CNC machining and 3D printing for machine builders | pros & cons

Machine builders aim to supply customers with the highest-quality product, at the lowest cost and as quickly as possible – while at the same time incorporating innovations that benefit the end user.  As difficult as that may be, making informed decisions from understanding the benefits and restrictions of each discipline will only help to achieve this.  

Traditionally, most components are machined from solid billets or fabricated from sheet metal. If machines are built in high volumes, plastic parts might be injection moulded or vacuum formed. In the new millennium, machine builders started to use additive manufacturing (AM) technologies, also referred to as rapid prototyping, rapid manufacturing and 3D printing.  

CNC machining has largely taken over from manual machining, enabling complex geometries to be machined more easily. Today, therefore, a choice can often be made between CNC machining or 3D printing. So, which one should machine builders use?   

Advantages of CNC machining

CNC machining can be carried out on many different types of machines, covering the principles of milling, turning, laser/water cutting, drilling and engraving. If the component has been designed using a 3D solid modelling system, then the CAD data can be used to help generate tool paths by using CAM software, making the CNC programming of complex geometries quick and simple.  

With those complex geometries, manual machining can become considerably more costly due to the time required for multiple set-ups and making fixtures; in contrast, CNC machines can often do everything in one set-up. With our five-axis VMCs (vertical machining centres) that are tended by robots, we can remove the part from the machine vice, turn it around and machine features that were previously inaccessible without any intervention by an operator. 

Another advantage of CNC machining over most 3D printing technologies is the inherent accuracy. Although we quote a general tolerance of ±0.1 mm on CNC machining, we can, in fact, achieve much tighter tolerances. If critical features need a specific tolerance, we can usually meet the customer’s requirements. Compare this with 3D printing, in which the general tolerance we quote is ±0.5 mm. Furthermore, CNC machining usually gives a better surface finish, especially if we apply our standard finish of vapour honing. Importantly, the accuracy of CNC machining is good for repeatability, low-volume runs and high-volume runs.  

For 3D printing, we offer a broad choice of materials, but these are all polymers. If machine builders need metal components – which they usually do – then CNC machining is the best option (see below for remarks on metal 3D printing and investment casting). Metals that we hold in stock for our Express CNC Machining service are aluminium alloy (grade 6082) and stainless steel (grades AISI 304 and 316). In addition, for our Standard and Economy services we often machine copper, brass, phosphor bronze, mild steel and tool steel. If required, we can source and machine high-performance engineering plastics, composites and some ceramics. 

Disadvantages of CNC machining

Although CNC machining can produce complex geometry on the outside of components, there are limits to what can be machined inside because the cutting tool requires access. If internal channels or voids are required, the components might have to be machined in two or more parts and then assembled, which could be costly and also compromise other characteristics of the part. In contrast, 3D printing can (within reason) create components with channels, voids and complex lattice structures. 

We offer an Express CNC Machining service with parts machined in three working days. This is fast enough for most customers, but it is not as quick as our Overnight Express 3D Printing service, where we print parts overnight. So, if you want parts very quickly, and the part can be made from a 3D printable material, then 3D printing will be quicker than CNC machining. 

CNC machining also has other limitations in terms of the geometry that can be machined; for example, pockets that are deep and narrow, and features with small internal radii or no internal radius. 

Although we say we can CNC machine almost any solid material, polypropylene (PP) is notoriously difficult to machine. If parts need to be made of PP, it might be better to 3D print them in a material with properties similar to those of PP. 

Advantages of 3D printing

3D printing is usually associated with product development and prototyping but it can be highly beneficial for end-use parts including those required by machine builders. Because 3D printing is an additive process, with parts built layer by layer, it is straightforward to create complex geometries, including internal passageways, voids or lattice structures for weight saving. 

If the machine is an automated assembly or test system for consumer products, it is easy to 3D print nests or gripper fingers with geometry that is a perfect match for the surface of the components being handled. 3D printing can also be used to make quick-change inserts if a machine needs to handle different product variants; snap fits or living hinges can be incorporated, and alphanumeric text and/or machine-readable codes can be 3D printed for clear identification of the inserts.  

Compared with CNC machining, where added complexity leads directly to increased costs, there is little or no cost penalty associated with more complex 3D printed parts. In fact, greater geometric complexity can reduce the part cost if less material is required. 

Another major advantage of 3D printing is the speed with which parts can be built. With our Overnight Express 3D Printing service, we print parts overnight. Alternatively, we offer a Standard or Economy service for customers who do not need parts as quickly. These other production speeds also provide sufficient time that we can apply a wider choice of finishes or provide additional services such as installing threaded inserts. 

Most 3D printed parts for machines will be rigid, in materials with properties similar to nylon, ABS, polypropylene, acrylic and polycarbonate. However, technologies such as PolyJet and DLP (digital light projection) can also build parts in elastomeric materials. This can prove very useful for a variety of applications throughout machinery, including custom seals and gaskets. 

PµSL (projection micro stereolithography) is unlike any other 3D printing technology. As its name suggests, this technology builds small, highly detailed parts with extreme accuracy and an exceptionally smooth surface finish. If a machine features micro-electro-mechanical systems (MEMS) or micro-fluidics, then PµSL is likely to be the best manufacturing technology. We offer a choice of materials, including one that withstands temperatures up to 114 °C. 

Something that 3D printing can do but CNC machining cannot is build ready-assembled mechanisms. In particular, SLS (selective laser sintering) builds parts from PA2200, a fine powder. Resultant parts have material properties similar to those of nylon, including good abrasion resistance and low friction. Such assemblies need to be designed with sufficient clearances so that unfused powder can be removed but, for some applications, it can be very useful to 3D print ready-assembled parts.  

