DTH Drill Bit Face Design: Concave, Convex, or Flat?

Three MosCut DTH drill bits displayed side-by-side highlighting the distinct geometries of flat, convex, and concave face designs

The geometric advantage: Selecting the correct face design dictates how percussive energy is transferred into the rock and how efficiently abrasive cuttings are evacuated from the hole.

In high-performance quarry drilling, there is no such thing as a “universal” drill bit. According to geotechnical literature published by the American Rock Mechanics Association (ARMA), the Uniaxial Compressive Strength (UCS) and fracture state of the rock formation demand specific tool geometry for efficient energy transfer. The “face” of the drill bit—whether it is concave, convex, or flat—dictates the trajectory of the percussive shockwave and the airflow path for rock cuttings. Utilizing the wrong face design in a complex geological formation can lead to a 40% loss in penetration rate and severe deviation of the borehole trajectory.

A marble extraction company in Turkey operating in heavily fractured and faulted geological zones consistently struggled with vertical pilot holes drifting off-target. This deviation made wire saw intersections nearly impossible. Upon inspection, MosCut field engineers discovered operators were using standard Flat Face bits, which tend to “skate” when hitting angled natural fissures. By transitioning their inventory strictly to Concave Face bits, the quarry leveraged the “self-centering” physical properties of the indented face. The bit tracked perfectly straight through the fractured zones, improving their hole straightness by 100% and drastically reducing wire saw setup times.

The Physics of Penetration: Why Face Shape Matters

You are fighting two enemies simultaneously: solid rock resistance and accumulated rock dust. The bit face determines which enemy you conquer first.

Every time the DTH hammer strikes, the tungsten carbide buttons shatter the rock. However, the subsequent challenge is immediate: how do you get those rock chips out of the way before the next strike? If the cuttings are not evacuated instantly, the bit begins to grind its own dust, a phenomenon known as ‘secondary crushing’, which rapidly destroys the carbide buttons and stalls penetration.

The geometric face of the drill bit is specifically engineered to direct high-pressure exhaust air. Certain face designs prioritize massive, wide air channels to blow out soft, sticky rock dust rapidly. Other face designs prioritize maximum steel mass and structural integrity to withstand the brutal, unrelenting impact of extreme high-quartz granite. Matching the face profile to the specific rock condition ensures optimal energy transfer and fluid dynamics at the bottom of the hole.

Directed airflow: The geometric grooves and flushing holes on the bit face are mathematically engineered to create high-velocity updrafts, clearing the rock face instantly.

The Core Four: Face Designs Explained

Understand the distinct mechanical advantages of the four primary DTH bit geometries to optimize your quarrying operations.

📏 Flat Face: The Hard Rock Hammer

Design: The front of the bit is completely flat. All inner buttons are on the exact same horizontal plane.

Characteristics: This is the most robust and durable design. Because the face is flat, it contains the maximum amount of solid steel mass to absorb severe percussive shockwaves.

Ideal Application: Extremely hard, highly abrasive, and solid (non-fractured) formations like high-quartz granite or basalt. When brute force is required over finesse, the Flat Face delivers.

⛰️ Convex Face: The Fast Penetrator

Design: The center of the bit protrudes outward (like a dome), placing the center buttons slightly ahead of the gauge buttons.

Characteristics: The domed shape creates exceptionally wide and deep flushing channels. This allows for massive volumes of air to rapidly evacuate heavy or sticky rock cuttings.

Ideal Application: Soft to medium-hard rock (like standard limestone and soft marble). It provides the highest penetration rate possible but is prone to drifting if it hits angled hard-rock faults.

🎯 Concave Face: The Straight Shooter

Design: The center of the bit is smoothly indented inward (bowl-shaped), creating a conical depression.

Characteristics: As it drills, the indented center leaves a small cone of rock in the middle of the hole. The bit continuously rides over this cone, creating a powerful “self-centering” physical effect.

Ideal Application: Medium-hard to hard rock that is heavily fractured, faulted, or jointed. When maintaining an absolutely straight hole is critical for wire saw intersections, the Concave face prevents deviation.

🕳️ Drop Center Face: The Aggressive Hybrid

Design: Features a wide, flat outer gauge ring with a sudden, deeply recessed center section.

Characteristics: Combines the massive outer steel strength of a Flat face with the extreme self-centering tracking of a Concave face. It generates the largest rock chips and blows them out instantly.

Ideal Application: Soft-to-medium but highly abrasive rock where both maximum penetration speed and strict hole straightness are required simultaneously. A favorite for deep-hole limestone drilling.

Button Profiles: Spherical vs. Ballistic

The face shape dictates the airflow, but the button shape dictates how the rock fractures.

