90 Focus | Machine tools component, thinking subtractively about how to get from the former to the latter, and correctly conceiving the order of operations in CNC programming, cannot be understated. The machine itself is essentially a mindless, blind device that will generally carry out the orders plugged into it in a very open-loop fashion. Thus, when cutting something like a cooling plate, one might for instance create a CNC program that starts with installing a face mill and using it to flatten and smooth the top surface of the block, then replacing it with a spot drill to bore some holes, before swapping in a tap drill to widen or reshape those holes. However, a cognisant mind remains vital even for such basic programs, particularly because the speed and order with which one removes material will affect the part’s dimensions relative to the target geometries and tolerances. Metals and plastics are formed by heat, and the friction of cutting inevitably introduces heat into workpieces. If thermal gradients and resultant material behaviours are not accounted for, one may find that, upon being de-clamped from the milling chamber, the workpiece deforms into an entirely unacceptable shape as it cools. Additionally, as the cutting tool spins against the workpiece, possible tool deflections are a perennial concern and cause of imprecise geometries and depths. Tool deflections can be caused by an excessively high cutting speed (a natural reaction to wanting a part finished faster) or by a variety of other factors such as insufficient tool strength for the job or excess overhand of the tool from its holder. Such factors might also cause inaccurate cutting and grinding owing to the tool bouncing unpredictably off the material. Again, machining experience is critical to predicting and avoiding such behaviours for consistent quality parts. 5-axis milling When looking for parts of higher design complexity than a 3-axis machine can cost-effectively provide, one can find CNC machines with up to 12 axes (anything beyond being very uncommon), and 3- and 5-axis machines are arguably the most widely employed – at least among the CNC machining services providers that we have seen – and thus the most useful with which to gain familiarity. Broadly, 5-axis machines can be separated into two categories. One may be thought of as ‘3+2’ axis machines and the other as full, simultaneous 5-axes-atonce machines. The former is effectively a 3-axis machine for cutting in the X-, Y- and Z-axes, but upgraded with additional A- and C-rotary axes via which the workpiece can be moved between milling stints. Thus, each block can be worked by the cutting tool in the three orthogonal axes from multiple different angles (but the workpiece cannot be rotated in both the A- and C-axes at once), without having to stop milling, open the machine chamber and reposition it manually. While this might sound like a minor change, it dramatically eases workflows for CNC machined parts and, by extension, enables far more geometrically detailed and complex parts in a given setup by virtue of that complexity no longer driving up labour hours per unit produced. For example, 90% of a complex component such as a turbocharger housing or cylinder head can be finished in a single stint and then moved to a simpler 3-axis machine for the final phase. A minor caveat is that preprogramming time increases with the number of axes used per machine, meaning that the 3-axis machine will get started quicker than the other, more costly, machines. However, those other machines will quickly speed past in terms of cutting progress, units finished per day and production costs per unit, particularly as order sizes increase. In the latter category, the relative angle between the workpiece and the tool can be changed in both the A- and C-axes simultaneously, meaning that the workpiece can be re-angled dynamically as the cutter spins. That enables the geometries necessary to account for fluid flow in complex, helical, CFD-optimised parts such as propellers, impeller wheels or gas turbine diffusers. Multi-spindle machines Given the extremely fine and performance-critical tolerances one must adhere to in producing impeller wheels, be they for gas turbine engines, turbochargers, superchargers, pumps or other uses, casting and forging such parts efficiently is challenging. August/September 2025 | Uncrewed Systems Technology The ability to mill more than three axes of a part simultaneously enables hugely complex metal parts to be produced in a timely and cost-effective fashion (Image courtesy of Gruppo Parpas)
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