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How to Select the Right DTH Bit Size and Type for Your Formation
Nov. 05, 2026
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Down-the-hole Drilling Tools must be selected according to the formation, hole diameter, drilling equipment, and project objective. The correct DTH bit is not always the largest or most aggressively designed option. It is the bit that delivers stable penetration, accurate hole quality, acceptable service life, and a reasonable cost per meter.
This guide explains how to choose the right DTH bit size and type step by step. It also covers the tools required for selection, formation testing, bit inspection, purchasing decisions, and common mistakes that reduce drilling performance.
Start by Defining the Drilling Requirements
First, confirm the final hole diameter
The required hole diameter is the starting point for DTH bit selection. The bit diameter must be large enough to create the designed hole, but excessive diameter increases air consumption, drilling resistance, and operating cost.
Confirm the required finished hole diameter from the construction drawing or drilling plan.
Check whether the hole must be enlarged to allow casing installation, grouting, blasting, or anchor placement.
Allow for normal gauge wear when the hole must remain within a strict diameter range.
Check whether the bit will be used in straight drilling, angled drilling, or a long-hole application.
Record the required bit diameter in millimeters or inches before comparing supplier quotations.
Second, identify the hole depth and drilling angle
Hole depth and angle affect the required bit design, flushing performance, and drilling stability. A bit that works well in a short vertical hole may perform poorly in a deep angled hole because cuttings can accumulate and reduce impact efficiency.
Short holes generally allow more flexibility in bit selection.
Deep holes require reliable flushing, sufficient air volume, and strong gauge protection.
Angled holes require stable bit guidance and good cuttings removal.
Long-hole drilling may require a stronger bit body and a design that limits deviation.
Underground drilling may require a compact bit and shank combination for restricted spaces.
Third, define the production target
Purchasing decisions should be based on total drilling cost rather than purchase price alone. A low-priced bit may have a shorter service life, lower penetration rate, or higher risk of hole deviation.
Target meters drilled per shift.
Target meters drilled per bit.
Maximum acceptable drilling time per hole.
Maximum acceptable cost per meter.
Required hole accuracy and wall quality.
Expected number of holes per day or per month.
Analyze the Rock Formation Before Choosing the Bit
Classify the formation by hardness and abrasiveness
Rock hardness and abrasiveness are related but different properties. Hard rock can damage buttons through impact loading, while abrasive rock can rapidly wear the gauge and carbide inserts. The correct bit must address both conditions.
Soft formations: clay, weathered rock, soft limestone, marl, and loose sediment.
Medium formations: sandstone, shale, medium limestone, and moderately weathered granite.
Hard formations: granite, basalt, quartzite, hard limestone, and massive igneous rock.
Highly abrasive formations: quartz-rich sandstone, abrasive granite, volcanic rock, and formations containing hard mineral particles.
Fractured formations: broken rock, fault zones, jointed formations, and unstable ground.
Check whether the formation is homogeneous or mixed
Many drilling projects pass through several rock types. A bit designed for one consistent formation may wear quickly when the hole alternates between soft and hard layers.
Review geological logs, previous drilling records, core samples, and nearby borehole data. Mark the depth of each major layer and identify zones that may cause sudden changes in penetration rate, vibration, or bit wear.
Use field indicators when geological data is limited
If a formal geological report is unavailable, use drilling behavior as an initial indicator. These observations are not a substitute for testing, but they help narrow the selection.
Rapid penetration with low vibration may indicate a soft or fractured formation.
Low penetration with high vibration may indicate hard rock, insufficient impact energy, or excessive bit wear.
Rapid gauge wear usually indicates abrasive rock, poor flushing, excessive rotation, or insufficient bit protection.
Blocked flushing holes may indicate inadequate air volume, wet cuttings, or unsuitable hole cleaning.
Repeated button breakage may indicate excessive impact pressure, incorrect rotation speed, poor alignment, or an unsuitable carbide grade.
