Top Considerations for Using Metal 3D Printing vs. CNC Machining
Designers everywhere must be well informed in order to determine the right process and materials for each project.
In recent years, designers and inventors alike have been keen to create their next part or product using metal 3D printing. With the latest advancements enabling a wider range of processes and materials that can be exploited with much greater speed and efficiency, metal 3D printing is all the rage. This is because it frees engineers to create complex, functional designs that would otherwise be unattainable using conventional means. But, is it the most practical solution for every application?
The alternative to metal 3D printing is of course conventional CNC machining. While CNC machining is viewed as a more traditional manufacturing method, it is also often the more cost-effective solution compared to metal 3D printing. Each process has its own unique benefits and limits. Thus, designers everywhere must be well informed in order to determine the right process and materials for each new project in their workflow. How can you determine the best method for your next metal project? Let’s explore.
Comparing metal 3D printing to CNC machining
In essence, metal 3D printing and CNC machining are opposite processes: additive versus subtractive. With additive manufacturing, a part is built one layer at a time. Metal 3D printing uses metal powders and, with energy from a laser or electron beam, layers of the chosen material are solidified on top of each other to form the finished part. With CNC machining, the user starts with a block of metal and then material is removed to create the finished part. Although different, both processes need 3D CAD modeling to design the functional programming of the machines.
For all 3D printing processes, a cutting program is first derived that takes the geometry of the finished part and slices it into a series of 2D cross-sections. These sections determine which areas of raw material will be solidified for each layer. Now, let’s look at a few of the main types of processes used in commercial printing.
4DMLS is one of several powder bed fusion methods. Here, a very thin layer of metal powder is spread across the surface of the build chamber. A laser is slowly and steadily moved across the surface to sinter this powder only at the locations appropriate for that layer. This causes the particles of metal to fuse together. Additional layers of powder are then applied and sintered, aided by mechanical compaction with a roller, thus ‘printing’ the object one cross-section at a time. Once the DMLS process has been completed, the semi-finished part is usually baked in a secondary oven to burn off the binding compound, create a more uniform surface finish, and relieve thermal stresses in the part.
The choice of which process to use is still driven by the traditional factors of cost, speed, volume, complexity and fitness for purpose. Four heat sinks are shown here.
4In SLM printing, a high-powered laser fully melts each layer of metal powder rather than sintering it, creating an extremely dense and strong part, nearly the equal of a forged part. SLM is a very high-energy process, as each layer of metal powder must be heated above the melting point of the metal, which can lead to stresses and dislocations inside the final product and compromise its physical properties. The technique can only be used with specific types of metals since many metals don’t have the right flow characteristics needed.
4Binder jetting is another powder bed fusion method; however no laser is needed. Rather, minute droplets of a binding agent are sprayed in a 2D pattern, compacted with a roller, and then recoated with fresh powder. The binding agent will later be baked off in an oven, leaving behind a solid part only in those areas that were sprayed. By controlling the application of the binding agent, support structures can easily be snapped off by hand after the build, saving considerable time, money and effort. It’s also possible to recycle powder for reuse, and to stack parts vertically, thereby making optimal use of the volume of the build chamber. A drawback is that, because this is a low-temperature and low-energy process, the finished part will lack the strength of its fully-welded counterpart.
With CNC machining, the user starts with a block of metal and then material is removed to create the finished part.
In contrast to metal 3D printing—with the various competing technologies, build strategies and attendant results—CNC machining is fully mature, highly refined and absolutely reliable. By using different mills and cutters, virtually any solid material can be machined with high accuracy, excellent surface finish and reliably precise tolerances. The main limitations are that only one part can be made at a time, and geometries are limited to straight lines and curves, with no hidden or buried structures possible.
Applications for each process
Metal 3D printing provides engineers with freedom of design and the ability to reduce the weight of parts. With this process, designers can make complex geometries and internal lattice structures for optimal strength. Using SLM and DMLS, designers can produce parts from a range of metals and metal alloys including aluminum, stainless steel, titanium, cobalt chrome and Inconel, which are primarily used in industrial applications such as rockets and nuclear components for aerospace or medical devices and equipment for the medical sector.
Metal 3D printing can also process precious metals, such as gold, platinum, palladium and silver, which are primarily used for jewelry making. The surface finish quality is still poor for metal 3D printing so post-processing is often needed. When considering this process, designers must consider the high cost of the metal powder. For instance, two pounds of stainless steel 316L powder costs approximately $350 to $450. As such, minimizing the part volume and the need for supports for the part or product is key to keeping costs low in production.
With CNC machining, designers can achieve greater dimensional accuracy. This process can also be repeated in exactly the same manner over and over again, making it ideal for mass production. CNC machining can be used in the production of many complex 3D shapes that need to be extremely robust, precise and heat-resistant. A commonly used metal in CNC machining is aluminum, which is often used by prototyping companies to create high-quality prototypes in a variety of industries. Other commonly used metals include stainless steel, magnesium alloy, zinc alloy titanium and brass. CNC machines can also handle precious metal such as gold and silver. CNC machining is also typically used in the automotive sector for shafts and gears and the equipment market for industrial hardware. When it comes to cost, two and a half pounds of 13-8 Mo stainless steel can cost $100 to $200 dollars. As the part or product needs to be created from the metal block, more material is typically needed.
With metal 3D printing, designers can make complex geometries and internal lattice structures for optimal strength.
Deciding between the two manufacturing methods
While metal 3D printing can create a wider range of finished parts, it is a much slower per-part process and would only be commercially viable for a small handful of parts and products. Although advances continue to be made in this field, we are still some years away from real, high-volume commercial applications for metal 3D printing. Until the new systems are put to the test and truly prove their claims, CNC machining will still be the preferred technique for volume production due to high throughput, reliability and accuracy. It’s also important to note that this need not be an either/or choice. Virtually all metal 3D printed parts will require some form of traditional post-machining to make accurate, threaded holes or very flat surfaces for example. Combining the strengths of the two approaches, as some hybrid machines are able to do, can offer the best of both worlds for the most sophisticated applications.
Until there is a giant leap forward in speed for 3D printing, along with a resulting reduction in cost, then CNC machining is still the fastest and most cost-effective way to make high volumes of parts in metal. So far, 3D printing makes the most sense when the part has a geometry difficult or impossible to make conventionally, needs a high strength-to-weight ratio, the volumes are relatively low, and the piece price is not an issue. 3D printing is also an ideal solution for medical orthotics, replacements or implants, since it’s possible to make customized parts quickly by doing a 3D scan of a patient’s unique physical structure and build a part to suit.
The choice of which process to use is still driven by the traditional factors of cost, speed, volume, complexity and fitness for purpose. The good news is that we now live in a time when there are readily available manufacturing solutions to make virtually any part that can be imagined, in almost any material.