DTH Hammer and Bit Selection

DTH Hammer and Bit Selection

DTH hammer and bit selection connects the finished bore, formation, mechanically compatible tooling, rig capability, air demand and effective cuttings removal. Reddy Compressors helps organise the compressor and supply enquiry around that complete duty. The guide explains what to compare, what to measure and what must be confirmed by the tooling and equipment suppliers.

Flow basis and compatibility checks before selection

Treat the hammer’s nominal designation as a family label, then verify the actual hammer body, bit diameter, shank, retaining arrangement and approved application envelope. Epiroc’s catalogue separates hammer selection, bit shanks and pipe selection [V2]. Do not convert a catalogue rule of thumb into a universal compatibility rule across manufacturers.

Hammer exhaust commonly provides the annular cleaning air. Compare its resulting local flow against the cleaning requirement rather than adding two figures that describe the same air at successive boundaries. Local flow changes with absolute pressure and temperature. Free-air delivery cannot be substituted directly into a bottom-hole velocity equation. Genuine separate simultaneous users must be counted on a declared common basis.

For a 152 mm illustrative hole and 89 mm pipe body, $A_a=\pi \left({0.152}^2-{0.089}^2\right)/4=0.011925\hspace{0.25em}{m}^2$. If local air volume at a stated annular section is assumed to be $0.30\hspace{0.25em}{m}^3/s$, the simple area-average gas velocity is $0.30/0.011925=25.16\hspace{0.25em}m/s$. That result is not a cuttings-transport acceptance threshold. It excludes slip, water, solids, washouts and local restrictions, and it is invalid if the 0.30 figure is actually a reference-volume flow.

Use the tooling manufacturer’s recommendations for lubricant identity, injection rate, rotation, feed and regrinding. The COP M6 operator instructions describe contamination and lubrication checks for that model [V3]; its particular values and procedures are not automatically transferable to a different hammer.

Detailed engineering guidance

Engineering the Hammer Bit Compressor Formation Interface for Efficient Down the Hole Drilling

The DTH hammer and drill bit form the energy-delivery interface between the compressed-air system and the rock formation. Compressor capacity alone cannot guarantee drilling performance if the hammer is incorrectly sized, the bit is unsuitable for the formation, or the combined hammer/bit arrangement cannot use the available airflow effectively.

The engineering chain is:

$\mathrm{Compressor}\to \mathrm{Airflow\; +\; Pressure}\to \mathrm{Drill\; String}\to \mathrm{DTH\; Hammer}\to \mathrm{Bit}\to \mathrm{Rock}\to \mathrm{Cuttings\; Removal}$

Every element must be compatible.

1 What Does a DTH Hammer Actually Do

A Down-the-Hole hammer is a pneumatically powered percussion mechanism positioned immediately behind the drill bit.

Compressed air enters the hammer and drives an internal piston through a rapid reciprocating cycle.

The piston repeatedly strikes the bit.

Therefore:

$\mathrm{Pneumatic\; Energy}\to \mathrm{Piston\; Kinetic\; Energy}\to \mathrm{Impact\; Energy}\to \mathrm{Rock\; Fracture}$

After performing work inside the hammer, the exhaust air passes through the bit and performs another critical function:

$\mathrm{Exhaust\; Air}\to \mathrm{Bit-Face\; Cleaning}\to \mathrm{Cuttings\; Entrainment}\to \mathrm{Annular\; Transport}$

The air therefore serves both percussion and flushing.

2 Hammer Size Is Not the Same as Hole Diameter

A common selection error is to assume:

“A 6-inch hole requires a 6-inch hammer.”

Nominal hammer designation, actual hammer-body outside diameter and approved bit diameter are different fields. A nominal size can match a marketed hole size; it does not itself verify the physical combination.

A hammer family normally operates with a specified range of compatible bit diameters.

The engineering relationship is:

$D_{bit}>D_{hammer}$

with the permissible combination determined by the hammer manufacturer.

The difference between bit diameter and drill-string/hammer outside diameter also influences the annular return area.

3 Start With the Required Borehole

Hammer selection should begin with the finished hole—not with the compressor already available.

