5-axis machining: when do you really need it?

A conventional machining centre moves the tool along three linear axes, X, Y and Z. A 5-axis machine adds two rotary axes, often labelled A, B or C, carried by the table, the spindle or both: this gives table-table, head-table or head-head kinematics. These rotations orient the part relative to the tool, or the tool relative to the part.

Two working modes hide behind the same term, and the difference in cost is significant.

• 3+2 axes (positional 5-axis): the rotary axes place the part in one orientation and lock, then machining is done in three axes. This gives access to several faces and inclined planes in a single set-up.
• Continuous (simultaneous) 5-axis: all five axes move together during the cut, to follow complex curved surfaces while keeping the tool in a favourable orientation.

The glossary explains the concept in detail. Above all, remember that many so-called “5-axis” parts are made in 3+2 and do not need simultaneous kinematics, which are more costly to programme.

• Fewer set-ups. Faces are machined in one clamping: handling and set-up time falls, and the repositioning error between two set-ups disappears.
• Better relationships between faces. Angled holes, inclined planes and relative positions hold together, without rework, provided the machine is correctly calibrated.
• Shorter tools. By tilting the part or the tool, you reach a deep area with a shorter tool, which is therefore stiffer, vibrates less and leaves a better surface finish.
• Better cutting conditions. On a curved surface, tilting a ball-nose cutter avoids cutting with its centre, where the cutting speed is zero.
• Access to shapes that are impossible in three axes: negative draft, blades, sculpted forms.
• Lower risk of distortion and scrap. A part re-clamped several times can distort or be badly repositioned; a single set-up limits these risks.
• Monolithic parts. A part machined from solid can replace an assembly of several components, with fewer fasteners and less play.

By contrast, 5-axis is not inherently more accurate than a good 3-axis machine. Accuracy comes from the machine, its calibration, the workholding, the tool and the inspection, not from the number of axes.

Not every 5-axis machine suits every part. Three kinematic layouts coexist.

• Table-table: a pivoting cradle carries a rotary table, and the spindle stays vertical. Very common on vertical centres, it suits small and medium-sized parts.
• Head-table: the spindle tilts and the table rotates. It is better suited to heavy or bulky parts.
• Head-head: both rotations are carried by the spindle and the part stays fixed. It is found on large parts, notably in aerospace.

Each layout has its limits: travel of the rotary axes, risk of collision between the tool, its holder, the workholding and the part, and stiffness of the pivoting table. The workshop simulates them in CAM before machining and often checks the position of the rotary axes by probing on the machine. It is therefore best to give it the mass and dimensions of the part: a part slightly too large for the pivoting table cannot be rescued by programming; it is a question of machine, not of set-up.

Continuous 5-axis is essential when the geometry requires it or when the tool cannot reach the surface any other way. Common examples:

• blades, turbines, propellers and pump impellers, whose curved surfaces are followed by the cutter;
• monolithic structural parts for aerospace, with inclined pockets and thin walls;
• implants and instruments for the medical sector, with organic shapes;
• free-form parts for optics, luxury goods or design, where surface appearance matters as much as dimensions;
• mould cavities, with deep recesses and polished surfaces;
• hydraulic blocks and manifolds with angled holes;
• lightweight single-block parts for robotics and racing.

These parts are often machined from solid, with a high rate of material removal: cutting time then weighs heavily in the price. 3+2, on the other hand, is enough for a large share of housings, brackets and casings whose faces are oriented differently but remain flat, with a few angled holes.

A 5-axis centre costs more to buy and maintain: its hourly rate is higher. As a guide, the guide on the price of CNC machining cites around €45 to €80 excl. VAT per hour for 3-axis and around €80 to €150 excl. VAT for 5-axis. But the price of a part is not read only by the hour.

Two effects pull in opposite directions. On one side, the higher rate and longer programming (toolpaths, collision simulation) weigh on fixed costs. On the other, a single set-up removes adjustments, dedicated fixtures and rework, which can reduce total time. On a five-face part, this often favours 5-axis. On a flat part with one or two faces, it only adds cost.

• 5-axis often works out cheaper for a part with four or five faces, for a short run where each extra set-up weighs heavily, or when positional tolerances between faces would be hard to hold across several set-ups.
• It works out more expensive for a prismatic part on one or two faces, for a large run where a dedicated three-axis fixture pays for itself, or when machine time outweighs everything else.

Size and mass also matter: the permissible diameter on the pivoting table and its maximum load limit the parts that can be handled. As with any machining, quantity spreads the fixed costs: see how a quote is calculated.

• Switch to 3+2: if the faces are flat and the inclined surfaces are planes, positional machining is enough.
• Add a fourth axis: a rotary axis on a vertical centre, or a horizontal-spindle machine with indexing, covers four-face parts well.
• Several three-axis set-ups, with precise locating (dowels, dedicated fixture): on a medium run, a well-designed fixture can cost less than 5-axis time, provided you accept an inspection of the position between faces.
• Redesign the part: remove an undercut, replace an angled hole with a straight one, split the part into several assembled components.
• Combine processes: a casting or forging blank, or a metal 3D-printed part, then machining only the functional surfaces; see 3D printing or CNC machining.
• Mill-turn for a part of revolution with milled features: see screw machining or CNC turning.

When in doubt, ask the workshop to price two variants: a 3+2 or multi-set-up solution, and a continuous 5-axis solution. The price difference informs the choice and lets you decide with full knowledge.

• A dimensioned drawing and a STEP file, with the revision index.
• The critical faces and functions: position dimensions between faces, datum references, GD&T if you use it.
• The blank: bar, plate, forged, cast or printed part.
• The material, its condition and any certificate required.
• The quantities, with price breaks, to gauge the weight of fixed costs.
• The tolerances, roughness and inspection expected: state whether a measurement report is required, since inspecting angled holes and inclined faces ties up a coordinate measuring machine.
• The acceptable clamping areas and the marks that can be tolerated.

Describe the function rather than imposing the process: if you think 5-axis is necessary, say why, and let the workshop propose the cheaper solution between 3+2 and continuous 5-axis. On Meaflow, posting a request is free and commission-free: several CNC machining workshops can send you their quote, which you compare before discussing through the messaging system.