The hammer–bit–compressor chain
A DTH hammer and its bit are the point where compressed air becomes rock-breaking energy. The compressor supplies air at pressure, the drill string carries it down, the hammer turns it into blows, the bit transmits those blows into the rock, and the exhaust air carries the cuttings away. Every link has to match the next one.
A large compressor cannot rescue a bit that is wrong for the rock, and an ideal bit cannot drill quickly if the hammer is starved of air. Most poor-performance complaints on borewell rigs come from one weak link: a hammer too large for the air available, a bit face that clogs in clay-rich weathered rock, buttons left unground until the bit loses gauge, or a slim drill pipe in a wide hole that leaves the cuttings in the annulus.
Selection therefore works in a fixed order: hole, formation, hammer, bit, string and rig, and finally air. Air is last not because it is least important, but because the other choices set what the air has to do.
Start from the finished bore and the formation
The finished bore diameter usually follows from the casing and pump the client needs. Agricultural and domestic borewells in Telangana and Andhra Pradesh commonly use 6½–8 inch bits, with larger diameters for production wells and industrial supply. The bit diameter then narrows the hammer choice, because each hammer size is designed for a range of bit diameters.
The formation decides how the bit should be built. Much of the Deccan plateau has a soft or weathered top zone over hard, abrasive granite and gneiss, while parts of the region drill through basalt. Many holes pass from loose overburden, through weathered rock, into fresh hard rock, often meeting water-bearing fractures on the way. A bit that suits the fresh rock may struggle in the clay-rich weathered zone, and overburden that will not stand usually needs casing to be advanced as the hole is drilled.
| Formation condition | Typical bit face | Typical button shape | Points to watch |
|---|---|---|---|
| Soft to medium, weathered or clay-rich | Concave or drop-centre with good flushing channels | Ballistic or conical for faster penetration | Clogging, balling of the face, need for high uphole velocity or foam |
| Medium-hard, mixed or fractured | Convex or concave, depending on deviation risk | Mixed spherical and ballistic | Hole deviation in fractures, gauge wear |
| Hard, abrasive granite and gneiss | Flat-face or convex | Spherical (dome) buttons for durability | Rapid gauge-button wear, regrinding discipline |
| Very hard, highly abrasive | Heavy-gauge flat or convex | Spherical, with heavier gauge rows | Bit life, need for higher hammer pressure |
Concave faces tend to help hole straightness because they centre the bit; convex faces resist gauge wear and flush well in hard rock. Button shape trades penetration against life: sharper ballistic profiles cut faster in softer rock, while spherical buttons survive hard, abrasive ground. The tooling maker’s application guidance for each bit family should settle the final choice.
Check mechanical compatibility before anything else
A nominal “6 inch hammer” is a family description. Before ordering, confirm each item below against the documents for the actual parts.
- Shank type. Bits are made to particular shank standards, and several are in common use. The bit shank must match the hammer’s chuck and retaining system exactly.
- Bit diameter range. The bit must lie within the hammer maker’s approved range for that hammer body.
- Foot valve and flushing. Some bits use a foot valve that is part of the hammer’s air cycle; a missing or wrong foot valve changes consumption and performance.
- Drill pipe and threads. The top sub thread must match the drill pipe, and the pipe diameter should suit the bit for cuttings removal.
- Pressure rating. The hammer’s rated pressure range must cover the pressure you intend to run.
- Back-head and check valve. These must be fitted and serviceable to stop water and cuttings entering the hammer when the air is shut off.
Do not assume that a compatibility rule from one manufacturer’s catalogue applies to another’s parts. Where there is doubt, ask the tooling supplier to confirm in writing.