Choose the Correct DTH Bit Type
Select the face design according to the formation
The bit face design controls penetration, cuttings evacuation, button support, and hole stability. Common DTH bit face designs include convex, concave, flat, and drop-center configurations.
Convex face bits: Suitable for faster drilling in medium to hard formations when high penetration is important. They may provide good cutting action but can be less tolerant of severe hole deviation or uneven ground.
Concave face bits: Suitable for hard and fractured formations where hole straightness and stable bit guidance are important. The concave profile helps maintain contact with the hole bottom.
Flat face bits: Suitable for general-purpose drilling and some soft to medium formations. They provide a balanced design when the formation conditions are not highly specialized.
Drop-center bits: Suitable for formations where center cutting and improved hole-bottom cleaning are required. They can help maintain penetration in certain hard or fractured conditions.
Choose button shape and carbide arrangement
Carbide button shape affects penetration rate, wear resistance, and resistance to breakage. Button selection should be matched to both formation hardness and abrasiveness.
Spherical buttons: Provide good impact resistance and are commonly used in hard or fractured formations.
Parabolic buttons: Can offer a balance between penetration and wear resistance in medium to hard formations.
Ballistic buttons: Can provide aggressive penetration in softer or medium formations, but may be more vulnerable to damage in severe hard-rock conditions.
Large buttons: Usually provide stronger support and better resistance to breakage.
Small or closely spaced buttons: Can improve cutting coverage but may wear faster in abrasive ground.
Match the bit to the DTH hammer shank
A DTH bit must match the exact hammer shank and connection system. A bit with the wrong shank cannot be installed correctly and may damage the hammer or create unsafe operating conditions.
Confirm the following specifications before ordering:
Hammer brand and model.
Shank type and spline or drive configuration.
Bit diameter and allowable diameter range.
Bit body length and shoulder dimensions.
Flushing hole configuration.
Button size, number, and layout.
Recommended operating pressure and air volume.
Compatibility with the drill pipe and drilling rig.
Follow a Step-by-Step DTH Bit Selection Process
Step 1: Record the project and machine data
Create a selection worksheet before contacting suppliers. Complete the basic operating information so that every quotation can be compared on the same basis.
Project location and application.
Rock type and expected formation changes.
Required hole diameter.
Required hole depth.
Drilling angle.
Drill rig model.
DTH hammer model.
Available working pressure.
Available air volume.
Drill pipe size and connection.
Target penetration rate.
Target bit life and cost per meter.
Step 2: Select the initial bit diameter
Choose a diameter that meets the final hole requirement while allowing for normal gauge wear. Avoid selecting a larger bit only to compensate for uncertain drilling performance. Oversized bits require more power and may reduce penetration.
When the hole must accommodate casing or grout, calculate the required clearance carefully. Confirm the clearance with the project engineer instead of relying on a general rule.
Step 3: Select the face profile
Use the formation assessment to select a starting face design. For hard and fractured rock, prioritize stability and button support. For softer formations, prioritize cutting efficiency and flushing performance.
If the formation changes significantly along the hole, consider using different bit profiles for different sections rather than using one universal bit for the entire project.
Step 4: Select the button configuration
Choose button shape, size, and density according to the balance between penetration and wear resistance. Hard, fractured rock usually requires stronger impact resistance. Abrasive rock requires effective gauge protection and adequate carbide volume.
Step 5: Verify hammer and air compatibility
Check that the selected bit is designed for the hammer model and available compressor capacity. Insufficient air volume can reduce impact power and prevent cuttings from leaving the hole. Excessive operating pressure can increase wear and button damage.
Ask the supplier for the recommended operating range rather than using the maximum pressure as the normal setting.
Step 6: Compare supplier specifications
Request written technical information from each supplier. Compare the following items instead of comparing price alone:
Bit diameter tolerance.
Compatible hammer models.
Face profile.
