What are the different types of CNC machine?

CNC milling machines and CNC vertical machining centres

People tend to think of a milling machine in terms of traditional manual mills (vertical mills, horizontal mills and universal mills), while a vertical machining centre (VMC) is considered to be a mill with ‘bells and whistles’. For example, a VMC will typically be fully enclosed and equipped with an automatic tool changer, powered vice, and it might well have more than three axes. Nevertheless, the terms milling machine and VMC are often used interchangeably.

The ’CNC’ element means that the axes are controlled fully automatically, with the tool following a toolpath generated by software such as Mastercam, which uses as its starting point the data from a 3D CAD model file. Alternatively, a CNC program can be written by someone working from 2D drawings or dimensioned sketches. With our five-axis VMCs, all aspects of the process are fully automated: spindle speed, motions (directions, distances, speeds and accelerations) in all axes, opening and closing of the vice, tool changing and coolant flow.

Furthermore, our five-axis VMCs are tended by robots, so we can leave the machines running by themselves to produce batches of identical parts. Also, by downloading suitable programs to the VMC and robot, and ensuring the correct blanks are in the pallet by the machine, we can produce a series of different parts (‘batch of one’) with no need for human intervention.

CNC milling machines and VMCs are so versatile that they can be used to machine almost any solid material and, depending on the technical requirements, they can prove cost-effective for anything from one-off prototyping to high-volume manufacturing.

CNC lathes and CNC turning centres

A CNC lathe is much like a conventional manual lathe but with automated control, whereas a CNC turning centre would typically be equipped with a tool changer and additional axes. This means a part that has axial symmetry can have extra features added by, effectively, milling operations with live tooling. For example, a series of flats can be machined around the outside of the part, as well as radial or offset plain or threaded holes. This is far more efficient than the traditional approach of carrying out the turning operations on a lathe, then transferring the workpiece to a dividing head on a milling machine for a second set of operations.

As with CNC mills and VMCs, CNC lathes and turning centres can machine almost any material and any batch size.

CNC routers

CNC routers comprise a table and a spindle mounted above for rotating the tool. This setup provides for two-dimensional (2D) profiling of various materials, including wood, plastics, composites, tooling board, modelling foam and stone. If the height of the spindle can also be controlled with the CNC, then this provides for 2.5D routing. With 2.5D routing, the program is normally run such that all cutting at one level (Z-axis position) is completed before the spindle is lowered to cut at the next level.

By using cutters (router bits) with different profiles, the workpiece can be given the required form around its periphery, or formed grooves can be cut in the surface.

CNC routers come in a range of sizes and with corresponding cutting capabilities. For instance, some are small enough to operate on a desktop while much larger CNC routers are used in furniture manufacturing. Their versatility also means CNC routers are used for everything from prototyping to volume production.

CNC drills

CNC drilling is useful when multiple holes need to be drilled in a component for functional or aesthetic reasons. Almost any solid material can be CNC drilled, from metals and engineering plastics, through to composites (eg for printed circuit boards), wood and ceramics.

CNC drilling is much faster and more accurate than manual drilling.

If more than one diameter of hole is required, a turret-type CNC drilling machine can be populated with drill bits of the appropriate sizes.

CNC drilling machines are often used in volume manufacturing but less so in prototyping. If multiple holes need to be drilled in a prototype part, the task might be performed on a CNC mill or VMC, as these are more widely available.

CNC laser cutters and profilers

We have a CNC laser cutting machine at Prototype Projects that cuts most non-metallic thin sheet materials, including plastics, cardstock, gasket material, fabric, felt and woven glass fibre. The same machine also marks and engraves surfaces; these can be metallic.

However, much larger CNC laser cutting machines are available for profiling thicker materials and metals. For instance, there are specialists who operate CNC laser cutting machines that will readily cut 30 mm stainless steel, 25 mm mild steel or 20 mm aluminium. Nesting software automatically selects the optimum layout for parts (known as nesting) to minimise material wastage.

As there is no need for tooling, CNC cutting is economical for one-offs and small batches, whether for prototyping or end-use applications. For higher volumes, it may be that dedicated punches and dies are more cost-effective, depending on the complexity of the part’s geometry.

CNC plasma cutters and CNC oxy-fuel cutters

Plasma cutting and oxy-fuel cutting (sometimes called flame cutting) are similar but plasma cutting is usually used on thinner material and achieves higher precision – and note that materials have to be electrically conductive for plasma cutting. Typically, CNC plasma cutting is used for material up to, say, 25 mm thick but it is possible to cut up to 150 mm mild steel with the right equipment. In contrast, oxy-fuel cutting can cut through mild steel up to 250 mm thick or more.