Define:

• required bore diameter
• target depth
• casing diameter
• casing programme
• geological formation
• expected groundwater
• required penetration performance
• rig capability
• available compressor capacity
• Then determine the hammer and bit combination

The preferred workflow is:

$\mathrm{Hole\; Requirement}\to \mathrm{Bit}\to \mathrm{Hammer}\to \mathrm{Air\; Requirement}\to \mathrm{Compressor}$

rather than:

$\mathrm{Available\; Compressor}\to \mathrm{Whatever\; Hammer\; Will\; Run}$

4 Formation Determines Bit Requirements

Rock formations vary enormously.

The bit must create an effective fracture mechanism for the actual formation.

Important geological characteristics include:

• compressive strength
• hardness
• abrasiveness
• fracture structure
• competency
• weathering
• inclusions
• discontinuities
• water conditions

A bit optimized for hard competent granite may not be optimal for fractured or softer formations.

Therefore:

$\mathrm{Correct\; Bit}=f\left(\mathrm{Rock},\mathrm{Hammer},\mathrm{Hole\; Diameter},\mathrm{Flushing},\mathrm{Drilling\; Objective}\right)$

5 Basic DTH Bit Anatomy

A typical DTH bit includes:

Bit Face

The working surface exposed to the formation.

Carbide Buttons

Transmit concentrated impact forces into the rock.

Gauge Buttons

Maintain borehole diameter and stabilize the outer cutting profile.

Face Buttons

Break the central and intermediate rock area.

Flushing Holes

Allow exhaust air to reach the bit face.

Flushing Channels

Guide cuttings away from the working face.

Bit Head

Supports the cutting structure.

Shank

Interfaces mechanically with the hammer.

Splines

Transmit rotational torque while permitting the required axial movement.

The bit is therefore simultaneously a:

percussion tool + rotational tool + flushing component + bore-diameter control element.

6 Bit Face Designs

Different bit-face geometries may be selected for different drilling conditions.

Common concepts include:

Flat Face

A relatively straightforward face geometry used across many formations.

Potential characteristics include:

• robust geometry
• broad application range
• stable drilling characteristics

Concave Face

The centre of the bit is recessed relative to the outer cutting region.

Depending on design and formation, it may assist:

• hole centering
• directional stability
• cuttings management

Convex Face

The central region projects forward.

Certain designs may be applied where aggressive penetration and particular rock-breaking characteristics are desired.

Drop Centre Recessed Designs

May improve centering or flushing behavior in particular formations.

There is no universally superior face geometry.

Selection should follow the bit manufacturer’s recommendations for the actual geology and drilling conditions.

7 Button Geometry

Carbide button geometry materially affects rock interaction.

Typical button forms include:

Spherical Ballistic Type Profiles

Different manufacturers use different terminology and geometries.

More rounded profiles can provide robust service in demanding formations.

More pointed or ballistic geometries can offer aggressive penetration in suitable rock but may respond differently to abrasive wear and impact conditions.

The correct choice balances:

$\mathrm{Penetration\; Rate}+\mathrm{Button\; Life}+\mathrm{Gauge\; Retention}+\mathrm{Rock\; Suitability}$

Selecting only for maximum initial penetration can increase total drilling cost if bit life falls sharply.

8 Gauge Protection Is Critical

The outermost buttons determine and maintain hole diameter.

As gauge wear progresses:

$D_{effective}\downarrow$

This can create problems when:

• changing to a new full-gauge bit
• withdrawing tooling
• running casing
• maintaining straightness
• continuing a long bore

Gauge condition should therefore be monitored rather than judging bit condition solely by face-button wear.

9 Bit Diameter Controls Annular Area

The bit establishes bore diameter.

Therefore changing the bit immediately changes the annular geometry.

If bit diameter increases while drill-pipe diameter and airflow remain unchanged:

$D_h\uparrow \Rightarrow A_a\uparrow \Rightarrow V_a\downarrow$

Thus a larger bit may require more airflow even if the same hammer can mechanically accommodate it.

This is one of the most important links between bit selection and compressor sizing.

10 Hammer Air Consumption

Every hammer should have a manufacturer-defined air-consumption characteristic.

Conceptually:

$Q_h=f\left(P_h\right)$

Where:

$Q_h$ = hammer air consumption,

$P_h$ = hammer inlet pressure.

As operating pressure changes, hammer consumption and performance may also change.

Therefore hammer selection must verify:

$Q_{compressor,available}\ge Q_{hammer,required}$

while verifying the same exhaust-flow path against the cleaning requirement. Do not add hammer flow and annular cleaning flow as independent loads unless the actual system has separate simultaneous consumers.

