The face of a down-the-hole (DTH) bit — the profile of its cutting front — has more influence on your results than most drillers give it credit for. It affects penetration rate, hole straightness, how cleanly cuttings evacuate, and how long the bit lasts in a given formation. Choose the wrong face design and you end up with slower drilling, crooked holes, or premature wear.
DTH bits come in three main face profiles: concave, convex, and flat. Here's how each behaves and when to run it.
Concave face: the all-around workhorse
A concave (dished) face recesses slightly toward the center. That geometry pulls the bit toward the center of the hole as it drills, which keeps holes straight and helps the bit track true.
Concave-face bits are the most common general-purpose choice because they balance penetration speed with hole accuracy. They perform well in medium-hard to hard, reasonably consistent formations — the conditions most quarry and construction drillers see day to day.
- Best for: straight holes in medium to hard rock
- Strengths: good centering, reliable deviation control, versatile
- Watch for: not the fastest option in soft ground
Convex face: speed in softer ground
A convex face bulges outward at the center. It's more aggressive and typically delivers faster penetration in soft to medium formations, with strong cuttings evacuation. The trade-off is hole straightness: a convex face is more prone to wander, so it's less suited to deep holes or jobs with tight deviation tolerances.
- Best for: fast drilling in softer, homogeneous formations
- Strengths: high penetration rate, good flushing
- Watch for: more hole deviation; not ideal for deep or precision holes
Flat face: control in hard and broken rock
A flat face does exactly what it sounds like. It shines in hard, abrasive, fractured, or interbedded formations where other profiles wander or wear unevenly. When you're drilling through broken ground, seams, or rock of varying hardness, a flat face resists deflection and keeps the hole on line.
- Best for: hard, abrasive, or broken/fractured formations
- Strengths: excellent straightness in difficult ground, even wear
- Watch for: generally slower than concave or convex in easy rock
How to choose in the field
A quick way to narrow it down:
- Need straight holes in typical rock? Start with a concave face.
- Drilling soft, uniform ground and want speed? A convex face will move faster.
- Fighting hard, abrasive, or broken formations? A flat face gives you control and even wear.
Face design is only part of the equation — button shape and carbide grade matter just as much, and we'll cover those in a future post. If you're unsure which combination fits your formation, that's exactly the kind of thing our team helps drillers dial in every day.
Talk to a rock drilling specialist
International Driller's Supply is the world's largest distributor of Mitsubishi Materials drilling products and stocks DTH bits in every common face design and diameter. Tell us your formation and hole requirements and we'll help you match the right bit. Call 615-255-1791 or email sales@internationaldrillerssupply.com.