In the civil engineering and special foundations sector, the technical specification process is a critical step. When the purchasing manager or plant manager prepares an order, the focus is usually on the nominal diameter and the type of cutting teeth, leaving vital structural parameters in the background. However, a mistake when sizing custom drilling tools can lead to a defective coupling with the rig’s Kelly bar, structural failures due to torsion, or site downtime with unacceptable costs.
Specialised companies know that a drilling bucket, an auger, or a core barrel are not mere accessories, but components subjected to enormous dynamic stresses. Therefore, consulting a catalogue is not enough; it is essential to coordinate the manufacturing drawings with the actual kinematics of the machine and the demands of the subsoil. Through our experience in specialised industrial steelwork, we have identified the recurring mistakes made in the purchasing or design phase and how to avoid them to ensure the compatibility and durability of the tools.
The true cost of an incompatible drilling tool on site
The economic impact of receiving an improperly sized tool on site goes far beyond the cost of the part itself. In deep piling, the production chain is sequential, and any interruption creates a bottleneck. If the connection (Kelly box) does not fit or the outer diameter rubs against the casing, the piling rig must stop.
A stopped piling rig means not only the loss of the daily depreciation of heavy-duty equipment, but also the cost of idle operators, delays in the concrete supply, penalties for missing deadlines, and, in the worst-case scenario, the need to mobilise additional cranes or emergency welding equipment to try to adapt the part in situ. For this reason, getting it right the first time through rigorous specification is the most profitable investment a purchasing department can make.
Mistake 1: Tolerance failures in the Kelly Box
The Kelly Box is the critical connection point between the piling rig and the tool. Its function is to transmit all the downward thrust (crowd force) and rotational torque from the Kelly bar to the tool. The most frequent mistake when ordering fabrication is providing approximate measurements without correctly defining the tolerances.
If the box is designed too tight, the tool can get stuck on the square drive of the bar due to mud accumulation or minor deformations from use. Conversely, excessive play generates a dynamic impact with every change in rotation direction or when starting to drill, which shears the locking pin and prematurely fatigues the welds connecting the box to the tool body.
How to measure a piling tool’s Kelly box?
To order a compatible tool, it is not enough to indicate that the machine uses a 200×200 mm or 130×130 mm Kelly bar. It is necessary to draw up a sketch or provide the following exact information to the steelwork fabricator:
- Internal clear dimension of the square: This must account for the actual measurement of the Kelly bar plus a reasonable technical clearance, generally between 5 and 10 mm per face depending on the size, to allow for a smooth coupling even in the presence of detritus.
- Height of the box: Essential to guarantee that the bar enters deeply enough to transmit the torsional moment without forcing the pin.
- Diameter and position of the pin hole: The exact distance from the internal support base of the box to the centre of the pin hole must be measured, ensuring that the pin will pass freely through the Kelly bar.
- Thickness of the box steel: The walls of the Kelly box must be oversized using high yield strength steel to resist the torque without bulging.
Mistake 2: Lack of coordination between rotational torque and structural design
Another common specification failure occurs when a tool is sent for manufacture using only the pile diameter as a reference, omitting the machinery’s power data. Designing a tool for a light piling rig delivering 150 kNm of torque is entirely different from designing one for a heavy rig exceeding 350 kNm working in rock formations.
When the fabricator does not know the machine’s maximum torque, they may manufacture a tool whose central tube, flight thickness, or internal reinforcements are insufficient. The immediate result on site is the torsion of the main shaft or the collapse of the tool upon hitting the first hard stratum, causing the premature wear of the deep drilling tool or its total fracture.
The impact of torque on the shaft and welds
The design must be concentric, and forces must be distributed evenly. To prevent breakages, it is essential that the drawings detail the reinforcements (ribs) connecting the Kelly box with the top plate of the tool and the sizing of the main tube (tool mast). Furthermore, the welding processes must account for the level of dynamic fatigue, often requiring full penetration welding and non-destructive testing if the project dictates.
Mistake 3: Ignoring the piston effect and relief valves in buckets
When extracting soil using drilling buckets in deep shafts, especially in the presence of plastic clays or below the water table, a fluid mechanics phenomenon known as the ‘piston effect’ or vacuum effect occurs. As the operator lifts the full bucket, the suction beneath the tool creates colossal resistance.
The purchasing mistake here lies in acquiring or ordering blind buckets or those with insufficient openings. A correct design must incorporate bypass channels, water passages, or relief valves (bypass) in the cylindrical body or on the top plate. These valves open during extraction, allowing water or bentonite slurry to flow towards the bottom of the excavation. Omitting this detail subjects the piling rig’s main winch to brutal overloads and destabilises the borehole walls due to the sudden change in pressure.
