DTH Bit Selection Guide

Shank first, then diameter, then face

A bit only fits the hammer family it was made for (for example K40, L3). Diameter follows the hole you need. Face design and button shape follow the rock: harder and more abrasive rock asks for more buttons and a more protective face.

That order is not arbitrary — it is the order in which mistakes become expensive. A bit with the wrong shank never seats properly and can leave the hole; a bit with the wrong diameter produces an out-of-spec hole or a stuck string; a bit with the wrong face and button configuration still fits and still drills, so it is only discovered weeks later in the cost per metre.

What the bit has to do

The bit is where the hammer's energy is turned into rock. Three jobs happen at the same face:

• Impact. The piston strikes the shank; the shank transfers that blow into the bit body and the buttons. Hard-rock buttons are tungsten carbide inserts set into the steel body. The energy per blow and the number of blows per second come from the hammer and its air supply — the bit only decides how that energy enters the rock.
• Flushing. Compressed air leaves through flush holes in the face and carries cuttings up the annulus. The number and position of the flush holes matter in soft, sticky ground where cuttings want to pack: too little flushing air across the face and the bit re-grinds its own cuttings, which halves button life and slows penetration.
• Gauge holding. The outermost buttons, called gauge buttons, cut the hole to diameter and keep it there. A bit with too little gauge protection drills undersized holes that then bind on the string. When a hole comes in under specification, this is where the cause usually is.

Face geometry is how these three jobs are balanced. Flat faces spread the blow across many buttons for maximum chipping; convex faces concentrate energy for hard rock; concave faces stabilise the bit in broken ground where the hole wall is irregular. Some of our families are shipped with a choice of head shapes — the L3 range, for instance, is available with flat (P), convex (T), concave (O) and double-edge (S) heads, and the rock decides which one you run.

Our DTH bit range (published parameters)

The table lists models with hole range and shank/type. Use it to confirm that the bit you order matches the hammer you run.

Our DTH bit range (published parameters)

The same range with the catalogue detail behind it

Three things in that table are worth pausing on.

The diameter spread inside one shank family is large. A K40 shank accepts bits from 115 mm to 150 mm, and K5 from 135 mm to 216 mm. That is deliberate: it lets one hammer serve several hole sizes, and it is why experienced buyers order a hammer and then buy bits across a spread rather than locking themselves to one diameter.

Variant part numbers are not marketing noise. The coal-seam bits, the eight-key M30B and the PDC bit in the K4 family exist because particular rock types defeat a general-purpose button layout. If your ground is soft but abrasive, or highly fractured, ask for the variant rather than accepting the standard bit and compensating with more of them.

Threaded families (R25, R32) are a different design logic. The R32 bits in our range are pilot or guide bits: they cut a 76, 89 or 102 mm hole around a 40 mm pilot, with an R32 thread. They belong in controlled-diameter work — piloting, utilities, some anchor work — not in blasthole production where a K-family button bit on a square shank is the standard tool.

Face and button selection by rock

The general rule from the top of this page has a practical form:

• Hard, abrasive rock (granite, basalt, quartz porphyry): more buttons, smaller button protrusion, strong gauge protection. The bit should look busy and slightly heavy. Running a light face here produces flat buttons and polished rock.
• Medium rock (limestone, sandstone, marble): general-purpose flat or convex face with a moderate button count; this is the default configuration for most of our bit models.
• Soft and sticky ground: fewer, larger buttons and generous flushing. The risk here is not wear but packing — cuttings that are not lifted quickly are re-drilled, and the bit wears from its own debris.
• Broken and fissured ground: concave face and heavier gauge; the concave shape keeps the bit centred when the hole wall is not.

Button angle and gauge row count are specified per family and per part number — the L2 example (5×12 gauge buttons at 42°) shows the level of detail. When you reorder, quote the part number rather than the diameter; the same diameter exists in several button layouts in most families.

Wear is a cost decision, not just a spec

Wear is a cost decision, not just a spec. Bit life and regrinding interval drive your cost per metre as much as the rig does. Keep a regrinding schedule and record metres per bit — the data will tell you when to change button type.

Two mechanics matter more than brand comparisons. First, a worn gauge row makes an undersized hole, which loads the string and the next bit in the sequence. Second, a flat button stops breaking rock and starts crushing it, which raises the required thrust and generates heat in the button. Both failures are visible early if someone looks: a gauge row that has lost its protrusion, a shiny flat spot on a button. A regrinding schedule based on metres, not on calendar, catches both.

Regrinding restores button protrusion and the gauge diameter, but only within limits. The number of regrinds available per bit is a function of the button height specified for that part number. When you record metres per bit across regrinds, you get a curve: the first regrind is nearly free, the last one costs almost as much penetration as it returns. That curve, not a catalogue figure, is what tells you when to change button type or move to a heavier face.

Selection steps

• Confirm the hammer model and its shank family.
• Fix the hole diameter and tolerance, including reaming practice.
• Name the rock: hardness, abrasiveness, fractures, water.
• Choose the face and button configuration for that rock.
• Check gauge protection against the hole tolerance you need to hold.
• Confirm flushing: holes across the face and enough air volume from the compressor.
• Check the regrinding equipment and schedule, and the number of spare bits on site.
• Record metres per bit and regrind cost from the first week, not from the first month.

Checklist

• Confirm hammer model and shank. 2) Hole diameter and tolerance. 3) Rock hardness and abrasiveness. 4) Bit face and button type. 5) Regrinding equipment and schedule. 6) Spare bits on site.

Common mistakes

• Ordering bits by diameter alone. Two suppliers' 130 mm bits can belong to different shank families.
• Ignoring gauge wear and blaming the rig for a hole that comes in undersized.
• Running a general-purpose face in highly abrasive rock, then buying more bits instead of a different layout.
• Leaving the same bit on across a regrinding interval, in the belief that regrinding costs production time. Flat buttons cost more.
• Skipping the flushing check when the compressor has just been resized. More air at the face with the same flush holes can still be the wrong distribution.
• Using threaded pilot bits (R32 class) for blasthole production work where a K-family button bit is the right tool.
• Recording cost per bit instead of cost per metre. The cheaper bit is often the more expensive one.

Conclusion

Shank first, diameter second, face third — and then treat the bit as the consumable that decides your cost per metre. For most 105–165 mm blasthole work the K4, K40 and K5 families cover the ground, with the K6 above 152 mm and K8 above 203 mm; the L2, L3 and M-family bits serve small-diameter and mid-pressure jobs. Keep the regrinding data, quote part numbers rather than diameters on reorders, and change button layout when the data says so.

Next: the DTH hammer selection guide for the hammer side of the match, drilling cost per metre for the full cost picture, and best compressor for DTH drilling for the air supply. Full ranges: DTH drill bits, DTH hammers and DTH drill rods.