Most plasma cutting is two-dimensional profiling, but the technology can also be used with six-axis robots for cutting complex profiles from material that is not flat, while other specialist applications use plasma cutting to prepare holes in tubes, rectangular hollow section (RHS) steel and so on.

As with plasma cutting, most oxy-fuel cutting is usually restricted to two-dimensional profiling, but robots can be equipped with oxy-fuel equipment for cutting on complex surfaces.

Both CNC plasma cutters and CNC oxy-fuel cutters can be fitted with multiple cutters so that, for example, a bevelled edge can be formed at the same time as the profile to prepare the material for welding. Plasma cutting and oxy-fuel cutting are widely used in production environments, but they can be equally suitable for prototyping.

CNC waterjet cutters

Waterjet cutting uses a stream of water at very high pressure (up to 90,000 psi or 620 MPa), with or without an abrasive media added to the water stream. Plain water is used for materials such as wood, rubber or even food substances, while adding an abrasive enables hard materials to be cut, including ferrous and non-ferrous metals, marble and granite.

Unlike thermal process, there is no heat-affected zone and no thermal distortion. Waterjet cutting is capable of being used on metals up to 150 mm thick and other materials up to 450 mm thick, though in most cases much thinner material is profiled.

In general, waterjet cutting is restricted to two-dimensional profiling but specialist five-axis CNC waterjet cutting is available.

Waterjet cutting is characterised by accuracy, cleanliness and a cut surface that requires little or no finishing. However, accuracy and surface finish depend on the speed of cut, so a better-quality cut might come at a cost, literally. In addition, there is no cross-contamination, so there it is not necessary to clean or change tooling when cutting different materials.

Waterjet cutting is widely used for cutting granite for kitchen worktops and similar applications where batch sizes can be as low as one, but it is also used across many diverse manufacturing industries. With no need for tooling, and the technology’s inherent accuracy and ability to cut almost any material, waterjet cutting is popular for prototyping as well.

CNC wire erosion

CNC wire erosion is a type of electrical discharge machining (EDM), so is often referred to as wire EDM or WEDM. In this process a high-accuracy two-dimensional profile is cut through metal by applying a voltage between the wire and the metal (the two acting as a pair of electrodes), separated by a small distance and the gap flooded with a dielectric liquid (usually water). As the current increases, the dielectric liquid breaks down and an arc forms. This arc causes material to be removed from the workpiece. When the current drops, more dielectric liquid flows into the gap and washes away the debris, ready for the arcing cycle to be repeated.

Originally wire erosion was developed for cutting hardened steel in toolmaking applications. The technology has evolved and today wires as thin as 0.021 mm can be used for cutting very intricate profiles, though wire diameters in the region of 0.25 mm are more common.

Depending on the machine specification, the maximum material thickness might be up to around 200 mm or, in some cases, 400 mm.

CNC wire EDM is still widely used for toolmaking but the technology is also applicable to prototyping if a high-precision profile needs to be cut in metal.

CNC grinders

CNC grinding is less common than the other types of CNC machining discussed so far. Nevertheless, CNC grinding is excellent for removing small amounts of material to achieve a high-quality, high-precision finish, particularly on hard materials.

CNC versions of all types of conventional grinder are available, including surface grinders, cylindrical grinders, centreless grinders and internal grinders. Specialist grinders are available for thread grinding and tool and cutter grinding, for example.

CNC grinders are used in manufacturing industries such as aerospace, automotive, medical, tooling, and mould and die manufacturing. Where required, it is also very useful for producing prototypes or one-offs.

3D printers

3D printing technologies range from stereolithography (SLA), digital light projection (DLP) and PµSL (projection micro stereolithography), through to PolyJet, selective laser sintering (SLS) and fused deposition modelling (FDM).

Regardless of whether a 3D printing technology works by using light to cure a photopolymer, lay down material in a defined position or fuse a powder at a precise location, all 3D printing technologies are computer numerical controlled (CNC). 3D printers run programs that cause the model to be built automatically, including with support structures where necessary. Note that 3D printing processes are additive whereas the other CNC technologies we have discussed above are subtractive.

While 3D printing is normally associated with prototyping, the technology is also suitable for end-use parts, though care is required to ensure the technology and material used suit the application requirements.

Talk to us

If you need CNC milling, CNC turning or 3D printing – or would like us to liaise with other CNC specialists on your behalf through our purchasing service, contact us on 01763 249760 or complete the enquiry form below.