The five cost blocks
- Capital or hire cost of rig and compressor. 2) Fuel and power. 3) Consumables: bits, hammers, rods, shanks. 4) Labour and site overhead. 5) Downtime and rework. Most projects focus on block 1 while blocks 3 and 5 decide the outcome.
Block 1 is the easiest to compare and the least informative. A machine that costs 20 per cent more to buy but drills 15 per cent more metres per shift pays the difference back inside the first contract. A machine that is cheaper because it is one size too small for the job will lose the money back through blocks 2, 3 and 5 for as long as it works.
The reason blocks 3 and 5 dominate is that they scale with metres and hours rather than with the purchase order. Bits and hammers are consumed by rock, and downtime is consumed by everything: hose changes, filter changes, waiting for a compressor to be repaired, redrilling a hole that was not straight. On a long campaign the consumable and downtime lines are usually larger than the depreciation line, and they are the lines nobody quotes at tender stage.
A per-metre cost is only meaningful if all five blocks are in it. The usual failure is to compare quotations on block 1 plus a fuel estimate, and to leave the rest out until the campaign is over. Build the sheet before the first hole, and the comparisons stay honest.
The cost blocks in practice
| Block | What drives it | How it shows up per metre |
|---|---|---|
| 1 Capital or hire | Machine class, mounting, ownership period | Fixed per month; falls as metres per month rise |
| 2 Fuel and power | Load factor, compressor size against hammer demand, engine efficiency | Rises with hours run, whether or not the rig is drilling |
| 3 Consumables | Rock abrasive strength, bit class, hammer pressure, hole diameter | Bit cost divided by metres per bit; hammer and shank life in metres |
| 4 Labour and overhead | Crew size, shift pattern, site services, supervision | Fixed per shift; falls as metres per shift rise |
| 5 Downtime and rework | Maintenance practice, spares on site, hole cleaning, hole straightness | Unplanned hours, plus the metres drilled twice |
The important property of that table is the asymmetry: block 1 improves as metres per month rise, and blocks 2 and 3 also improve as metres per hour rise, because the fuel and the bit are both spent per metre. Downtime is the only block that is pure loss, and it is the one most often left out of the estimate.
Why the compressor is a running cost
A DTH rig consumes air all day. A compressor that is larger than the hammer needs burns fuel for no benefit; one that is too small costs you metres per hour. Size it from the hammer, not from habit.
The compressor is the clearest example of how a sizing decision becomes a per-metre cost. Three failure modes appear again and again:
- Oversized machine. Fuel is burned to move air that the hammer never sees, and the extra flow is dumped or throttled. The purchase price was higher, and so is every hour of the campaign.
- Undersized machine. The hammer runs below its rated pressure, blow energy falls, and metres per hour drop. The bit stays in the hole longer for fewer metres, so bit life per metre falls as well β blocks 2 and 3 both get worse from one wrong number.
- Right size, wrong pressure point. A compressor rated at 25 bar and one rated at 18 bar at similar flow are not interchangeable for a hammer that wants 24 bar. Compare flow at the pressure the hammer will actually see; the open-pit compressor guide sets out how to read those figures and which hammer classes sit in each pressure band.
Machinery that carries its own air changes the arithmetic. An integrated rig puts a factory-matched compressor on the carrier, so the air is neither oversized nor undersized in normal service, and there is no separate engine to fuel or service. The trade is that the air you have is the air the carrier was built with, which matters if the hole programme later grows.
The same reasoning applies to every machine that runs while the rig works. Fuel per hour is a function of hours, not of metres, so anything that runs all shift should be sized for the duty it is doing at the moment, not for the largest duty it might ever do.
Building the record sheet
Cost blocks and the data to record
Use the checklist below to set up a simple record sheet for your site.
The sheet needs three sections and nothing more:
- Machine section: rig class, compressor model and rated flow and pressure, hammer model, bit model and diameter. These come straight from the specification sheets and never change during a campaign.
- Shift section: metres drilled, hours drilled, hours down (split into planned and unplanned), fuel issued, bits changed, and the reason for each bit change. One line per shift, filled in by the operator, checked by the supervisor.
- Cost section: unit prices for fuel, bits, hammer parts, rods and shanks, plus the labour and hire or depreciation rates. This is where all five blocks get a number.
Metres per bit is the number that separates a good programme from a bad one. Record the reason every bit comes out of the hole β worn gauge, broken buttons, dropped or cracked β and the pattern shows up within weeks. A bit that fails on gauge in abrasive rock is a bit selection or pressure question; a bit that fails on buttons is usually a hammer energy or hole-cleaning question. Those two diagnoses come from the same shift log and lead to different fixes. The selection side is covered in DTH Bit Selection and DTH Hammer Selection.
Metres per hour must be recorded against the hole size it was drilled at. A 115 mm hole and a 200 mm hole drilled by the same rig are different operations, and averaging them produces a meaningless figure. Keep the sheet segmented by diameter and by bench or well.
Record these four numbers
Metres per hour, fuel per hour, bit metres, and downtime hours. With four weeks of honest data you can compare configurations instead of guessing β and negotiate from evidence.
Four numbers, one month, and the arguments end:
- Metres per hour, by hole diameter. This is the productivity number.
- Fuel per hour, by machine. Fuel issued per shift divided by hours run, not by metres. It tells you whether an oversized compressor or an idling rig is eating the budget.
- Metres per bit, by bit model and rock type. Replaces opinion about bit brands with a number.
- Downtime hours, split into planned and unplanned. Planned service is a cost of doing business; unplanned downtime is the number to attack.
From those four, the derived figures fall out: fuel per metre, bit cost per metre, and the share of available hours that actually produced metres. When you change a compressor, a hammer or a bit, change one thing at a time and let the four numbers tell you what happened. Two changes at once and the data is useless.
Cost comparisons also belong in the tender, not only in the report. A contractor who can show a client four weeks of metres per hour against hole diameter, and fuel per metre against compressor size, can justify a specification and defend it. The same sheet is the evidence for a machine that costs more but drills more, which is the only argument that survives contact with a client's purchasing department.
Common mistakes
- Comparing rigs on purchase price alone. Depreciation is one of five blocks, and on a long campaign it is usually not the largest.
- Quoting fuel per metre when the contract changes hole sizes. Fuel per hour is the stable number; fuel per metre changes with every diameter.
- Judging a compressor by its rated flow without checking the pressure point. Two machines with the same flow at different pressures are not the same air supply.
- Letting the bit change without recording why. The failure reason is the useful data; the count on its own only tells you what you already paid.
- Averaging metres per hour across diameters. It hides the effect of hole size, which is usually the single biggest driver.
- Treating planned maintenance as downtime. Mixing planned and unplanned hours hides the real availability of the machine.
- Changing two variables at once. Whether it is a hammer and a bit, or a compressor and a pressure setting, the comparison is lost.
Checklist
- Metres drilled per shift. 2) Fuel consumed per shift. 3) Metres per bit and regrind interval. 4) Unplanned downtime hours. 5) Rework or redrilled holes. 6) Consumables stock accuracy.
Conclusion
Drilling cost per metre is decided by metres per hour and by hours lost, and both of those are engineering outcomes rather than purchasing outcomes. Size the air to the hammer, record four numbers honestly for a month, and change one variable at a time. The rest is arithmetic β and it is arithmetic that can be argued from evidence. For the machine side of the same question, see How to Choose a DTH Drilling Rig and the application notes for open-pit mining and quarry drilling.