Match the hammer to the air you can supply
A larger hammer delivers more energy per blow and generally drills larger holes faster, but it consumes more air. The hammer must be fed at a pressure within its rated range and with enough flow at that pressure, after water back-pressure and air-path losses are taken off. Typical borewell practice in India runs from 600–900 cfm at 200–250 psig (13.8–17.2 bar) for smaller work, through 800–1,200 cfm at 250–300 psig (17.2–20.7 bar) for the common 6 inch hammer, to 1,000–1,500 cfm at 300–400 psig (20.7–27.6 bar) for deep and large-diameter holes. Special units reach up to 35 bar (507.6 psig).
The exhaust from the hammer is also the hole-cleaning air, so do not add a hammer figure and a flushing figure together. Instead, check uphole velocity: the free air divided by the annular area between the hole and the pipe.
As an illustration, a 152 mm hole on 89 mm pipe has an annular area of about 0.0119 m². If 0.30 m³/s of free air were moving up that annulus near the collar, the area-average velocity would be about 25 m/s. That is a simple average: it ignores slip between air and cuttings, water inflow, washouts and local restrictions, and air lower in the hole is compressed and moving more slowly. It is a check, not an acceptance figure. Dry-air practice commonly works in the region of 15–25 m/s near the collar.
Our pages on DTH hammer air requirement and drill string and air path review go further into this calculation.
Rig capability: rotation, feed and pull-back
The rig has to turn the bit at the right speed, hold the right feed force and lift the string out of a deep or collapsing hole. DTH bits need only modest feed force compared with rotary drilling; too much feed increases wear without improving penetration, and too little lets the hammer bounce and lose energy. Rotation should index the buttons onto fresh rock between blows. Too slow and the buttons strike the same spot; too fast and gauge buttons wear quickly.
The hammer and bit makers publish recommended rotation and feed ranges, which take precedence over any general rule. Pull-back capacity matters in deep holes and in ground that may close in, and torque capacity matters with large bits in fractured rock.
Bit wear, regrinding and lubrication
Bit life is mostly a matter of discipline. Buttons that wear flat lose penetration and put more load on the hammer; gauge buttons that wear unchecked let the hole lose diameter, so the next bit jams. Common drilling practice is to regrind buttons before the wear flat reaches about a third of the button diameter, and to check gauge diameter after each hole. Follow the bit maker’s own limits where they are given.
Hammers need a steady feed of rock drill oil carried in the air. Too little oil leads to scoring and early wear; too much wastes oil and can affect water quality in the bore. Use the type and feed rate the hammer maker specifies and keep the lubricator full. Air from the compressor should be free of dust, and excess water should be separated where the arrangement allows. See DTH lubrication and air treatment interfaces.
A selection checklist
- Fix the finished bore diameter, depth and casing plan with the client.
- Describe the formation from local drilling records: overburden, weathered zone, fresh rock, expected water.
- Choose the hammer size for the bit diameter range.
- Choose the bit face and button shape for the dominant formation, with a plan for the weathered zone.
- Confirm shank, foot valve, threads and pressure rating against the documents.
- Confirm the rig’s rotation, feed, torque and pull-back for the deepest case.
- Read the hammer’s air consumption at the intended pressure and add back-pressure and losses.
- Check uphole velocity for the hole and pipe sizes.
- Select or confirm the compressor on delivered flow at the required discharge pressure in summer conditions.
- Set a regrinding and lubrication routine before drilling starts.
How Reddy Compressors helps
Hammers and bits come from tooling suppliers; our part is the air behind them. Reddy Compressors supplies all types, all brands and all models of borewell compressors: new machines, used and refurbished units, rentals from our own fleet, service and spare parts. We work across ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.
When a rig owner is changing hammer size or moving to deeper, larger-diameter work, we check whether the existing compressor can still supply the new duty, and if not, which pressure–flow class will. From our base at Tarnaka, Secunderabad, our service teams cover Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India. We will tell you plainly where the tooling supplier, not us, must confirm a choice.
What to send us
- Planned bore diameter, depth and casing plan
- Hammer make and model you use or are considering, and bit diameter
- Drill pipe size and rig model
- Formation description and expected water level, if known
- Your current compressor and the pressure it holds while drilling
- Whether you want to buy, hire or upgrade your compressor
Frequently asked questions
Which DTH bit is best for hard granite?
