Raise Bore Machine: Answers to the Questions That Actually Decide Your Project

A raise bore machine, often called a raise borer or a raise boring rig, is one of the most powerful and precise pieces of equipment in underground mining and civil tunnelling. It drills a pilot hole from one level to another and then reams it back to full diameter from the bottom up, delivering a finished shaft or raise without a single person ever entering the opening. The technology looks simple in principle, yet the choices around it – capacity, cutter design, ground conditions, maintenance and safety – decide whether a project makes money or burns it. This article walks through the questions engineers, contractors and buyers actually ask, in plain terms, before and during a raise boring project.

1. What does a raise bore machine really do inside the shaft?

A raise boring rig works in two clearly separated stages. In the first stage, a small-diameter pilot bit is rotated and pushed downward from the top (or, in some layouts, upward from the bottom) to drill a slim pilot hole along the designed centreline. This pilot hole is normally 200 to 350 mm in diameter and is drilled with ordinary rotary drilling tools, including drill pipes, stabilizers and a tri-cone or PDC pilot bit. The hole is surveyed as it advances so that the finished raise does not wander off line.

Once the pilot bit breaks through at the far end, the drill string is retrieved and a reamer head is attached at the bottom. The reamer head is pulled upward while the machine rotates the drill string from the top, so the reamer cuts the rock from the bottom towards the surface. Rock cuttings fall to the bottom of the raise, where they can be removed, and the ground ahead is never occupied by men or machines. The finished product is a smooth, circular, full-diameter shaft that needs no blasting, no mucking during the actual ream, and no personnel exposure to a working face.

2. When does a raise borer beat conventional shaft sinking?

Conventional raise construction used to mean one of three methods: drilling and blasting from a working face, Alpine miner (roadheader) excavation, or long-hole drilling and benching. All of them expose crews to the excavation face, produce overbreak, and are slow in hard rock. A raise bore machine replaces most of that cycle. The decisive advantages are worth listing because they drive the business case.

• Safety first: nobody works inside the raise while it is being cut, which removes the single largest source of shaft-sinking fatalities.
• Speed: a well-run raise boring project can complete a 2 to 3 metre diameter, 200 metre raise in a few weeks, against months for drill-and-blast.
• Ground support: the circular, smooth profile is much easier to line with concrete or shotcrete, and the hole stays open better in soft or jointed ground.
• Accuracy: surveyed pilot holes keep the raise on its design alignment within tight tolerances.
• Lower labour intensity: far fewer people are needed on site, and the crew does not face drilling, blasting, mucking and scaling operations one after another.

The method shines for ventilation raises, ore passes, chutes, escape ways, and pipeline or cable shafts in hard, abrasive, or unstable rock. It becomes less attractive in extremely soft or running ground, where the pilot hole may be difficult to control, and in very shallow or heavily inclined openings that do not justify mobilising a large rig.

3. Which components make up a complete raise boring system?

A raise boring system is more than the machine sitting on the upper level. It is a chain of equipment that must be matched. The main links are:

• The raise borer itself: a carrier with a rotation head (power head), thrust and pull cylinders, a clamping system, and a control console. Machines are rated by maximum reaming diameter, hole depth, pull force and torque.
• The drill string: high-torque drill pipes plus stabilizers that keep the pilot hole and the string running true, especially in long or inclined raises.
• The pilot assembly: pilot bit, reamer pilot, and the surveying equipment used to guide the pilot hole.
• The reamer head: the tool that cuts the final diameter. It carries rolling cutters (rock roller bits) or PDC cutters, depending on rock hardness, and is often fitted with stabilizer blocks to keep the reamer centred.
• Power and auxiliary units: the hydraulic power pack, pumps, hoses, and the muck handling arrangement at the bottom.

A common mistake is to focus only on the rig and ignore the cutter package. In many projects the reamer and its cutters cost as much as or more than the machine, and they decide penetration rate and tool life. Matching the cutter to the rock is therefore as important as choosing the correct borer size.

4. How do you size a raise bore machine for your project?

Sizing a raise borer is driven by four numbers, and they interact with each other. The table below summarises what each parameter means and what pushes it up.

Two practical rules govern selection. First, the machine’s rated diameter and torque must cover the largest raise on the project with margin, because cutter load grows roughly with diameter squared. Second, the pull force must overcome the rock’s resistance plus the weight of the drill string; underestimating either leads to stalls and cutter damage. It is usually wiser to select a rig one size above the nominal requirement than to run it constantly at its limit.

5. What operating parameters decide penetration rate and cutter life?

Once the machine is on site, the three levers an operator controls are rotation speed, pull (thrust) and, for pilot drilling, flushing. Getting them right is the difference between a smooth campaign and a string of broken cutters.

• Rotation speed: softer, less abrasive rock allows higher RPM; hard, abrasive rock needs lower RPM to protect the cutters and avoid overheating the bearings.
• Pull force: it must be high enough to make the cutters penetrate, but not so high that the cutters stall, glaze, or the drill string buckles. Modern machines let the operator monitor actual pull versus depth and torque in real time.
• Flushing: during pilot drilling, water or air removes cuttings and cools the bit; inadequate flushing causes bit wear and bore deviation.

