92 High-quality CNC machining services are therefore crucial for either cutting impellers from an initial block of metal or grinding a cast part to the final dimensions. Furthermore, owing to the complexity of impellers, they are often regarded as the most difficult components to mill, and the ability to do so competently is regarded as a mark of quality. In addition to standard 5- or more-axis CNC machines, multi-spindle machines are highly useful in producing parts of such detail and curvatures. Mill-turn machines, for instance, can integrate two opposite lathes facing each other, with an axis in between integrating live tooling, for milling, turning or drilling a workpiece clamped into one of the two lathes. As the tooling cuts the workpiece, that lathe can rotate it about a given axis, enabling the higher than standard milling detail typical of a 4-axis CNC machine. Moreover, the lathe can then ‘hand off’ the workpiece to the other lathe at a certain point, thereby enabling the tool to access the workpiece’s back side, and therefore cut all sides of a part to spec in one set-up. In a high-end multi-spindle machine, the central axis may constitute a multiaxis milling head, such as a five-axis turnable tooling system, be it all five axes simultaneously or a 3+2 system where the cutter rotates in a single axis at a time. Either way, perfectly square parts (if so desired) or intricate, delicate parts designed with symmetry in many axes can be machined in such solutions without any stops to manually reposition or re-clamp the workpiece. Impellers, particularly, could be milled to spec with exact gradients of all inducer and splitter blades, without any interaction from technicians or machine operators following the first cut into the raw metal bar stock – especially if the multi-spindle machine comes with a parts catcher, for taking hold of the finished part from the lathes and mechanically placing it outside of the work chamber in an automated fashion. Challenging materials As one might expect, the more mechanically robust a metal, the more of a challenge it is to machine-cut it precisely or cost-effectively. Stainless steel is arguably the prime example of this, being well-known as a material with high strength and high corrosion resistance; properties that naturally make some stainless steel grades very difficult to cut. Moreover, being a ductile material, its strength increases during permanent deformation, a process known as work hardening or strain hardening. This process can occur when cutting stainless steels and other high-end metals similarly sensitive to work hardening. For example, titanium is also prone to work hardening. Additionally, owing to its very low thermal conductivity, it can easily induce localised heat buildups during the machining process. Both properties will have an impact on the tool as titanium is milled to the customer’s desired specifications; in general, titanium will get cut much more easily than steel, as long as the machinist is using an extremely sharp tool and holding the workpiece firmly. Fortunately, significant advancements in tools for cutting titanium have been made in recent years. However, in any event, the tool will dull at an accelerated pace compared with that of a tool used to cut easier, cheaper block materials less susceptible to heat buildups or work hardening, and there is little to no means of remotely sensing that degradation. Ultimately, knowing when to replace a tool before a breakage (and possibly an irrevocable mistake in the workpiece) occurs comes down to the machinist paying close attention and using instinct derived from either experience or good training. Inconel alloys are known to have similar mechanical strength, thermal resistance and applications as titanium alloys, and it follows that they are similarly problematic for CNC machines. Hence, good CNC workshops capable of working with such material are a rare find because it takes weighty experience, specialised tools and expensive machinery to do so properly. Beyond metals, many CNC workshops work with a variety of plastics, such as Teflon, which are widely cut to shape for underwater seals. However, while Teflon is easily bent and pressed into shape, it does not yield to cutting easily: instead, it will tend to deform away from the implement while being cut, returning somewhat to its original shape once the tool withdraws from the plastic. Hence, Teflon and some other plastics can be extremely challenging (and thus potentially costly or time-consuming) to machine-cut with tight tolerances. On top of that, some plastics are prone to absorbing water during the cutting process (particularly when water-aided tooling is used). Hence, some workshops will perform a dry cut or otherwise keep an industrial oven for drying finished plastic parts to extract moisture from them. Such parts may also be shipped in a vacuum-sealed bag filled with a desiccant material to prevent moisture absorbance during transport. Moreover, glass- and carbonreinforced plastics – otherwise known as composites – might also be subject to CNC machining, such as the boring of holes for fasteners or cables. As most of our readers will know, carbon composites are among the highest strength-toAugust/September 2025 | Uncrewed Systems Technology Being able to precisely machine an impeller wheel is often considered a mark of excellence among CNC machines and those operating them (Image courtesy of Kagoshima Seiki Co., Ltd.)
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