Two rigs of the same make on the same bench can turn in very different metres per shift, and the difference is rarely the rig. It is a compressor for DTH drilling that cannot keep its hammer fed, or a bit run a week past the point where it should have been reground. Both are cheap to get right. Both are expensive to ignore, because the bill arrives as slow drilling, short bit life and holes that wander.
The figures here are typical ranges from manufacturers' data and drilling handbooks. The hammer maker's chart for your hammer, at your pressure, is the number to work from.
What the compressor does for a DTH hammer
In a down-the-hole drill, compressed air does two jobs at once. It drives the piston that strikes the bit, and the same air, exhausted through the bit face, blows the cuttings up the gap between the drill tubes and the wall of the hole. A hydraulic top hammer rig separates the two: oil drives the drifter and a small on-board compressor only flushes. That is why air matters most on DTH rigs and pneumatic wagon drills, and why the drilling method decides how big a compressor has to stand behind the rig.
DTH hammer air pressure and volume: what each does
Pressure is energy per blow. The force on the piston is the air pressure times its area, so a hammer at 20 bar hits far harder than the same hammer at 10. More pressure gives more impact energy and more blows a minute, and penetration rises with it, though not in strict proportion: the bit has to turn that energy into broken rock, and the cuttings have to get out of the way. DTH hammers are generally designed to work at 10 to 24 bar. Older and smaller hammers on wagon drills run at 7 to 10 bar, and a few high-pressure designs at 32 to 35 bar.
Volume is what holds the pressure. A hammer behaves like an orifice on the end of the string. It passes a certain volume of air at a certain pressure, and the two rise together. A 4 inch class hammer typically takes somewhere between 4 and 9 m³/min of free air depending on pressure, and one maker rates its 6 inch hammer at 26.5 m³/min at 24.1 bar. A compressor that cannot deliver the volume cannot hold the pressure, whatever its nameplate says, and the gauge at the rig settles wherever supply and demand balance.
Pressure is also lost on the way. Long or undersized hoses, leaking couplings and a clogged line filter each take a little, so the pressure that counts is the one at the hammer, not the one at the receiver. Free air delivery falls with altitude and heat too, so a compressor chosen at its rated figure will be short on a hill site in May.
Flushing and bailing velocity
Bailing velocity is the speed of the air travelling up the annulus between the drill tube and the wall of the hole. It has to be high enough to carry the heaviest cutting out. The older rule of thumb for air drilling is 3,000 to 7,000 feet a minute, roughly 15 to 35 m/s. A US Army Corps of Engineers manual gives 20 to 25 m/s as satisfactory for many materials, with an upper limit of about 45 m/s. Dense rock, coarse chips and wet holes need the upper part of the range.
Annulus area = π ÷ 4 × (0.115² − 0.076²) = 0.00585 m²
Free air through the hammer = 9 m³/min = 0.15 m³/s
Velocity = 0.15 m³/s ÷ 0.00585 m² = 25.6 m/s
Same air on 89 mm tubes: 0.15 ÷ 0.00417 = 36 m/s
Same air where the hole has caved to 130 mm = 17 m/s
The gap sets the velocity as much as the compressor does
Free air volume is the right figure to use because the air in the annulus is close to atmospheric pressure. Two things follow from the sum. The drill tube should be as large as the hole allows, since a small tube in a big hole needs far more air for the same lift. And a hole that has caved, or passed through a cavity or a clay band, has a wide spot where the air slows and the cuttings stall.
Why a starved hammer regrinds its own cuttings
When the air cannot lift the chips, they stay at the bottom of the hole. The next blow lands on a bed of loose cuttings instead of fresh rock, and the bit spends its energy crushing them finer until they are light enough to leave. Penetration drops. The returns turn to dust. The bit body and the hammer casing wear from the outside as abrasive cuttings grind past them, and the string is at risk of sticking when the air is shut off to add a tube. From the operator's seat it looks like hard rock. It is not.