Carbide grade and button dimensions.
Steel body material and heat treatment.
Flushing hole design.
Gauge protection method.
Recommended drilling parameters.
Expected service life in similar formations.
Replacement and warranty policy.
Lead time and available stock.
Step 7: Run a controlled field trial
Test the selected bit on several comparable holes before placing a large order. Keep the hammer, air pressure, rotation speed, and operator procedure as consistent as possible.
Record the following results:
Drilling time per meter.
Total meters drilled.
Air pressure and compressor output.
Rotation speed and feed pressure.
Bit condition after each hole.
Hole diameter and straightness.
Frequency of rod changes or blockages.
Cost per meter.
Step 8: Approve the final bit specification
After the field trial, confirm the final specification in writing. The purchase order should identify the diameter, shank, face design, button arrangement, flushing design, quantity, inspection requirements, and delivery schedule.
Prepare the Required Tools and Information
Tools for formation assessment
Geological report or rock classification record.
Core samples or representative rock samples.
Previous drilling logs.
Nearby borehole records.
Rock hardness and abrasiveness test data when available.
Camera or mobile device for documenting formation and bit wear.
Tools for bit and hammer compatibility checks
Caliper or vernier caliper.
Steel ruler or measuring tape.
Thread and shank identification chart.
Hammer model manual.
Drill rig and compressor specifications.
Bit drawing or supplier dimensional drawing.
Inspection checklist.
Tools for operating data collection
Pressure gauge.
Airflow measurement device or compressor output record.
Rotation speed display.
Feed pressure display.
Depth counter.
Timer or drilling data logger.
Cost tracking spreadsheet.
Information to send to a DTH bit supplier
Complete technical information allows the supplier to recommend a bit more accurately. Send the following details with the inquiry:
Required hole diameter.
Hammer brand and model.
Rock type and hardness.
Expected abrasiveness.
Hole depth and angle.
Drill rig model.
Compressor pressure and air volume.
Current bit type and performance problems.
Photos of the worn bit and carbide buttons.
Target production rate and expected bit life.
Optimize Drilling Parameters After Selecting the Bit
Set air pressure and airflow correctly
Air must provide sufficient hammer operation and remove cuttings from the hole. If airflow is too low, cuttings can remain at the bottom and cause regrinding, reduced penetration, and excessive bit wear.
Monitor for these signs of inadequate flushing:
Dust or cuttings return decreases during drilling.
Penetration rate falls without a change in formation.
Bit temperature increases.
Flushing holes become blocked.
Cuttings return is weak or inconsistent.
Adjust rotation speed and feed pressure
Rotation speed should allow each button to strike fresh rock without excessive sliding. Feed pressure must keep the bit engaged with the hole bottom without overloading the hammer or carbide buttons.
Excessive rotation can cause gauge wear and button polishing.
Insufficient rotation can create uneven button loading and poor cutting coverage.
Excessive feed pressure can break buttons or damage the bit body.
Insufficient feed pressure can reduce impact transfer and cause the bit to bounce.
Adjust one parameter at a time so that the effect can be measured.
Inspect the bit at regular intervals
Do not wait until the bit stops drilling. Inspect it during rod changes and after each completed hole. Early inspection can prevent hammer damage and help identify formation changes.
Check for:
Rounded, chipped, cracked, or missing buttons.
Gauge diameter loss.
Cracks in the bit body.
Blocked flushing holes.
Uneven wear on the face.
Excessive wear on the outer row of buttons.
Damage around the shank and shoulder.
Calculate Total Cost Instead of Comparing Unit Price
Use cost per meter as the main purchasing metric
The most useful comparison is the total cost required to drill one meter. This calculation combines the bit price with drilling performance, service life, labor, downtime, and related operating costs.
A simple calculation is:
Cost per meter = Total bit and operating cost divided by meters drilled
Include these cost factors:
Bit purchase price.
Freight and import cost.
Downtime for bit replacement.
