In short
- Choose the drill from the hole the blast design wants: diameter first, then depth, then rock.
- A top hammer strikes the rod string at the surface, so energy is lost on the way down and deviation grows with depth. A DTH hammer sits directly behind the bit.
- Trade references put the usual crossover at about 100 mm of hole diameter and about 15 m of bench. Treat that as a guide and check the rig maker's range.
- A jackhammer drills holes of 27 to 40 mm to a practical 5 or 6 m. It is a tool for low benches and trimming.
- A pneumatic or DTH drill is only as productive as the compressor behind it.
Drill suppliers sell on metres per hour. A quarry is buying something else: a hole of the right diameter, to the right depth, that ends up where the blast design put it. The wrong machine shows up weeks later as boulders in the muck pile, or as a rig standing idle for want of air.
The quick answer. For small holes on low benches a top hammer machine is the usual choice, anything from a hand-held jackhammer to a hydraulic crawler rig. Once the design calls for holes larger than about 100 mm, or benches deeper than about 15 m, a DTH hammer generally drills straighter and holds its penetration better. Rotary rigs take over at the largest diameters.
How each machine breaks rock
All but one of these are percussive. A piston hammers a bit, the bit turns a little between blows, and compressed air blows the cuttings up and out of the hole. What differs is where the piston sits and what drives it.
- Jackhammer. A hand-held pneumatic drill, often steadied on a pusher leg. The hammer strikes a short steel from above. It is light, cheap to move and limited to small, shallow holes.
- Pneumatic wagon drill or crawler. The same top hammer idea with a heavier air-driven drifter on a feed beam, carried on wheels or tracks. The carriage supplies the feed force and holds the angle. Extension rods are coupled on as the hole deepens.
- Hydraulic top hammer rig. A crawler with a hydraulic rock drill on the feed. Oil delivers the blows in place of air, which makes it the quick machine in small and medium holes. It still needs air, but only to flush.
- Down-the-hole hammer. The hammer is a pneumatic piston at the bottom of the drill string, striking the bit directly. The tubes above it only rotate, feed and carry air down. The hammer's exhaust air does the flushing.
- Rotary. No hammer at all. A heavy rig pushes down on the bit and turns it, crushing and cutting the rock. It is used for the largest holes and in some very abrasive or sandy formations.
The five side by side
| Machine | Where the hammer sits | Hole size | Straightness over depth | Air or power | Where it suits |
|---|---|---|---|---|---|
| Jackhammer | On top, hand-held | Small: 27 to 40 mm typical, 32 mm common | Shallow holes only, to about 5 to 6 m in practice | Compressed air | Low benches, trimming, corners a rig cannot reach |
| Pneumatic wagon drill | On top, on a feed beam | Small to medium | Fair on short benches, falling off as rods are added | Compressed air for hammer and flushing | Short-bench quarrying and rock cuttings |
| Hydraulic top hammer | On top, on a crawler | Small to medium: generally under about 100 mm | Good on benches under about 15 m, deviation growing beyond | Diesel-hydraulic, plus flushing air | Fast production drilling on low and medium benches |
| DTH | Down the hole, behind the bit | Medium to large: generally over about 100 mm | Holds its line better at depth | High-volume compressed air | Deeper benches and larger holes |
| Rotary | No hammer | Large | Stiff, heavy string for long holes | High pull-down from a heavy rig | The largest holes, and some very abrasive or sandy ground |
Size and depth guides come from supplier specifications and a trade handbook, not from any regulation. Check the maker's stated range for the model.
Why the compressor decides the shift
Air does two jobs in a pneumatic or DTH drill. It drives the piston, and it carries the cuttings up the hole. Short of either pressure or volume, the hammer slows and the cuttings stay at the bottom, where the bit grinds them a second time. Penetration drops, bits wear faster and steel sticks.
A compressor sized for one rig will not run two, and a tired one turns a drilling shift into a waiting shift. That is why our ten drilling machines are compressor-supported. The compressor's diesel is also one of the larger lines in the cost of a drilled metre.
Straight holes and depth
With a top hammer the blow has to travel the whole length of the rod string before it reaches the rock. Every coupling takes a share, and a long, slender string bends under feed force. Energy loss and deviation both grow as the hole gets deeper. On a short bench neither matters much, which is where top hammer rigs earn their keep.
A DTH hammer strikes the bit directly, so the blow that reaches the rock at the bottom of a deep hole is the blow it delivered at the collar. The tubes are stiffer and closer to the diameter of the hole, which leaves the bit less room to wander. How much straighter is specific to the site, and we would not print a percentage: published comparisons differ, and the ground matters as much as the machine. How hole deviation becomes oversize works through what a small angle does by the time it reaches the toe.
Matching the drill to the bench, the rock and the hole
Start from the blast design, not from the machine in the yard. Diameter comes first, and it is tied to bench height. One published first estimate is that bench height in metres should be at least the hole diameter in millimetres divided by 15. On that basis a 32 mm jackhammer hole belongs on benches from about 2 m, a 100 mm hole wants roughly 6.7 m or more, and a 150 mm hole about 10 m.
Trial blasts refine that estimate, but it explains why a jackhammer and a large DTH rig are not alternatives. They are for different benches. Burden, spacing and sub-drilling shows how the rest of the pattern follows from the diameter.
Rock is the second question. Hard rock slows every machine and abrasive rock eats bits, and the name of the rock tells you less than you would hope: one published table gives compressive strengths of 78 to 412 MPa for basalt and 100 to 275 MPa for granite. Our crews drilled hard granite on the Challakere to Hiriyur road in Karnataka, under Sansar Infra LLP, and it wanted different bits and gave different penetration rates from the sandstone of Bundelkhand.
The bench itself is the third. A crawler rig needs a floor it can tram across and level on, and on broken or tight ground a smaller machine that can be carried to the mark may drill more in a shift.
That is the order we work in on our drilling contracts: the design fixes diameter, depth and inclination, and the machine is picked from the fleet to hold them.
Standards and sources
- Supplier specifications for hand-held pneumatic rock drills: hole diameter and practical depth
- Pit & Quarry, P&Q University Handbook, drilling and blasting chapter: application ranges for top hammer and down-the-hole drilling
- Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010): bench height against hole diameter rule of thumb, and rock strength table
- Fleet and project record: SM Infra's own operating record
Your contract and its technical schedules override anything written here. Check the clause before you build to it.