Introduction: why disc cutter performance matters
The growing use of Tunnel Boring Machines (TBMs) has radically transformed the tunnel construction industry, offering innovative solutions but posing new challenges in terms of safety, efficiency, and cost sustainability. In this scenario, a crucial role is played by disc cutters, the tools mounted on the TBM cuttinghead that come into direct contact with the rock. Their design, materials used, and construction processes significantly influence TBM disc cutter performance, maintenance cycles, and overall quality of the work.
A high-quality cutter reduces downtime, extends the life of the TBM, and improves overall productivity. Conversely, a poorly reliable tool leads to premature wear, frequent replacements, increased costs, and delays in progress. For this reason, design, metallurgy, and quality control should not be considered secondary aspects, but rather true strategic factors for the success of a tunnelling project. It’s no coincidence that our RingX TBM cutters are manufactured according to the same requirements outlined below.
The role of disc cutters in mechanized excavation
The disc cutter is the primary tool for rock crushing. Mounted on the rotating head of the TBM, the disc compresses and fractures the rock, forming concentric circular grooves. The lateral and radial fractures propagating from the contact point cause the material to disintegrate and produce material chipping.
The effectiveness of this process depends on the integrity of the cutter. As excavation progresses, the cutter inevitably wears, resulting in reduced penetration and reduced productivity. When wear becomes significant, the TBM requires tool replacement. These are complex and costly operations, often performed in hostile and pressurized environments, which significantly impact advance times.
TBM disc cutter wear types
Normal Wear
– This is uniform along the profile of the cutting ring.
– It is predictable and manageable with scheduled maintenance.
– This is the ideal condition, as it allows for planning and production continuity.
Uneven Wear
– This is caused by irregular and unpredictable phenomena such as chipping, buckling, or partial breakage.
– It can compromise the cutter’s functionality and cause secondary damage to the excavation head.
– It is more common in complex terrain and heterogeneous geological conditions.
The main causes of abnormal wear include:
– high rock abrasiveness (measured by the CAI – Cerchar Abrasivity Index),
– high compressive strength,
– grain size heterogeneity, and lithological discontinuities.
In tunnels with complex geology, cutter life can be drastically reduced, in extreme cases reaching just a few hours of use. This leads to increased maintenance costs and a drastic drop in productivity.
Strategies to reduce wear and improve efficiency
To combat these phenomena, specific strategies have been developed over the years:
– Increasing disc diameter: This allows for greater penetration and stress distribution, reducing specific wear.
– Optimizing spacing: The arrangement of cutters on the cuttinghead influences the energy required to remove a given volume of rock; optimal configurations reduce consumption and improve efficiency.
– Development of higher-performance cutter rings: thanks to advanced metallurgies and optimized profiles.
These advances, combined with careful design and increasingly stringent controls, have led to a significant improvement in the service life of disc cutters.
Steel for disc cutters: requirements and characteristics
– abrasion resistance,
– compressive strength,
– toughness,
– fatigue resistance.
Chromium-molybdenum-vanadium (Cr-Mo-V) tool steels are particularly suitable, as the carbides formed during heat treatment offer high hardness and wear resistance, while maintaining sufficient toughness to prevent brittle fracture.
The carbon and alloying element content must be carefully balanced to ensure the formation of a sufficient number of carbides without compromising hot formability.
Hot forming and manufacturing processes
The cutter manufacturing process begins with round steel bars. They are transformed by forging or hot stamping.
– Reduction ratio: This parameter expresses the deformation undergone by the material. Higher values result in a finer and more uniform microstructure, improving mechanical performance.
– Fiber orientation: Stamping allows the metal fibers to be oriented to increase impact resistance and reduce the likelihood of chipping.
After hot forming, the parts are cooled in a controlled manner to prevent cracks and thermal shock.
Preliminary heat treatments
Before final hardening, the steel is subjected to intermediate thermal cycles with specific objectives that directly impact the abrasion resistance and service life of the cutter. A fine microstructure acts as a barrier to wear damage, increasing the tool’s durability.
Profile Design
The cutting edge profile is one of the most important variables for the efficiency and life of the cutter.
– Hard rocks: narrow profiles (5/8”–3/4”) with rounded tips, capable of concentrating loads and optimizing rock fracturing.
– Friable or heterogeneous rocks: Wider profiles (≥1”) with more open angles (~78°), which ensure a resistant section to reduce the risk of breakage.
Profile design concerns not only the tip, but the entire geometry, including the interference fit with the hub. The cutter is shrink-fitted onto the hub: the interference fit must ensure proper locking and correct torque transmission. A careful design ensures uniform wear and reduces the risk of sudden detachment.
Final heat treatment
The quenching and tempering heat treatment gives the cutter its final mechanical characteristics.
– Vacuum hardening: offers high precision and uniformity, preventing surface oxidation thanks to the absence of oxygen.
– Conventional hardening: in oil or salts, it is still used but presents greater variability in results.
– Tempering: stabilizes the microstructure, reduces stresses, and ensures the right balance between hardness and toughness.
A carefully controlled thermal cycle is essential for obtaining high-performance tools, avoiding deformation or premature failures.
Quality control
The production process concludes with rigorous quality controls, essential to ensure product compliance with required standards.
– On the raw material:
verification of EN 10204 certificates,
chemical analysis,
hardness testing,
evaluation of micro-inclusions,
metallographic analysis of the microstructure.
– On the finished product:
dimensional checks,
hardness testing on multiple cross-sections,
evaluation of the hardening structure,
measurement of the austenitic grain size (ASTM E112),
analysis of the reduction ratio obtained,
destructive tests on representative samples of the batch.
These tests guarantee the quality of the cutters, minimizing the risk of failure during operation.
Conclusions
The overall efficiency of a TBM depends crucially on the quality of its disc cutters. Their design, the materials used, the production processes, and the final inspections represent an inseparable whole that transforms a tool into a strategic component.
A carefully designed and manufactured disc cutter guarantees:
– longer life,
– reduction of downtime,
– improved advancement speeds,
– reduced overall excavation costs.
In a competitive market like tunneling, investing in high-quality cutters is not just a technical choice, but a competitive advantage that allows you to successfully tackle even the most complex projects.
Learn more about RingX disc cutters
Our Ring X TBM disc cutters are engineered to meet the highest standards in design, metallurgy, and quality control.
Contact us to request a technical consultation and discover how our solutions can boost your tunnelling efficiency.