This continues the engineering framework established in the hammer air-demand guide — Air Requirements for DTH Hammer Drilling.

11 Working Pressure and Impact Energy

Within the approved operating envelope, increased hammer pressure can alter piston velocity, blow frequency and/or impact energy depending on hammer design.

But:

$P\uparrow \ne \mathrm{Unlimited\; Performance\; Increase}$

The hammer has defined pressure limits.

Operating beyond manufacturer limits can increase:

• mechanical stress
• seal loading
• thermal stress
• lubricant demand
• bit loading
• component wear
• failure risk

Maximum available compressor pressure is therefore not automatically the correct hammer pressure.

12 Hammer Compressor Matching

Three broad conditions can occur.

Condition A Compressor Too Small

Possible consequences:

• hammer operates weakly
• insufficient working pressure
• poor flushing
• low penetration
• cuttings accumulation

Condition B Correct Match

The compressor supplies adequate:

• mass flow
• operating pressure
• flushing air
• pressure reserve
• This is the target condition

Condition C Compressor Excessively Large or Incorrectly Controlled

Potential consequences include:

• unnecessary fuel consumption
• excessive operating pressure
• inappropriate hammer loading
• increased wear
• excessive system cost
• Correct matching matters more than simply maximizing compressor capacity

13 Hammer Bit Compatibility

The bit and hammer must be mechanically compatible.

Verify:

• shank type
• spline configuration
• hammer model
• retaining system
• bit diameter range
• bit head geometry
• flushing arrangement

A bit that is approximately the correct diameter but has an incompatible shank is not interchangeable.

Shank compatibility should be confirmed from verified manufacturer information.

14 Drill Pipe Diameter Also Matters

The drill string connects the compressor system to the hammer.

Its internal diameter influences pressure loss.

A simplified pressure-loss relationship is:

$\Delta P_{pipe}=f\left(L,D_i,Q,\rho ,\mathrm{roughness}\right)$

Where:

$L$ = drill-string length,

$D_i$ = internal diameter,

$Q$ = airflow,

$\rho$ = air density.

Increasing depth means increasing drill-string length.

Therefore:

$L\uparrow \Rightarrow \Delta P_{pipe}\uparrow$

all else equal.

A high-performance hammer cannot compensate for severe upstream pressure loss.

15 Rotation Is Essential

A DTH hammer does not simply pound one point repeatedly.

The drilling rig rotates the hammer and bit between impacts.

This distributes successive impacts across the bottom-hole pattern.

The combined action is:

$\mathrm{Impact}+\mathrm{Rotation}+\mathrm{Feed}+\mathrm{Flushing}=\mathrm{DTH\; Drilling}$

Incorrect rotation can reduce performance even when compressor and hammer sizing are correct.

16 Rotation Speed

Optimum rotation depends on variables including:

• hammer diameter
• bit diameter
• impact frequency
• rock type
• penetration rate
• button layout

If rotation is inappropriate, successive button impacts may not interact with the formation efficiently.

Too little rotation can repeatedly impact similar locations.

Excessive rotation can increase:

• button wear
• spline wear
• unnecessary mechanical loading

Manufacturer and drilling-tool recommendations should establish the operating range.

17 Feed Force

The hammer must remain appropriately engaged with the bit and formation.

Insufficient feed can result in:

• inefficient energy transfer
• unstable operation
• unnecessary vibration

Excessive feed can:

• restrict effective hammer action
• increase bit loading
• contribute to deviation
• increase wear

Therefore:

$\mathrm{Correct\; DTH\; Performance}=\mathrm{Air}+\mathrm{Rotation}+\mathrm{Correct\; Feed}$

18 Flushing Design Is Part of Bit Selection

A bit is not selected solely for carbide geometry.

Its air passages must efficiently remove fractured material.

Poor flushing can cause:

• cuttings recirculation
• regrinding
• reduced penetration
• elevated bit temperature
• accelerated wear
• increased risk of sticking

The objective is:

$\mathrm{Fracture\; Rock}\to \mathrm{Immediately\; Remove\; Fragments}$

Efficient drilling requires both processes.

19 Regrinding Is Wasted Energy

When cuttings are not removed efficiently, the bit may repeatedly break already-fractured particles instead of attacking fresh rock.

This consumes:

• compressed-air energy
• fuel
• bit life
• drilling time

Thus:

$\mathrm{Good\; Hole\; Cleaning}\to \mathrm{More\; Energy\; Applied\; to\; Fresh\; Formation}$

This is why annular velocity and bit flushing cannot be separated from hammer selection.

