DTH Bit Face Design Guide: Convex vs Concave vs Flat

DTH Bit Face Design: Convex vs Concave vs Flat — Which Should You Choose?

Short answer: there is no single DTH bit face design that is best for every formation. A convex face is commonly shortlisted when penetration and service life in hard, abrasive rock are priorities. A concave face is a strong all-round option when hole stability and straightness matter. A flat face remains a practical general-purpose choice, especially where broken ground, straightforward maintenance, or easy resharpening are important. The final choice still has to match the exact hammer shank, hole diameter, button geometry, flushing layout, formation, and operating objective.

This guide is for quarry, mining, water-well, foundation, and drilling-service teams comparing DTH drill bits before purchase. It focuses on the decision variables that change the answer rather than treating convex, concave, and flat faces as universal rankings.

1. Why DTH Bit Face Design Changes Drilling Performance

The face of a DTH bit is the working geometry that presents the carbide buttons to the rock and provides space for flushing and cuttings movement. Changing the face profile changes how the bit is supported at the bottom of the hole, how impact is distributed across the face and gauge, and how broken rock is cleared away.

Major DTH manufacturers therefore offer more than one face design for the same general hammer class. Boart Longyear, for example, lists flat, concave, and convex configurations within the same DTH product families, while Sandvik emphasizes both face design and flushing layout as contributors to penetration, tool life, and cuttings removal. That is an important procurement lesson: hammer size or bit diameter alone does not define the correct face.

2. Convex vs Concave vs Flat: Quick Comparison

These are selection tendencies from current OEM guidance, not guaranteed performance rankings. Two bits with the same face style can behave differently because their shank, diameter, button count, button shape, gauge protection, steel design, and flushing arrangement are different.

3. When Does a Convex DTH Bit Make Sense?

A convex face projects the center or working area forward relative to the surrounding face. Mincon positions its convex design as particularly effective in hard, abrasive rock and describes it as providing a useful combination of penetration rate and service life. Boart Longyear likewise associates convex designs with higher penetration rates.

That makes convex a logical candidate when the commercial objective is to maintain productive drilling in competent, abrasive formations without choosing a bit only for initial speed. Typical examples include quarry production holes, mining blast holes, and other applications where the formation is relatively hard and operators can compare wear and cost per meter over a meaningful interval.

Do not choose convex from rock hardness alone

Hardness is only one variable. If hole deviation is expensive, if the formation is strongly fractured, or if cuttings are not being cleared efficiently, a different face may produce a better total result. Check the complete drilling system: hammer, bit shank, diameter, compressor capacity, drill string condition, feed, rotation, and flushing.

4. When Is a Concave Face the Better Starting Point?

A concave face has a recessed center that helps the bit seat and stabilize in the bottom of the hole. Mincon describes concave as an all-round design for both soft and hard formations and highlights stability, straightness, and low vibration. Boart Longyear also positions concave designs for straighter holes.

This makes concave worth shortlisting where deviation creates downstream cost: blast patterns that depend on accurate burden and spacing, deeper holes where small angular errors accumulate, water-well or foundation drilling where alignment matters, or jobs where the operator is trying to improve consistency rather than maximize short-run penetration alone.

Concave does not guarantee a straight hole

Bit face is only one contributor to deviation. Drill-string stiffness, worn threads, feed pressure, rotation, collaring practice, formation changes, and rig alignment can all dominate the result. If straightness is the priority, treat concave as one design variable inside a full hole-deviation control plan.

5. Where Does a Flat-Face DTH Bit Fit?

A flat face presents a comparatively simple front geometry. Mincon describes flat face as a traditional general-purpose design and notes its usefulness in very broken ground, hard formations, and softer formations that tend to over-drill. Boart Longyear highlights easy sharpening and maintenance as a practical advantage.

That combination can make flat-face bits attractive for fleets that value straightforward maintenance, jobs with variable or broken formations, and operations where the team wants a familiar baseline against which other face designs can be field-tested.

Flat does not mean “basic” or automatically slower. A well-matched flat bit with the correct buttons, gauge protection, and flushing can outperform a theoretically more aggressive profile that is mismatched to the rock or hammer.

6. Face Profile Is Only One Layer of DTH Bit Selection

Procurement errors often happen when a buyer chooses a face design before confirming the variables that determine whether the bit can even run correctly on the hammer.

Face design and button shape are not the same decision

Epiroc and other major suppliers offer multiple button geometries for different drilling needs. A buyer should not assume that every convex bit uses the same button shape or that every concave bit behaves identically. Face profile determines the overall front geometry; button geometry determines how individual carbide inserts contact and break the rock.