Mistake 4: Poor distribution of cutting diameter and overcut
The cutting diameter of the tool should never be exactly equal to the nominal diameter of the pile, especially when working with recoverable casing. If a tool with an incorrect outer diameter is requested, the tool may get jammed inside the metal casing or fail to excavate a sufficient diameter to allow the casing to descend.
The concept of overcut is fundamental. The perimeter teeth or cutting picks of the tool must protrude radially just enough to clear the ground below the casing shoe, without colliding with the inside of the tube during the entry and exit manoeuvres.
Relationship between side teeth and nominal diameter
When preparing the requirements, the technical buyer must provide the fabricator with the free internal diameter of the casing and its wall thickness. With this data, the geometry of the outer tooth holders can be arranged to guarantee the necessary annular space, thus optimising the choice of drilling tools according to the ground and facilitating the manoeuvre.
Mistake 5: Adapting the site to a standard tool instead of the tool to the site
The market offers numerous standardised or modular options that promise to cover 80% of standard jobs. However, geotechnics is unpredictable, and machinery requirements are highly specific. The great mistake made by purchasing offices is forcing the adaptation of an off-the-shelf part to a particular situation, compromising on performance or safety.
The difference when working with specialists in custom steelwork manufacturing is conceptual: Relente manufactures bespoke products according to the client’s actual drawings, measurements, manoeuvres, and conditions. The advantage over modular tools or moulds is that neither the machine nor the site has to be adapted to the part; it is manufactured directly for the required geometry. In foundation projects, this tailored approach accelerates the start of the works and prevents subsequent on-site adjustments, ensuring that the design is born to solve the exact demands of the operation.
Final checklist: What to send your steelwork fabricator for custom drilling tools
To avoid delays, cost overruns, or unnecessary emails before requesting a quote for deep drilling tools, ensure you compile and send the following set of documented technical data:
- Type of tool required: Clay bucket, rock bucket, continuous flight auger, rock auger, core barrel, or cleaning tool.
- Exact dimensions of the Kelly Box: Sketch of the internal section, total height, diameter, and elevation of the hitch pin.
- Piling rig specifications: Maximum rotational torque (in kNm) and crowd force, for calculating plate thickness and structural reinforcements.
- Operating diameters: Nominal pile diameter and internal diameter of the casing (if any) to calculate the actual cutting diameter.
- Geological characteristics: Type of ground to be drilled to define the pitch of the flights, the cutting lip inclination, and the type of teeth (earth teeth, tungsten rock picks, roller bits, etc.).
- Hydrological conditions: Presence of a water table or drilling under slurry to design the relief valves in the buckets.
Frequently asked questions about drilling tool specification
What to consider when buying a drilling bucket?
Besides the load capacity (internal volume), it is vital to consider the bottom opening system (single or double), the type of hinges, and the robustness of the locking latch. Likewise, the type of ground will dictate whether flat teeth are needed for soft soils or tungsten carbide picks for hard, abrasive strata.
Why does the drilling tool connection break?
The breakage of the connection is usually caused by excessive play in the Kelly Box or material fatigue after absorbing cyclic torsional impacts. It also occurs if the box design lacks the appropriate triangular reinforcements (gussets) to transfer the torque to the central body, concentrating all the stress on a single weld line.
What happens if the Kelly box has too much play?
Excessive play between the internal square of the box and the outside of the Kelly bar causes sudden impacts every time the piling rig starts or reverses rotation. This constant hammering prematurely wears the fixing pin, ovalises the hole, and eventually cracks the piece’s structural welds due to repetitive dynamic impact.
Is it better to buy standard tools or manufacture custom ones?
It depends on the project’s variability. If it is a highly standardised job, catalogue options might resolve an urgency. However, for complex terrains, special diameters, or demands where site downtime is not an option, custom drilling tools guarantee the exact fit and optimised durability for the actual stresses, avoiding field adaptations.
Designing a drilling tool is not a guessing game. At Industrias Relente, we analyse the mechanics of your equipment and the demands of your project to deliver a robust component that fits the first time, without excessive play. If you need to renew your equipment or are about to start a new civil engineering project, we encourage you to read more on our technical blog or to request fabrication according to your drawings. Our technical team will evaluate your requirements and advise you on the most suitable dimensions, reinforcements, and cutting types to maximise your piling rigs’ productivity.