20 Hard Rock Selection

In hard, competent formations, engineering priorities may include:

• adequate impact energy
• durable carbide configuration
• gauge retention
• suitable button geometry
• sufficient working pressure
• reliable flushing
• But “hard rock” alone is not a complete bit specification
• Abrasiveness and fracture characteristics must also be considered

21 Abrasive Formation

Abrasive formations can accelerate:

• carbide wear
• gauge wear
• bit-body erosion
• flushing-channel erosion
• The lowest-cost bit may therefore produce the highest cost per metre

A better commercial metric is:

$C_m=\frac{C_{bit}+C_{downtime}+C_{support}}{\mathrm{Metres\; Drilled}}$

where $C_m$ is effective drilling-tool cost per metre.

22 Fractured and Unstable Formations

Fractured formations introduce different problems:

• hole deviation
• unstable walls
• loss of return
• cuttings accumulation
• jamming
• difficult casing advancement

In such conditions, bit/hammer selection may need to be coordinated with:

• casing system
• drilling method
• flushing strategy
• borehole stabilization plan
• Compressor capacity alone cannot solve geological instability

23 Water Bearing Formations

Water introduces hydrostatic backpressure:

$P_w=\rho gh$

The hammer still requires adequate differential pressure after overcoming this head.

Therefore:

$P_{source}\ge P_{hammer}+P_w+\sum \Delta P+P_{margin}$

As water head increases, the selected hammer/compressor system may eventually require a high-pressure compressor or booster configuration.

24 Bit Wear Monitoring

Important wear indicators include:

• button profile
• carbide damage
• cracked buttons
• missing buttons
• gauge loss
• bit-body erosion
• flushing-hole condition
• spline condition
• shank damage
• Bit wear should be evaluated systematically

Continuing to drill with severely degraded tooling can reduce penetration and increase risk to other components.

25 Button Sharpening Regrinding

Depending on bit design and manufacturer recommendations, carbide buttons may require regrinding before excessive flattening develops.

The purpose is to restore an effective cutting profile.

However:

Regrinding limits, methods and discard criteria must follow the tool manufacturer’s instructions.

Incorrect grinding can damage carbide or create unsuitable geometry.

26 Hammer Lubrication

DTH hammers commonly require correctly controlled rock-drill lubrication according to their design.

Lubrication affects:

• piston surfaces
• internal wear
• corrosion protection
• operating reliability

Lubrication requirements can vary with:

• hammer design
• airflow
• operating pressure
• ambient temperature
• drilling conditions
• Insufficient lubrication can rapidly damage a hammer
• Excessive or incorrect lubricant can also create operational problems

Follow the hammer manufacturer’s lubricant specification and feed-rate instructions.

27 Contamination Control

Compressed air entering the hammer should not carry harmful contamination beyond what the equipment is designed to tolerate.

Potential problems include:

• debris entering during pipe connections
• corrosion products
• unsuitable water contamination
• dirty lubricator systems
• damaged hoses
• Clean drilling practices can materially extend hammer life

28 Selection Matrix

This matrix is conceptual. Final selection requires verified equipment data.

29 DTH Selection Workflow

A disciplined engineering sequence is:

$A_a=\frac{\pi }{4}\left(D_h^2-D_p^2\right)$

This prevents isolated equipment selection.

30 Total Drilling Economics

The fastest instantaneous penetration rate does not necessarily produce the lowest project cost.

Consider:

$C_{metre}=\frac{C_{fuel}+C_{compressor}+C_{hammer}+C_{bits}+C_{rig}+C_{labour}+C_{downtime}}{\mathrm{Metres\; Completed}}$

A properly matched system can reduce cost by improving:

• bit life
• hammer life
• fuel efficiency
• penetration consistency
• hole quality
• equipment utilization

31 Common Selection Mistakes

Mistake 1 — Selecting hammer only by nominal size

The complete operating envelope matters.

Mistake 2 — Selecting bit only by hole diameter

Formation, flushing and hammer compatibility matter.

Mistake 3 — Assuming more pressure always means faster drilling

Pressure must remain within approved operating conditions.

Mistake 4 — Ignoring annular velocity

A powerful hammer cannot compensate for inadequate hole cleaning.

Mistake 5 — Ignoring water head

Deep water-bearing drilling can fundamentally change pressure requirements.

