where he wants to go if he doesn’t use them well. Manufacturers have seen plenty of poor drilling techniques. They offer tips.

“Not selecting the proper bit is an ineffective drilling habit. That causes a lot of problems and is very expensive for the driller,” Davis says. “And drilling fluids — it’s very, very important that a driller understand the difference in drilling fluids and use them properly.”

Quisenberry suggests drillers “choose a bit for a little harder rock than you actually expect to drill in. People select a bit for average hardness, but when they meet something harder, it’s difficult to keep the hole going.” He also frequently sees bits wearing out prematurely because drillers are spinning the bit too fast, exhausting bearings without speeding up penetration. “You should make a hole by spinning the bit at 40 to 65 rpm and applying 2,000 pounds of thrust. You’ll see a dramatic increase in production.”

Zylstra’s tip is one word: patience. “That’s the best overall advice for rock drilling. Too many drillers get frustrated and make aggressive moves that sometimes are more than what a machine is designed to do. Usually there is damage to your pilot tooling.”


Rock drill bit manufacturers press ahead with new bits: some designs springing from their own laboratories, and others resulting from customer feedback after in-the-field experience. “We listen to our customers,” Davis says. “When they tell us what might work best in the industry where they are, we design a product for them.”

The bit makers believe there are engineered solutions still awaiting discovery. “We are constantly innovating,” Zylstra says. “We are focused on incorporating new cutting tools and cutting parts, using different materials, different placement of carbide and teeth, and things like that. We constantly are learning.”

Ditch Witch was a pioneer in all-terrain drilling and bit design. Davis acknowledges continued progress is key to competition. “We are the leaders in the field. We are still going at it pretty darned hard in our design of new products to stay out in front of the gang.” Its latest bit is the RockMaster backreamer with patented edge protection and wear resistance.


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