Milling

CNC precision milling

CNC milling is a chip-removal machining process performed on a part clamped to a pallet by means of a rotating tool.
Unlike turning, where the part rotates and therefore only cylindrical, conical or spherical geometries are generated, milling makes it possible to produce virtually any geometry. Planes, slots and pockets can be obtained and, with CNC machines, also cylinders, cones and three-dimensional profiles such as helixes, spirals and complex surfaces.

The complexity of the component to be produced requires the use of specific machines:

• 3 axes when machining the part in a single setup is sufficient
• 4 axes for radial operations
• 5 or 6 axes for complex geometries

The use of multi-axis machines allows several surfaces to be machined at the same time when the references between the surfaces are important, because the errors caused by repositioning during workholding are eliminated.

Machining several surfaces at the same time also brings an economic benefit, because operator intervention is reduced.

Precisely in order to exploit this freedom in producing shapes, machining centres provide a significantly higher number of tools than lathes. In some cases there can be machines with hundreds of tools.

The centres can be equipped with multiple pallets to achieve high productivity or flexibility targets. The use of a large number of pallets combined with a large tool magazine makes it possible:

• to keep a large number of different part numbers set up
• to run continuous, unattended production to maximise productivity.

Types of milling

The main operations that can be performed on a CNC machining centre are often combined within the same cycle.

• Facing machines flat surfaces with tangential-insert cutters and is the starting point for support planes, reference surfaces and mounting bases. Contouring profiles the external or internal outline of the part along 2D or 3D paths: shapes, profiles, O-ring seats.
• Pockets and cavities are produced by clearing out closed volumes with trochoidal or spiral strategies, for weight reduction, insert seats and functional cavities. Drilling, tapping and boring complete the cycle with through or blind holes, internal threads and H6–H7 tolerance holes for bearing seats and guides.
• Continuous 5-axis milling is the most advanced operation: the two additional rotary axes make it possible to produce sculptured surfaces, blades, marine propellers, moulds and tool holders in a single setup, eliminating multiple repositioning and improving overall geometric accuracy.

Workable materials

The choice of cutter, coating and strategy varies according to the material.

Precision and tolerances

The achievable tolerance depends on the rigidity of the machine, the quality of the tool and the thermal stability of the process.

Industrial applications

Milled components are used in sectors where geometric complexity and positional tolerances are decisive requirements.

• Hydraulics: valve bodies, manifold blocks, manifolds with cross-drilled holes and sealing seats milled on multiple faces.
• Machine tool equipment: vices, spindle bodies, clamping systems, tool holders with tight positional tolerances.
• Industrial machinery: base plates, flanges, supports, housings up to 1,000 mm in C45, Ergal, Anticorodal.
• Medical: titanium and stainless steel components with controlled finishes and no burrs.
• Marine: marine propellers in aluminium bronze, 5-axis milled.

Difference between CNC milling and turning

The fundamental difference lies in the cutting kinematics. In milling the tool rotates and the part is stationary: the process is ideal for prismatic shapes, slots, pockets and 3D profiles. In turning the part rotates and the tool traverses: ideal for cylindrical geometries such as shafts, bushings, pins and rods.

Many components require both processes. A hydraulic valve body, for example, is turned for its external diameters and main bores, then milled for the side seats and cross holes. Performing turning and milling within the same facility ensures control of the positional tolerances between turned and milled features.

The Gamba milling department

Gamba SRL’s milling department has 15 vertical ISO 40 machining centres from three manufacturers (Doosan, Kitamura, Deckel Maho): 6 five-axis centres with tables up to 630 mm for complex geometries in a single setup, 4 centres with a twin pallet for series production with automatic part change, and 5 three/four-axis centres with tables up to 1,000 mm for medium-to-large parts.

Advantages and limits of CNC milling

The main advantages are: complex geometries in a single setup (5-axis), batch-after-batch repeatability, automatic tool change for dozens of tools within the same cycle, and high productivity with twin-pallet systems. From prototype to series, the same program guarantees consistent tolerances.

The main limit is efficiency: for simple cylindrical parts, turning is faster and cheaper. Very deep cavities require long tools at reduced speeds. On materials above 60 HRC, grinding is more cost-effective for finishing. The cost of the CAM program weighs on very small batches (1–5 pieces).

Quality control

On the machine, measuring probes detect the part position after clamping and compensate for positioning errors. At the end of the cycle they check the critical dimensions. For components with tight tolerances, final verification takes place on the coordinate measuring machine (CMM) in the temperature-controlled metrology room.

Complete cycle: from milling to the finished component

A component that requires milling follows this flow, managed in-house:

1. CNC turning: if the part has cylindrical features
2. CNC milling: slots, pockets, cross holes, 3D profiles
3. Heat treatment: hardening, case-hardening or nitriding
4. Surface grinding: finishing of planes with tolerances < ±0.005 mm
5. Final inspection: CMM verification and certification

Integration within the same facility eliminates transfers between suppliers and ensures dimensional consistency between turned and milled features.