Mistake 6 — Ignoring compressor derating

Site conditions can change actual available capacity.

Mistake 7 — Ignoring tool wear

A worn bit can make an adequately sized compressor appear unable to support the duty because worn cutting and flushing features change the operating result. Record actual bit condition before concluding that compressor capacity is the constraint.

32 A selection decision that can be verified

The selected combination should have an identifiable hammer model, approved bit shank and retention arrangement, bit diameter and manufacturer performance basis. Record the rig capacity, air-delivery evidence, formation assumptions, water case and limits that must be reassessed if the job changes. A buying decision is incomplete when any of those fields is only a verbal assumption.

An alternative bit or hammer must be rechecked against this record. A supplier may offer a mechanically compatible alternative with different air consumption, flushing arrangement or formation suitability. Mechanical fit is one acceptance gate; the complete operating requirement is another.

Project delivery and verification

The hammer and bit establish important boundaries for a DTH air enquiry. A useful selection record identifies the exact assembly, its intended hole range and the manufacturer’s operating information. Describing tooling only by nominal inch size leaves too much unresolved for a reliable equipment proposal.

Begin with an identifiable assembly

Record hammer make, model, variant, serial or asset identity where available, bit diameter and shank type. Include the connection to the drill string and any installed options that affect the configuration. The OEM DTH catalogue separates hammer variants, recommended hole sizes, shank styles and working-pressure ranges; these are model-specific entries, not interchangeable family labels.

For an existing assembly, ask whether components have been changed since its original purchase. A photograph can help establish identity, but worn markings or an undocumented rebuild may require inspection. The enquiry should distinguish confirmed details from assumptions.

Match tooling to the work

Give the drilling specialist the target hole geometry, available formation description, water observations and rig limits. Request a reasoned tooling recommendation for those conditions. Avoid choosing a bit solely because it fits the hammer or selecting a hammer solely because a compressor is already owned.

When comparing alternatives, request the applicable air-consumption information and operating guidance for each. Confirm how the manufacturer defines the quoted pressure and airflow. Tooling changes should trigger a review of both compressor duty and the return passage around the drill string.

Make wear and maintenance visible

Keep tooling identity, service history and observed condition with performance records. If results change, record the hole interval and operating circumstances before attributing the change to the compressor. The review may need to examine tooling condition, configuration and operating method as well as the air supply.

Replacement decisions should use the manufacturer’s inspection and service criteria. This guide does not assign generic wear limits, sharpening procedures or overhaul tolerances across hammer brands.

What to send with a request

Provide clear photographs of markings, the existing bit and connections, plus the manual or parts reference. State whether the enquiry concerns a new selection, a replacement bit, a service issue or an air-package change. For spare parts, identify the exact component rather than ordering by appearance.

Start a tooling-related air enquiry: combine these details with the compressor duty and site brief. Use DTH Hammer Air Requirement for demand interpretation and Bore Diameter vs Air Requirement for geometry.

DTH Hammer and Bit Selection decision worksheet

Complete the evidence field and record the reviewer decision. Blank fields or an unresolved item mean the selection remains open. This worksheet organises evidence; it does not calculate an equipment approval.

Frequently asked questions

Is nominal hammer size enough to choose a bit

No. Verify the manufacturer-approved hammer model, bit diameter, shank and retaining arrangement, plus the application envelope.

Should hammer flow and cleaning flow be added

Not when they describe the same air passing through the hammer and then the annulus. Assess the actual flow paths and separately identify simultaneous additional users.

Can more compressor pressure compensate for the wrong bit

Pressure must remain within the approved equipment limits. Formation suitability, compatibility, feed, rotation, condition and cleaning also govern performance.

How should bit wear be assessed

Inspect gauge, buttons, body, flushing passages, shank and retention features against the applicable manufacturer instructions.

Is one face design best for every formation

No. Compare the actual rock, abrasiveness, fracture conditions, hole objective and supplier recommendation.

What information is needed for selection

Bore/casing objective, geology and water case, hammer and bit identities, rig capability, compressor performance evidence and operating records.

Start a duty based enquiry

Share the completed worksheet, machine and tooling nameplates, available performance evidence and site details with Reddy Compressors. Sales, rental and service scope will be confirmed for the assignment. High-pressure equipment requires supplier confirmation; no drilling-depth, savings, delivery-time or emergency-response guarantee is made.