Labor for removal and installation.
Compressor and fuel consumption.
Hammer maintenance.
Cost of poor hole quality or deviation.
Cost of lost production caused by low penetration.
Compare performance under similar conditions
A bit that drills 20 meters in soft rock cannot be directly compared with a bit that drills 10 meters in hard abrasive rock. Compare products only when the formation, hole size, hammer, air pressure, and operating method are reasonably similar.
Consider supply reliability
Purchasing groups often need predictable delivery more than a small unit price reduction. Evaluate the supplier's ability to maintain consistent dimensions, repeat the same carbide configuration, provide technical support, and deliver replacement bits when production is urgent.
Avoid Common DTH Bit Selection and Operating Mistakes
Mistake 1: Selecting only by diameter
Diameter determines hole size, but it does not determine whether the bit will perform well in a particular formation. Always specify the face profile, button design, shank, flushing layout, and operating range.
Mistake 2: Choosing the cheapest bit without a field trial
A low purchase price may result in high cost per meter if the bit has poor penetration, short life, or inconsistent dimensions. Test a representative quantity and measure performance before approving a large order.
Mistake 3: Using one bit type for every formation
Soft, hard, abrasive, and fractured formations impose different loads on the bit. A general-purpose bit may be acceptable for mixed ground, but specialized formations often require a more specific face and button configuration.
Mistake 4: Ignoring the hammer and compressor
A high-performance bit cannot compensate for an undersized hammer or insufficient air supply. Confirm the complete system, including hammer, bit, drill pipe, compressor, and flushing capacity.
Mistake 5: Operating with excessive feed pressure
More feed pressure does not always mean faster drilling. Excessive pressure can cause button breakage, body cracking, abnormal gauge wear, and hammer damage.
Mistake 6: Failing to clean the flushing holes
Blocked flushing holes reduce cuttings removal and can make a good bit appear ineffective. Inspect and clean the bit according to the supplier's maintenance procedure.
Mistake 7: Continuing to use a worn bit
Once the gauge is undersized or the buttons are badly worn, drilling efficiency can fall rapidly. Continuing to use the bit may damage the hammer and produce an undersized or deviated hole.
Mistake 8: Providing incomplete information to the supplier
Suppliers cannot make a reliable recommendation without the hammer model, hole diameter, formation, air pressure, and operating history. Include photos and actual drilling records whenever possible.
Use a Practical Purchasing Checklist
Technical checklist before ordering
Required hole diameter confirmed.
Hammer shank and model confirmed.
Formation hardness and abrasiveness documented.
Hole depth and drilling angle documented.
Bit face profile selected.
Button shape and arrangement selected.
Flushing design verified.
Rig, compressor, and air requirements checked.
Bit drawing approved.
Field trial results reviewed.
Supplier evaluation checklist
Consistent manufacturing tolerances.
Reliable steel heat treatment.
Suitable carbide quality.
Traceable quality inspection.
Clear technical documentation.
Responsive after-sales support.
Stable production capacity.
Reasonable lead time.
Transparent warranty terms.
References from similar drilling applications.
Final decision checklist
Confirm that the bit matches the formation.
Confirm that the bit matches the hammer.
Confirm that the compressor can provide the required air.
Confirm that the bit reaches the required hole diameter.
Confirm that the trial data supports the expected cost per meter.
Confirm delivery quantity and replacement availability.
Approve the final technical drawing and purchase specification.
The right DTH bit size and type should be selected through a combination of formation analysis, equipment matching, controlled testing, and total cost evaluation. By following these steps, purchasing teams can reduce incorrect orders, prevent premature wear, improve drilling productivity, and obtain more consistent hole quality.
For project-specific recommendations, provide your hole diameter, hammer model, formation description, air pressure, and drilling results to Haichuang Unite. Our Down-the-hole Drilling Tools are selected and configured to support reliable drilling performance from the first test hole through regular production.