For hard, abrasive granite and gneiss, flat-face or convex bits with spherical (dome) buttons are the usual choice because the buttons resist chipping and wear. Gauge rows take the most wear, so regrinding discipline matters as much as the bit design. Run the hammer at a pressure within its rated range, since hard rock responds well to higher blow energy.
What size hammer do I need for an 8 inch borewell?
An 8 inch (about 200 mm) bore is commonly drilled with a 6 inch hammer, provided the bit is within that hammer’s approved diameter range. Confirm the shank type and foot valve with the tooling maker. A hole this size on standard drill pipe usually needs around 900–1,200 cfm for good hole cleaning, at a pressure set by depth and water.
When should DTH bit buttons be reground?
Common practice is to regrind before the wear flat on a button reaches about a third of its diameter, and to check the gauge diameter after each hole. Waiting too long slows penetration, overloads the hammer and lets the hole lose gauge, which can jam the next bit. Follow the bit maker’s own limits where they are published.
Why is my DTH bit clogging in soft rock?
Clay-rich weathered rock can pack into the bit face and flushing holes, especially when water is present and uphole velocity is low. A concave or drop-centre face with generous flushing channels helps, as does more air or foam injection. Check that the compressor is delivering its rated flow and that the drill pipe suits the hole size.
Can I use any bit with my DTH hammer?
Not always. The bit shank must match the hammer’s chuck and retaining system, the bit diameter must be within the hammer maker’s approved range, and any required foot valve must be correct. Several shank standards are in use, and parts from different makers do not always interchange. Ask the tooling supplier to confirm compatibility for the exact parts.
Does a bigger compressor make a DTH hammer drill faster?
Only up to a point. Higher pressure within the hammer’s rating increases blow energy, and adequate flow keeps the hole clean, so an undersized compressor does hold a rig back. Beyond what the hammer and hole need, extra capacity mostly burns fuel. Match the compressor to the hammer, bit and depth rather than buying the largest available.
Related pages
Field guide
The hammer and the bit: Where the compressor’s energy meets the rock
The energy path in DTH drilling is: compressor, compressed air, hammer piston, bit, carbide buttons, rock fracture. Every joint in that chain can lose energy, so the compressor is only as useful as the tooling it feeds.
Inside the hammer
Air admitted to the hammer creates a pressure difference across the piston, which accelerates, strikes the bit, and returns. This repeats many times a second. Piston kinetic energy is ½mv², so a small change in piston speed changes impact energy a lot. Actual performance depends on pressure, flow, port timing, internal leakage, back-pressure, lubrication and blow rate. Impact power is roughly energy per blow times blows per second. Different designs trade energy for frequency, so judge a hammer by how well it fractures your rock, not by frequency alone.
Lubrication matters: the hammer works under rapid reciprocation, impact, high airflow, water and abrasive dirt. Too little speeds up piston and liner wear, scoring and seizure; too much or the wrong oil also causes problems. Use the hammer maker’s oil type, viscosity and injection rate. See lubrication and air treatment.
The bit is more than a steel body with buttons
| Button shape | Tendency |
|---|---|
| Spherical | Tough and wear-resistant; suits hard, abrasive rock and long intervals |
| Ballistic | Aggressive penetration in selected medium-hard rock; more vulnerable under severe impact or abrasion |
| Semi-ballistic | A balance between the two |
No profile wins everywhere; the choice follows rock strength, abrasivity, brittleness, hammer energy, rotation and the tool life you want. Face buttons break the bottom of the hole; gauge buttons hold the diameter. A bit with worn gauge can still drill while the hole shrinks, drag rises and the next tool jams, so judge condition by more than whether it still penetrates.
Flushing holes
Restricted flushing passages produce a telling pattern: pressure up, returns down, penetration down. It is easy to misread that as a need for more compressor pressure when the real fault is a restriction at the bit.