Penetration rate is a function of the cutters biting into the rock at the right load. A good field practice is to track metres per hour, cutter hours per set, and torque draw together. If torque rises sharply while penetration stalls, the cutters are likely dull or the pull is too low; if penetration is high but cutter wear is extreme, speed is probably too high. None of these numbers can be optimised without good instrumentation, so a rig with reliable torque, pull and depth readouts is not a luxury – it is the tool that protects the investment.

6. Which field problems come up most often, and how are they solved?

Raise boring is reliable, but it is not immune to trouble. The most frequent problems, and their practical fixes, are:

The common thread is that most problems trace back to a mismatch between the tooling and the ground, or to running the machine outside its design envelope. A disciplined start-up – recording ground conditions, cutter condition and operating parameters on every shift – usually makes these problems visible before they become expensive failures.

7. What does routine maintenance of a raise borer involve?

A raise borer works hard and is expensive to repair, so maintenance is scheduled around the drill string and the reamer as much as around the machine itself.

• Drill string: inspect pipe threads, shoulders and tool joints before every make-up; replace worn joints to avoid twist-offs.
• Reamer and cutters: check cutter rotation by hand, inspect bearings and seals, replace dull or seized cutters before they damage the reamer body; keep stabilizer blocks measured and adjusted.
• Hydraulics: maintain oil level, filtration and temperature; inspect hoses and fittings daily, because a hydraulic failure stops the whole campaign.
• Carrier and power head: lubricate, check clamp and chuck wear, and verify pull and torque calibration.

Cutter life is the biggest consumable cost. Keeping a spare cutter set on site, rotating cutters to balance wear, and recording cutter hours per position extend tool life measurably. Downtime in raise boring is rarely cheap, and a small maintenance habit can save a full shift of lost production.

8. What safety rules matter most in raise boring?

Raise boring is chosen partly for safety, but it still demands strict control. The rules that matter most are:

• Keep personnel clear of the raise collar and of the bottom area while the reamer is working or being retrieved; falling rock from above is the main residual hazard.
• Never enter the raise until it is fully supported or otherwise verified stable.
• Secure the rig against reaction torque and vibration; check anchoring and clamping before every ream.
• Monitor hydraulic pressure and pull so that a stuck or failing reamer is detected before the string is overstressed.
• Control access to the working area and train every crew member in emergency and rescue procedures.

Modern rigs integrate torque, pull and depth monitoring, alarms and remote control so that the operator stays away from the highest-risk zones. Safety in raise boring is not an add-on; it is designed into the machine, the method and the shift routine.

9. What should you ask before you commit to a raise bore machine?

Buying or renting a raise borer is a major decision. The questions below sort out whether a machine fits the project:

• What is the maximum reaming diameter and total raise length on the project, and what margin do I need above them?
• What ground conditions will the reamer meet – rock hardness, abrasiveness, fracturing, water inflow – and which cutter type is matched to them?
• Do I need a full reaming system (rig, string, reamer, cutters, stabilizers, spares) or just the machine?
• What pull force and torque are required for the hardest raise, and does the rig carry real-time monitoring?
• How mobile does the machine need to be, and what are the transport, foundation and headroom constraints on each level?
• What is the supplier’s local support – spares availability, field service, operator training and cutter supply?
• What is the total cost of ownership, not just the purchase price, including cutters, maintenance and downtime risk?

Buyers who answer these honestly before signing usually avoid the two classic mistakes: buying a rig that is technically capable but undersized for the hardest raise, or buying one that is oversized, expensive and idle for most of the year.

10. How do you justify the investment and estimate the cost?

The economics of a raise borer are straightforward when the comparison is fair. Conventional sinking carries the recurring cost of a large crew, explosives, mucking, support and rework; raise boring converts most of that into machine time, cutters and a small crew. The payback calculation should weigh:

• Capital or rental cost of the rig and tooling against crew, explosive and mucking cost avoided on each raise;
• Cycle time saved, which for a mine or tunnel drives earlier access to production;
• Cutter and maintenance cost, typically the largest operating item and heavily dependent on ground conditions;
• Mobilisation and site preparation, which can dominate for a single short raise but amortise quickly across several.

A useful rule of thumb is to model at least the hardest and the most common raise on the programme, because average numbers hide the decisive case. If the machine is on a mine that will sink many ventilation raises and ore passes over several years, the business case almost always closes. If it is a one-off civil contract, renting with a tooling package may beat owning. Either way, the decision should be made on total cost per metre of finished raise, not on the sticker price of the machine.

Conclusion

A raise bore machine is a proven, safe and fast way to build vertical and steeply inclined shafts, but it rewards preparation. Sizing the rig to the hardest raise, matching cutters to the ground, monitoring torque and pull, maintaining the string and reamer, and asking the right questions before purchase are the habits that turn a powerful machine into a profitable project. For any engineer facing a programme of raises, the numbers, the ground and the support network matter far more than the brand on the machine.