The signs are easy to read: fine powder instead of chips at the collar, a penetration rate that falls with depth more than the rock explains, and wash wear on the steel of the bit below the buttons. The cures, in rising order of cost, are fixing leaks and hoses, matching the tube to the hole, blowing the hole clean before each tube change, and a compressor of the right size. The opposite fault exists as well. Too much velocity erodes the wall of the hole and sandblasts the hammer and tubes.
Button bit types and face designs by rock
A button bit offers two choices: the shape of the carbide buttons, and the shape of the face they are set in.
| Feature | Type | Suits | Trade-off |
|---|---|---|---|
| Button | Spherical (dome) | Hard and abrasive rock such as granite and quartzite | Slowest of the shapes, longest life |
| Button | Ballistic | Soft to medium, less abrasive rock such as limestone and shale | Faster; wears and breaks sooner in hard, abrasive ground |
| Button | Semi-ballistic or parabolic | Medium-hard, moderately abrasive rock | A compromise, often used on the gauge row |
| Face | Flat | Hard to very hard and abrasive rock; fractured ground | Strong and forgiving; not the fastest |
| Face | Concave | Medium-hard, fairly uniform rock; the all-round choice | Good hole straightness and flushing |
| Face | Convex | Soft to medium-hard rock where speed is wanted | Fast, but the poorest at holding a straight hole |
| Face | Drop centre | Soft to medium-hard and fissured rock | Good flushing and straightness; less suited to very hard rock |
Catalogue guidance differs a little between makers. Treat it as where to start, then let the wear on the first few bits decide.
Drill bit selection for hard rock turns on abrasiveness as much as strength. Rock that is hard but low in quartz, as basalt usually is, will often take a more aggressive button than its strength suggests. Hard and abrasive together, as in a quartz-rich granite, punishes anything but spherical buttons on a strong face and wears the gauge row first. We have drilled both, basalt on the Samruddhi Mahamarg and granite on the Challakere to Hiriyur road, and our project record notes the second for its different bits and different penetration rates.
Regrinding
Carbide buttons wear flat. As the flat grows, the button stops being a point load and becomes a pad, and the same blow breaks less rock. The usual rule from bit makers is to regrind when the flat reaches about one-third of the button diameter, and some set it tighter, at a quarter. Run to a half and the bit is over-drilled: Secoroc's own figure for the loss of penetration at that stage is 30 to 40 per cent, and the buttons begin to crack and shear.
Three other things to watch. In non-abrasive rock, buttons polish instead of wearing and develop a fine surface crazing, often called snakeskin, which has to be ground off before the cracks run deeper. Gauge buttons wear faster than face buttons and decide the diameter of the hole, so a bit that has lost gauge drills a hole the next new bit will jam in. And if the steel between the buttons washes away faster than the carbide, the buttons are left standing proud and unsupported, which is a flushing problem showing up as a bit problem.
Penetration rate against bit life
The two pull in opposite directions, and the right balance depends on what a drilled metre is worth to the site. More pressure, more feed and a sharper button profile drill faster and wear the bit sooner. Spherical buttons and a conservative set-up last longer and drill slower. Where a plant is waiting on rock, the rig is the dear item and the bit the cheap one, so metres per shift should win. The measure that settles it is cost per drilled metre with the rig's hourly cost included, not bit life in metres taken alone.
Whichever way it is tuned, rotation and feed have to follow. A bit turned too fast in hard rock scrubs its gauge buttons away. One fed too lightly lets the hammer fire without the bit hard against the rock, and it beats its own shank and chuck instead. A worn or badly matched bit also pulls the hole off line, which is taken up in blast hole deviation.
What to tell the driller
For a drilling quote that means something, send the rock type and any test results you have, the hole diameter and depth, the bench access, and the altitude if it is a hill site. We run ten drilling machines, rotary and percussion, with high-pressure compressor support. Bit selection, feed pressure and penetration rate change with the rock, and compressor capacity is what separates a machine that drills all shift from one that waits. The service is on the drilling services page, and how the holes are laid out is in burden, spacing and sub-drilling.