When to change a bit
As buttons flatten, contact area grows, impact efficiency falls, and penetration drops while fuel per metre and hammer hours per metre rise. The economic question is not “can it drill another 50 m?” but “is continuing cheaper than changing or regrinding now?” Track cost per metre in the productivity guide.
Do not chase the maximum setting
- Too little feed gives poor engagement and bouncing; too much overloads the bit, hurts rotation and wear, and destabilises the hammer.
- Too slow a rotation re-attacks the same spots; too fast wastes wear.
- More pressure helps the hammer but costs power, fuel, heat and tooling stress.
- A 10% faster rate bought with 25% more bit wear and 15% more fuel per metre can cost more per metre. The aim is the most sustainable productive metres at the lowest total cost.
Mechanical specific energy (energy used per unit volume of rock removed) separates a tougher formation from a failing tool or compressor. If energy per volume and bit wear both rise, suspect the tool; if energy per volume rises and penetration falls together, the rock or the settings may have changed.
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Annular area and return velocity
In drilling, the air must lift the cuttings up the annulus. Check the return velocity from the hole and rod size.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Hole diameter (mm): 115
- Rod outside diameter (mm): 89
- Free air supplied (m³/min): 12
- Bottom-hole back pressure (bar g): 1
- Target return velocity (m/s): 15
Result
- Annular area: 41.7 cm²
- Return velocity: 24.16 m/s = 4,757 ft/min
- Free air needed for the target velocity: 7.45 m³/min = 263 cfm at 15 m/s
- Assessment: Within the commonly quoted range
Method. Annular area = π/4 × (D_hole² − D_rod²). Actual flow at the back pressure = free air × (P_atm / (P_back + P_atm)). Velocity = actual flow ÷ area. A return velocity often quoted for efficient cuttings lift is around 15–25 m/s; confirm for your material.
What to take from it. Low return velocity leaves cuttings in the hole; very high velocity wears the rods and hammer. Both are warnings, not verdicts.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
DTH hammer duty against two compressors
Check whether each of two compressors can supply a DTH hammer’s documented air consumption at the pressure the hammer needs, after hose, string and water-column losses, and with what margin.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Hammer working pressure at the hammer (bar g): 21
- Hammer air consumption at that pressure (cfm): 950
- Pressure loss from compressor to hammer (bar): 1
- Water column above the hammer to be lifted (m): 0
- Compressor A: maximum working pressure (bar g): 17
- Compressor A: data-sheet flow at the required pressure (cfm): 1150
- Compressor B: maximum working pressure (bar g): 24
- Compressor B: data-sheet flow at the required pressure (cfm): 1000
- Site derating of delivered flow: heat, altitude, condition (%): 0
Result
- Hammer working pressure: 21 bar g = 305 psig
- Water column allowance: 0 bar
- Required compressor discharge pressure: 22 bar g = 319 psig
- Hammer consumption: 950 cfm = 26.9 m³/min
- Compressor A: Fails: cannot reach 22 bar g (maximum 17 bar g); its flow does not count
- Compressor B: flow available at 22 bar g: 1,000 cfm = 28.3 m³/min
- Compressor B: margin: +5.3 % (covers the duty)
- Result: Only compressor B covers the duty
Method. Required discharge pressure = hammer working pressure + pressure loss from compressor to hammer (hose, swivel, drill string, check valves) + water column to be lifted (0.0981 bar per metre of water). A compressor whose maximum working pressure is below the required pressure fails, whatever its flow. Available flow = data-sheet flow at the required pressure × (1 − site derating). Margin (%) = (available flow − hammer consumption) ÷ hammer consumption × 100. Use the same flow basis (FAD or scfm) for hammer and compressor. 1 bar = 14.504 psi; 1 m³/min = 35.31 cfm. Hole cleaning (annular velocity) is a separate check.
What to take from it. Compare flows at the same pressure. More cfm at a lower pressure does not run a hammer that needs higher pressure.