Some parts of a machine may be difficult to design right first time, so 3D printing can be quick and cost-effective for prototyping and developing parts such as guides. Once the geometry has been proven with 3D printed parts, the final component can be CNC machined if the robustness of metal parts is needed. 

Depending on the machine’s function and whether it is a one-off or being built in quantities, the designer might wish to combine parts or functions to save weight, reduce the parts count and cut costs. 3D printing might be a better option for manufacturing the combined part than CNC machining due to the complexity of the geometry. 

Finally, 3D printing can be more cost-effective than CNC machining if, say, the machine builder is producing ten machines over the course of a year. For CNC machining, the cost-per-part will be lower if ten parts are ordered as a single batch, but this requires many of those parts to be held in stock for months. On the other hand, 3D printing enables the machine builder to order only the parts required, when needed, with no cost penalty and no need for stockholding. 

Disadvantages of 3D printing

Compared with CNC machining, 3D printing is less accurate. Whereas our quoted general tolerance on CNC machined parts is ±0.1 mm (and we usually achieve better than that), we quote a general tolerance of ±0.5 mm on 3D printed parts – other than PµSL parts, for which we quote ±25 µm. 

For 3D printing, the choice of materials is limited to plastics, most of which are rigid though some are elastomeric. This can be a serious limiting factor for machine builders needing parts made from metals, composites, ceramics and so on. 

Depending on the 3D printing technology and material, parts can degrade if subjected to ultraviolet light, elevated temperatures, humidity or if they come into contact with certain chemicals. 

Also dependent on the technology and material is the surface finish. SLS parts have a relatively poor surface finish, while SLA and DLP parts have a better finish but can still exhibit the ‘staircase’ effect on surfaces. These shortcomings can be overcome with secondary finishing operations, but this adds time and cost. In comparison, CNC machining leaves a very good surface finish, and all our CNC machined parts also benefit from vapour honing as standard, which leaves a very uniform surface appearance with no visible machining marks. 

Something else to be aware of with 3D printing is anisotropy, especially with technologies such as SLS. Parts tend to be slightly weaker in the z-build axis, so the build orientation has to be chosen with care. Furthermore, because technologies like SLS fuse the material by applying heat, parts can have residual stresses that cause them to warp or distort over time or if the temperature rises in the operating environment. 

If machine builders are used to designing parts that are to be machined from solid, it can take a while to become familiar with designing for 3D printing. If the full advantages of 3D printing are to be enjoyed, it is necessary to design the part for that manufacturing technology, which requires a different mindset to be adopted. 

Other technologies to consider

CNC machining and 3D printing are not the only options for machine builders. For example, investment casting can be a good choice for metal parts with complex geometries. Today, wax patterns can be 3D printed using QuickCast, which uses a specialist material to 3D print patterns with SLA. This technique can be particularly useful if only relatively small quantities of parts are needed. 

Vacuum casting is another option for producing small quantities of parts. The material is polyurethane resin but its properties can be specified so the parts have material characteristics ranging from elastomeric to rigid. This versatile production technology also enables castings to incorporate accurately machined metal inserts, or parts can be over-moulded so they feature soft elastomeric areas, including integral seals and gaskets. 

Laser cutting uses a high-powered laser to produce prototypes and end-use parts from thin sheet material. The same machine can also mark or engrave parts. As the technology is non-contact, it is good for cutting materials that are usually difficult to handle, such as thin sheets for gasketing. 

We offer QuickCast, vacuum casting and laser cutting in-house but there are two other technologies that we can subcontract on behalf of customers. One is metal 3D printing and the other is ceramic 3D printing. Both of these can be ideal for niche applications but they are specialised technologies that can be costly and not as quick as either CNC machining or 3D printing in plastics. 

We have already discussed laser cutting of thin sheets but higher-specification lasers can cut heavier/thicker materials, including sheet metal. Other CNC profiling systems use cutting technologies ranging from plasma and oxy-fuel, to wire EDM (electrical discharge machining) and waterjetting. Note that waterjet with added abrasive particles can cut metals, granite, marble, wood, plastics and more. When used without abrasives, waterjet can cut thin and delicate materials without causing any heat-related damage to the cut edges. 

Mix and match

Just as CNC machining and 3D printing are not the only manufacturing options, they are also not exclusive. There are times when the best way to produce a part is to 3D print it, and then use CNC machining to add accurate features. This enables a part to benefit from the best of both worlds: a complex geometry created with 3D printing, and high-accuracy CNC machined features. 

Alternatively, a part can be 3D printed and have an accurately CNC machined insert fixed in place. 

How to decide?

For some parts, it is obvious that either CNC machining or 3D printing is the better option, perhaps because of the material requirements or geometric complexity, respectively. However, it can sometimes be more difficult to decide which technology is best. 

Depending on the geometry and the part’s intended purpose, it may be that small changes to the design can improve its machinability or 3D printability, without compromising its performance.  

If in doubt, we can give impartial advice because we have both CNC machining and five different 3D printing technologies in-house. We can discuss the pros and cons of CNC machining and 3D printing, and we can even quote for both if neither offers a clear technological advantage for the part in question. If necessary, we can advise on design changes that could reduce the cost without sacrificing quality or performance. 

Talk to us

If you are a machine builder and need CNC machined or 3D printed parts, or want to discuss the pros and cons, contact us on 01763 249760 or request a quote.