DTH Hammer and Drill Bit Selection: Size, Pressure and Matching
A DTH hammer is only as good as its bit and the air supply behind it. This guide walks through the three factors that decide penetration rate and cost per meter: size, pressure and matching.
1. Choose the right hammer size
Hammers come in casing sizes from 3 inches to 12 inches. The hammer size is dictated by the hole diameter you need, not the other way around.
- **3" (89 mm):** small blastholes, domestic water wells down to 100 mm.
- **4" (110 mm):** blastholes up to 115 mm, cost-efficient entry point.
- **5"-6" (135 mm):** the workhorse for blastholes and water wells to 165 mm.
- **8" (203 mm) and up:** deep water wells, geothermal and big-diameter production.
Rule of thumb: the bit face should be 20-60 mm wider than your largest hole requirement? No - the bit diameter defines the hole. Choose the hammer whose bit range covers your hole size with a margin for hard rock.
2. Match air pressure and volume
A DTH hammer converts compressed air into impact energy. Both pressure (bar) and volume (mΒ³/min) matter:
- **Low pressure (6-10 bar):** soft to medium rock, shallow holes, lower cost per hour.
- **Mid pressure (10-18 bar):** the sweet spot for most mining and water well work.
- **High pressure (18-25 bar):** hard abrasive rock, deep holes, best penetration rate but highest compressor cost.
Volume must match the hammer's air consumption table. An undersized compressor starves the hammer; an oversized one wastes fuel. Always match flow at maximum working pressure - not at idle.
3. Pick the bit for your rock
- **Flat face:** most versatile, good in medium-hard rock.
- **Convex face:** best penetration in soft-medium rock.
- **Concave face:** self-centering for large diameters, excellent in hard rock.
- **Dome face:** maximum penetration in highly abrasive rock.
Bit buttons also matter: spherical buttons for general use, ballistic for hard rock, and retraction-style for soft/abrasive formations.
Matching chart (quick reference)
| Hole diameter | Hammer | Recommended pressure | Compressor suggestion |
|---|---|---|---|
| 100-115 mm | 3"-4" | 10-18 bar | 13-21 mΒ³/min @ 10-14 bar |
| 115-165 mm | 5"-6" | 14-18 bar | 21-30 mΒ³/min @ 14 bar |
| 165-254 mm | 8" or larger | 20-25 bar | 30-40 mΒ³/min @ 20 bar |
Bottom line
Size the hammer to the hole, supply the pressure and volume the hammer demands, and choose the bit face and buttons for your rock type. Get these three right and your DTH cost per meter drops dramatically. Our engineers can build a matched package of rig, compressor and tooling - ask us.
Frequently Asked Questions
Which DTH hammer is best for rock drilling in hard formations?
For hard, abrasive rock such as granite, basalt and quartzite, use a high-pressure DTH hammer rated for 17β24 bar fitted with a tungsten-carbide button bit, and match both to your hole diameter and your compressor's free air delivery. High-pressure hammers deliver more energy per blow and hold up better in hard rock, while low-pressure hammers (7β10 bar) suit soft to medium formations. A hammer is only 'best' when hammer, bit and air supply are matched β mismatching any one of the three is the usual cause of slow penetration and short bit life.
What air pressure does a DTH hammer need?
Low-pressure hammers work at roughly 7β10 bar (100β150 psi); high-pressure hammers typically need 17β24 bar (250β350 psi). Raising pressure increases both the impact energy and the blow frequency, which is why hard-rock projects almost always run high-pressure hammers. Always check the hammer's air consumption at your actual working pressure, not at the compressor's rated pressure.
How do I match a DTH bit to a hammer and to the rock?
Three things must line up: the bit shank must match the hammer model, the bit diameter must match the hole size with the correct clearance, and the button design must suit the rock. Use spherical buttons for hard, abrasive rock, and ballistic or parabolic buttons for softer or medium formations where faster penetration matters more than wear resistance.
Why does the penetration rate drop as the hole gets deeper?
The most common causes are: air volume at the bit falling off (leaks, an undersized compressor, or hole-cleaning velocity too low to lift cuttings), a worn bit with blunt buttons, and cuttings not being cleared from the hole bottom. Check the compressor output at working pressure, confirm the hammer's air consumption, and replace the bit when the buttons are worn flat or the gauge is under size.