I tre numeri che controllano ogni foro DTH: rotazione, avanzamento e coppia.
Every DTH drilling operation comes down to three parameters. Rotation speed, feed pressure, and rotation torque. Get all three right and the hole drills itself. Get any one wrong and you're either wasting bit life, bending the string, or not advancing.
The problem is that these three numbers are interdependent and constantly changing — with depth, with rock hardness, with bit wear. Here's how to think about each one, with the field formulas and rules of thumb that experienced drillers actually use.
Rotation Speed: Match the Indexing to the Impact
The rotation's job is to move the bit inserts to fresh rock between piston blows. The bit shouldn't hit the same spot twice, but it also shouldn't rotate so far that it leaves unbroken ridges between impact craters.
The sweet spot, based on field testing in hard rock: each impact should index the outer gauge inserts by one-third to one-half of the insert's own diameter. Less than that and you're re-hitting the same rock. More than that and you're leaving ridges.
There's a formula for this: rotation speed equals impact frequency times insert diameter, divided by pi times hole diameter.
n = f × d / (π × D)
Where n is rotation speed in RPM, f is impact frequency in blows per minute, d is the gauge insert diameter, and D is the hole diameter.
The logic is simple: the insert needs to move one insert-diameter per impact, and the circumference of the hole determines how many insert-diameters fit in one rotation. The formula converts that geometry into an RPM.
In practice, for water well drilling, the working range is usually 10-30 RPM. Slower in hard rock where each impact needs full contact time to fracture, faster in soft rock where penetration is deep and the bit can index further between blows.
Feed Pressure: The Range Is Wider Than You Think
Feed pressure's job is to keep the bit seated against the rock so the piston's energy goes into fracturing instead of rebounding. The right value depends on hammer type, rock hardness, and air pressure.
The working range is 6 to 14.6 kilograms per millimeter of bit diameter. At the low end, for soft rock and low air pressure. At the high end, for hard rock and high air pressure.
A concrete example: a 152 mm bit running at less than 1.7 MPa air pressure needs about 6 × 152 = 912 kilograms of feed force. In harder rock, push toward the upper end of the range — 10, 12, even 14 kg per millimeter — watching the bit's behavior as you go.
The deep-hole correction from the previous article applies here too: actual downhole feed force equals the theoretical value minus the weight of the drill string, the hammer, and the bit. As the hole deepens, subtract the hanging weight from the gauge reading to keep the actual bit force constant.
And the same warning: more feed doesn't mean more penetration. The piston does the breaking. Feed is just contact maintenance. Overfeed and you wear inserts, load splines, and bend string for zero gain.
Torque: The Number Nobody Calculates
Torque gets the least attention because it's usually whatever the rotation motor delivers. But it matters, especially in deep holes and hard rock.
The theoretical requirement is about 1.06 newton-meters per millimeter of bit diameter. In the field, allow more — about 2.7 N·m per millimeter — to account for friction in the hole, cuttings in the annulus, and the increasing resistance of a long string.
A 152 mm bit needs roughly 2.7 × 152 = 410 N·m of rotation torque. That's the working figure, not the theoretical minimum. In deep holes, add more. In hard, abrasive rock, add more. The torque demand grows as the string gets longer and the rock gets harder.
If the rotation motor is laboring — surging, stalling, or running hot — the torque demand has outrun the supply. That's a sign to check the hole: cuttings packing, wall collapse, or bit jamming. Don't just throttle through it.
The Three Parameters Together
Here's the framework that ties all three together:
Rotation speed controls how far the bit indexes between blows. Too slow wastes impact energy on already-broken rock. Too fast leaves ridges and wears inserts.
Feed pressure controls whether the bit stays seated. Too little bounces the hammer and blank-fires the piston. Too much grinds inserts and loads splines.
Torque controls whether the bit can rotate at all. Insufficient torque stalls the string in the hole. Excessive torque demand signals a hole problem.
They interact. High feed pressure increases the torque demand. High rotation speed increases the wear on inserts. High torque in soft rock means the bit is packing up. The three numbers are a system, not a checklist.
The driller who understands this system doesn't need a manual. They read the hole — the penetration rate, the return air, the rotation motor sound, the vibration through the controls — and adjust all three parameters together, constantly, in small increments. That's the difference between drilling by the book and drilling by the hole.