Once you select the correct face geometry, you must choose the shape of the Tungsten Carbide (TC) inserts. The two primary profiles used in stone quarrying are Spherical and Ballistic.

Spherical Buttons (Domed): These are semi-circles. They possess the highest structural strength and are incredibly resistant to chipping and breaking. They are mandatory for extremely hard, abrasive granite where impact forces are immense. While they drill slightly slower, their lifespan is vastly superior in brutal conditions.

Ballistic Buttons (Parabolic): Shaped like a bullet, these buttons penetrate deeper into the rock on every strike. They generate significantly faster drilling speeds in medium-hard rocks like marble and limestone. However, if used in extreme hard rock or fractured zones, the prominent tip is highly vulnerable to sideways shear forces and will easily snap off.

Close up comparison of a spherical domed tungsten carbide button versus a pointed ballistic button on a drill bit

Crushing vs. Piercing: Spherical buttons (left) provide maximum durability for crushing hard granite, while ballistic buttons (right) offer aggressive penetration in softer limestone.

The Ultimate Selection Matrix

Use this chart to instantly match the geological reality of your quarry with the optimal MosCut bit configuration.

Rock Hardness	Rock Condition	Recommended Face Design	Recommended Button Shape
Hard (Granite / Basalt)	Solid / Uniform	Flat Face	Spherical
Hard (Granite / Quartzite)	Fractured / Faulted	Concave Face	Spherical
Medium (Marble / Hard Limestone)	Solid / Uniform	Flat or Convex Face	Semi-Ballistic or Spherical
Medium (Marble / Hard Limestone)	Fractured / Jointed	Concave Face	Spherical
Soft (Travertine / Soft Limestone)	Solid / Porous	Convex Face	Ballistic

Equip Your Rig with the Perfect Tool

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Frequently Asked Questions on Drill Bit Selection

1. Can I use a Convex face bit in hard granite to drill faster?

It is not recommended. While the penetration might initially seem faster, the prominent center structure of a Convex bit is highly susceptible to structural fatigue in extreme hard rock. The center buttons will wear out rapidly or shear off, ruining the bit prematurely.

2. Why does a Concave bit prevent the hole from deviating?

The indented center of a Concave bit naturally shapes the bottom of the borehole into a small peak or cone. Because the bit constantly rides over this physical cone, it is continuously forced back into the absolute center of the hole, resisting the urge to slide sideways when hitting angled rock layers.

3. What is a “Drop Center” bit?

A “Drop Center” is another industrial term for a Concave face bit. The center section is “dropped” or recessed below the outer gauge ring of the drill bit.

4. Should I choose larger or smaller flushing holes on the bit face?

Larger flushing holes are better for softer, sedimentary rocks (like limestone) that produce massive volumes of coarse chips. Smaller flushing holes create higher velocity air jets, which are excellent for blowing out the fine, heavy dust produced by extremely hard granite.

5. How often should I re-grind the carbide buttons on my DTH bit?

In abrasive rock, operators should inspect the bit every 50 to 100 meters. You must regrind the buttons as soon as the spherical dome wears down to a flat surface that is roughly 1/3 the diameter of the button. Waiting too long causes the drilling speed to plummet and risks shattering the carbide.

6. Can I mix button shapes on the same drill bit?

Yes! A highly effective and common configuration for medium-hard rock is to use Spherical buttons on the outer gauge (for maximum durability against the rock wall) and Ballistic buttons in the center (for faster downward penetration).

7. What happens if the gauge (outer) buttons wear completely flat?

If the gauge buttons wear flat, the bit loses its diameter clearance. The steel body of the bit will begin to grind directly against the rock wall, causing extreme heat, binding the rotation motor, and eventually getting the DTH hammer permanently stuck in the hole.

8. Is a Flat face bit harder to pull out of the hole than a Convex bit?

Generally, yes. Because Flat face bits have narrower flushing channels, rock dust is more likely to accumulate behind the bit head when drilling deep. Operators must ensure constant, high-volume air flushing (pull-back flush) when extracting a Flat face bit.

9. Why do some DTH bits have slots cut into the sides?

These are called “exhaust splines” or “side clearance grooves”. They provide a fast, unobstructed vertical pathway for the heavy rock cuttings and compressed air to travel upward past the body of the bit and into the main borehole annulus.

10. Does a Concave bit require a different feed pressure than a Flat face bit?

Typically, a Concave bit requires slightly less aggressive feed pressure than a Flat face bit, because its self-centering action inherently holds it in place. Pushing too hard on a Concave bit in hard rock can excessively stress the outer gauge buttons.