Flushing can decide whether the bit performs as expected

Sandvik explicitly links optimized flushing design with improved cuttings removal and reduced wear. If cuttings remain under the face and are repeatedly crushed, penetration can fall while heat and wear rise. When comparing two face profiles, record the flushing-hole arrangement and the compressor / drilling-fluid conditions rather than evaluating the face in isolation.

7. A Practical Seven-Step Selection Workflow

1. Identify the exact hammer and shank. Record the hammer brand, model, shank standard, chuck / retaining interface, and existing bit details. Similar outside diameter does not establish compatibility.
2. Define the hole requirement. Confirm target diameter, tolerance, depth, and whether deviation has a direct cost consequence.
3. Describe the formation. Note hardness, abrasiveness, fracture pattern, weathering, water, and whether conditions change by bench or depth.
4. Rank the performance objective. Decide whether penetration rate, hole straightness, bit life, easy maintenance, or lowest total cost per meter is the first priority.
5. Shortlist the face design. Use convex, concave, or flat as a hypothesis based on the formation and priority—not as a final answer based on a catalog label.
6. Match buttons and flushing. Check button geometry, gauge protection, flushing holes, compressor capacity, and cuttings volume.
7. Run a controlled field comparison when the economics justify it. Compare similar holes and record penetration rate, meters drilled, deviation, gauge / face wear, button condition, and grinding or replacement interval. The winning bit is the one that delivers the best total operating result, not necessarily the fastest first hole.

8. Six Common DTH Bit Face Selection Mistakes

1. Ordering from a product photo. Face appearance does not confirm the shank or retaining interface.
2. Using rock hardness as the only criterion. Abrasiveness, fracture pattern, hole depth, and straightness requirements can change the preferred face.
3. Confusing face profile with carbide profile. Convex / concave / flat and spherical / ballistic are different variables.
4. Ignoring flushing. A bit that cannot clear cuttings efficiently may show poor penetration and abnormal wear regardless of face design.
5. Standardizing one face across every bench. Formation changes can make a previously successful design inefficient.
6. Judging only initial penetration rate. Track usable life, deviation, grinding frequency, downtime, and cost per drilled meter.

9. What Should You Send in an RFQ for a DTH Drill Bit?

PerfoMax's current DTH drill-bit selection process is based on the complete operating condition, not only a nominal bit size. Send:

• hammer make, exact model, and shank standard;
• old-bit photos, shank drawing, or dimensional reference if the interface is uncertain;
• required hole diameter and tolerance;
• hole depth and application: quarry, mining, water well, foundation, geothermal, or other;
• rock strength / hardness, abrasiveness, fracture pattern, and representative photos if available;
• working air pressure and compressor air volume;
• water / foam injection details if used;
• current face design, button shape, flushing layout, and the wear problem you want to improve;
• required quantity and destination.

Start with the current PerfoMax DTH Drill Bits, review the DTH Hammer Selection Guide, browse the DTH Tools collection, or send the operating data through the Request a Quote page.

FAQ

Is concave always the best DTH bit for straight holes?

No. Concave faces are commonly positioned by OEMs for stability and straighter drilling, but hole direction also depends on the drill string, rig alignment, operating parameters, and formation. Use it as a strong candidate when straightness matters, then validate the whole system.

Is a convex DTH bit best for hard, abrasive rock?

Convex is a well-supported starting point for hard, abrasive formations. Mincon specifically highlights that application, and Boart Longyear associates convex designs with higher penetration. Final selection should still be validated against actual wear, straightness, and cost per meter.

When should I choose a flat-face DTH bit?

Flat-face designs are useful as general-purpose options and are often considered for broken ground, hard formations, and some softer formations. They can also be attractive where simple resharpening and maintenance are important.

Does the DTH bit face determine hammer compatibility?

No. Compatibility is primarily controlled by the shank, spline, chuck, and retaining interface. Two bits can have the same face design and diameter but fit different hammers. Confirm the exact hammer model and shank before ordering.

Should I choose face design or button shape first?

First lock the hammer / shank and required hole diameter. Then use the formation and performance objective to select the face and button geometry together. Also verify flushing because cuttings removal directly affects how the selected combination performs.

Technical References

• Mincon — DTH Drill Bits and Bit Face Designs
• Boart Longyear — DTH Bits
• Sandvik — Down-the-Hole Bits
• Epiroc — COP Gold Series DTH Tools

Next Step: Match the Face to the Complete Drilling System

Do not buy a DTH bit by face shape alone. A reliable selection starts with exact hammer compatibility and hole diameter, then matches the formation, straightness requirement, wear objective, carbide geometry, and flushing system. Send PerfoMax the hammer model, old-bit reference, hole specification, formation information, air supply, and current wear problem so the quotation can be built around the actual drilling condition.