On-site facing and boring of large cast iron components: when to call in a mobile machinist

When a rolling mill crosshead, a press frame or a turbine casing made of cast iron develops a bearing surface that falls outside tolerance, the question is not always "how to machine it?" but first "where to machine it?". Dismantling and transporting a multi-tonne component to a fixed workshop is not always the right answer. The mobile machinist — working directly on-site with portable machine tools — addresses a specific need: restoring a surface or bore to its required dimensions without keeping the installation out of service any longer than necessary.

Why workshop machining is sometimes not an option for large cast iron parts

Three objective parameters govern the decision to keep a component in place or move it.

Weight and geometry

Beyond a certain weight — often between 5 and 15 tonnes according to specialist hauliers — moving a bare or assembled component requires an abnormal load transport plan, regulatory permits and an unavoidable lead time. Some grey cast iron or ductile iron (spheroidal graphite) components are, moreover, built into fixed structures: press columns, lathe beds anchored to the floor, rolling mill frames bolted to their foundations. In such cases, full dismantling is as costly as the transport itself.

Downtime

In sectors with continuous production — paper, steel, food and beverage — every hour of downtime translates into a quantifiable loss of output. A mobile intervention, planned in advance, can start within a few days, whereas organising a round-trip to a workshop typically takes several weeks.

Transport risk

Cast iron is an inherently brittle material that does not handle the shocks and vibrations of road transport well. A crack sustained in transit on a component already weakened by fatigue can result in total scrap. This risk tips the balance towards on-site intervention whenever the metallurgical condition of the part is uncertain.

On-site facing: principles, tooling and achievable tolerances

Facing large flat surfaces in cast iron involves portable rotary-head milling units, portable planers or router systems mounted on guide rails. The machine is clamped directly to the workpiece or to the adjacent frame; travel is provided by rack-and-pinion or leadscrew carriages.

Realistic tolerances under site conditions

In a workshop, facing relies on a calibrated surface plate and a thermally stable environment. On-site, disturbances are numerous: vibrations transmitted by nearby machinery, thermal expansion caused by day/night temperature swings or process heat sources, and the absence of an absolute reference datum. In practice, skilled technicians achieve flatness in the range of 0.05 to 0.1 mm/m by adopting compensatory strategies: measurement with an autocollimator or precision spirit level, dividing the surface into zones, and frequent intermediate checks. Values below 0.05 mm/m are achievable but require well-controlled site conditions — no heavy traffic nearby, night-time work if necessary.

Cast iron characteristics in mobile facing operations

Grey cast iron (lamellar structure) and ductile iron (spheroidal graphite) behave differently under the cutting tool. Grey iron produces short, brittle chips that are abrasive to cutting edges; it machines readily but releases graphitic dust that must be extracted. Ductile iron, being tougher, generates higher cutting forces and can cause localised heating that is detrimental to surface geometry if depth of cut and feed rate are not properly adjusted. In both cases, the absence of generous coolant supply — a common constraint in process environments — calls for cutting geometries with large clearance angles and moderate cutting speeds.

On-site boring: reconditioning worn bearing seats and bores

Mobile boring is primarily aimed at restoring out-of-tolerance bearing seats, housing bores, bush seats or locating bores that have suffered wear, thermal damage or deformation under load.

Equipment used

The portable boring machine is mounted cantilevered inside the bore or supported on external concentric supports. It rotates a cutting tool about the theoretical bore axis with motorised axial feed. For large diameters — from 80 mm to over 2,000 mm depending on the equipment — adjustable boring bars cover a wide range without the need to change machines.

Accuracy and inspection

Concentricity and cylindricity depend on the quality of the initial bar alignment and the overall rigidity of the workholding and machine assembly. A skilled technician checks diameter and profile every 20 to 30 mm of axial travel using a dial indicator or air gauges. IT7 tolerances (ISO) are routinely achievable; IT6 is possible on short bores in a calm environment.

Practical cases that justify calling in a mobile machinist

• Paper industry: oval-worn bearing seat on a grey cast iron wet press. Removing the roll would require heavy lifting teams and the shutdown of several adjacent lines. The mobile machinist rebores the seat in under two shifts.
• Energy sector: facing the joint face of a steam turbine casing after thermal damage. The ductile iron casing shows localised distortion. On-site machining avoids dismantling the turbine and transporting components weighing several tens of tonnes.
• Steel industry: reconditioning of rolling mill bearing housings anchored in a concrete foundation block. Transfer is physically impossible; only mobile intervention allows the equipment to be returned to service.
• Food and beverage: facing the joint face of an industrial cast iron mixer whose seal integrity has been compromised. The work must comply with strict cleanliness requirements, which means continuous chip extraction throughout the operation.

These interventions are carried out across many industrial areas throughout France. The Lyon, Saint-Étienne and Clermont-Ferrand regions have a particularly high concentration of heavy industries that use this type of service. Areas such as Annecy, Cluses and Oyonnax, with their strong mechanical engineering tradition, also call on mobile machining for integrated production equipment.

Technical constraints and precautions specific to on-site cast iron machining

Brittleness under rework

A cast iron component that has been subjected to thermal or mechanical stress may contain internal microcracks that are not visible at the surface. Reworking by machining — particularly if accompanied by high vibration levels — can propagate these defects. Dye penetrant or magnetic particle inspection prior to machining is recommended on critical areas.

Clamping and distortion under fixation

Positioning the portable machine on the workpiece or frame must not introduce parasitic stresses that would distort measurements and therefore the machined dimension. Technicians use independent support jacks and verify the absence of deflection after clamping.

Thermal management

Machining without generous coolant supply, which is common in process environments, causes localised heating of the cast iron. A temperature rise of a few tens of degrees can induce expansion of around 10 to 15 µm/m/°C — significant when working to tight tolerances. The technician must alternate between machining and thermal stabilisation periods, particularly during finishing passes.

How a mobile intervention unfolds: preparation, fixation and inspection

1. Pre-visit assessment: site visit, recording of existing dimensions (wear, misalignment), identification of access constraints and vibration sources. This step determines feasibility and forms the basis of the quotation.
2. Work area preparation: clearing the zone, isolating energy sources (electrical, hydraulic, pneumatic), installing the supports and clamping devices for the machine.
3. Machining: roughing and semi-finishing passes with intermediate checks. Finishing is carried out under thermally stable conditions.
4. Final inspection: flatness survey using an electronic level or autocollimator, cylindricity check using a dial indicator or gauge, delivery of a dated inspection report.
5. Reassembly and trial: the machinist can assist with reassembly to validate the fit of reconditioned components.

Criteria for selecting and commissioning a mobile machining contractor

Selecting a contractor goes beyond geographical availability. The key points to verify are:

• The range of equipment available (bore diameters covered, facing lengths, own lifting capacity).
• Documented experience on cast iron — both grey and ductile — and on components of comparable weight.
• The ability to produce a metrological inspection report (not merely a completion certificate).
• Certification or qualification relevant to your sector (pressure equipment, food contact, nuclear, as applicable).
• Responsiveness: mobilisation lead time from the initial call to the start of work, particularly in an emergency.

For large, critical components, it is advisable to request a witnessed survey before and after the intervention, carried out with traceable metrology instruments.