Compare two quotations for the same bit and the cheaper one looks like the obvious decision. It usually is not, because the price on the invoice is only a part of what a bit actually costs you. The number that decides whether a drilling operation is profitable is cost per metre – and it is arithmetic anyone can do in five minutes.
The formula
Cost per metre is the total cost of everything consumed while drilling one metre of hole:
Cost per metre = (bit cost + rod cost share + regrind cost + downtime cost) divided by metres drilled per bit, plus the rig’s own running cost per metre.
The rig’s own cost – wages, fuel or power, depreciation, maintenance – is unavoidable and applies to slow drilling and fast drilling equally. That is the key insight: any bit that drills slower is charging you rig time, whether or not it appears on the invoice.
A worked example
Take an illustrative case. Bit A costs 100 and drills 300 m before it is finished. Bit B costs 250 and drills 900 m. Assume a rig cost of 120 per hour and a penetration rate of 30 m per hour.
- Bit A: 100 divided by 300 m = 0.33 per metre of bit cost. Rig time: 10 hours for 300 m, so 1,200 of rig cost, which is 4.00 per metre.
- Bit B: 250 divided by 900 m = 0.28 per metre of bit cost. Rig time: 30 hours for 900 m, again 4.00 per metre.
On those numbers the two are closer than the price tags suggest, and the difference lives entirely in drilling speed and in the number of stops. Change one variable – say Bit B holds its penetration for the whole 900 m while Bit A slows by a third in its last 100 m – and Bit B wins by a wide margin, because the extra money bought metres drilled by a sharp bit instead of metres drilled by a hot one.
Hidden terms most comparisons forget
- Regrinding. Two or three regrinds per bit is normal for button bits, each with a cost and a rig stop. Add both.
- Bit changes. Every change costs the same as a few metres of drilling. A bit that lasts three times as long cuts changes by two thirds.
- Stuck bits and lost holes. A bit that wanders or seizes costs far more than its price: recovery time, occasionally a lost hole and a re-drill.
- Rod and coupling wear. A bit that runs hot or off-centre wears everything behind it. Bits and rods should be costed together – our notes on wear forms explain the mechanism.
- Stock cost. Every failure point in your replacement interval is stock you have to hold. Longer life means less capital tied up in a warehouse.
How to run the comparison properly
You do not need new accounting. Run a controlled trial: same rig, same operator, same ground, two bit types over a shift each, and record metres, hours, regrinds and stoppages. Four lines on a sheet. Two things make the result trustworthy – interchange the bits so the same ground is drilled by both, and record the gauge at retirement so you know when the bit actually stopped cutting rather than when it broke.
One warning from experience: a trial run in unusual ground proves nothing. If you test in a soft patch you will conclude that the cheapest bit is best, and then discover the truth in the next seam. Run the trial in ground you would describe as typical or worse – and read the three traps behind low prices before you buy the cheapest quote in the mailbox.
Frequently asked questions
Is cost per metre the same as cost per hole?
No, and cost per hole hides more than it reveals. A hole that is drilled faster but requires a second pass to reach depth is a slower hole overall.
Should I include regrinding labour?
Yes, if your own staff do it. The wheel, the coolant and the hour all belong in the calculation.
What if penetration rate matters more than bit life?
Then say so in the specification. In a bottleneck operation where the rig is the constraint, a faster bit is worth a shorter life – but do the arithmetic in advance rather than discovering it after the order. See reducing downtime in taphole drilling for how that trade-off plays out in a steel plant.
Send us your current cost per metre, the rock you drill and your rig’s hourly rate, and we will show you which part of the string is actually costing you money.