Subtractive manufacturing differs from additive manufacturing in that it removes material to produce parts of the required dimensions. They are now deployed in many industries such as aviation, automotive, metallurgy, etc. This article in Antarctic Bear carefully discusses the differences between subtractive and additive manufacturing, and how they complement each other
First of all, there are many types of subtractive manufacturing methods, just like additive manufacturing includes multiple processes. For example, you may have heard the term “CNC machining”. It is an acronym for Computer Numerical Control and covers material removal techniques based on digital programs that control machine tools, such as drilling, turning, milling or boring. CNC machining is now compatible with a wide range of materials, from plastics to metals. That said, not all subtractive manufacturing techniques use digital procedures, and we’ll also look at some of the most popular cutting methods below..
Photo credit Protolabs
What is CNC machining?
CNC machining consists of several processes, turning and drilling were introduced for the first time, and the machining processes are directed at moving parts on rotating axes. The energy required to machine the part is provided by the movement of the part itself. These methods make it possible to manufacture any part modeled by rotation. Specifically, it is a two-dimensional drawing that is copied infinitely along an axis of rotation, thereby forming a three-dimensional model. Despite their different names, turning and boring are very similar because they both use the same principles. The main difference is that boring works on the inside of the part, while turning works on the outside, allowing work on wood, metal and some plastic materials. Turning and boring machines are now found in many industries such as the automotive, aerospace, medical and even decorative sectors.
Let’s move on to another technology, milling. Milling is an extremely important manufacturing tool due to its extreme accuracy, ability to create parts with internal cavities and the ability to machine parts. This technology uses a milling cutter to cut the material laterally. This means that when the milling cutter is in the material, it can move vertically and remove material along its path. Milling is compatible with many materials due to its suitability for various tasks and material processing. Nonetheless, this method also has some disadvantages that reduce its ease of processing. For example, the machining of a part is usually divided into several steps/operations, which require regular tool changes.
milling process
Drilling and milling use very similar tools but should not be confused. Drilling is only used to make round holes with a drill bit. While milling offers greater manufacturing freedom, specifically allowing for making holes or shapes that are larger in diameter than itself, drilling allows you to drill holes that are equal to the diameter of the drill bit. Despite its many advantages, milling is much slower than drilling when it comes to making holes, which is why drilling is more suitable for making several identical cuts in a row. While CNC machining encompasses many technologies, subtractive manufacturing is not limited to these. Other methods, such as cutting, are also used by many industries.
Different cutting techniques
Some use laser cutting, such as the automotive manufacturing industry that uses lasers to produce precision parts such as brake pads, while others choose EDM. EDM is electrical discharge machining. This technology has three forms: wire cutting, sinker cutting and rapid drilling. It is different from traditional drilling and can use electrodes to make thinner and deeper holes. While these different methods are relatively slow, what differentiates them from other methods is precision. Electrical discharge machining can be used in the production of parts from concrete, stone, metal and many other materials.
In addition to electrical discharge machining, laser cutting is also a commonly used cutting technology. This technology uses lasers with hundreds of watts of power to cut several materials quickly and accurately. For example, cutting parts up to 1 square millimeter has a tolerance of plus or minus 20 microns. In addition to cutting, laser machines can also engrave. Laser cutting machines come in two forms. Carbon dioxide lasers and fiber lasers, which are used to cut and engrave metal. Although laser cutting machines have many advantages, there are also some disadvantages: Harmful gases may be released when operating such machines. Additionally, because lasers use heat to cut materials, the laser itself can overheat. Therefore, it is necessary to cool it, which often requires additional auxiliary equipment.
Finally, among several major subtractive manufacturing processes and more specific cutting technologies, we find waterjet cutting. The process is based on the use of water jets, accompanied by abrasives, which impact the surface of the material at high speeds. This causes the material to shrink and separate, forming the desired final shape. A high-pressure compressor is required to produce this water jet. The nature of the jet and the material involved will affect the depth and speed of the cut. This technology is popular in the aerospace, automotive and mechanical industries. It is highly accurate, compatible with many materials, and unlike various subtractive processes, it is non-hazardous. While other processes produce chips, dust or fumes, waterjet cutting eliminates all this waste because the water jet carries it away in its path.
Waterjet cutting process (Image: Fogepack Systèmes)
Disadvantages and advantages compared to additive manufacturing
As mentioned previously, the main advantage of subtractive manufacturing technology may be its ability to provide high dimensional accuracy. Unlike most 3D printing processes that rely on thermal energy to operate, subtractive manufacturing allows parts to be made at room temperature, thus avoiding dimensional accuracy issues associated with the working (shrinkage) of the material. To ensure dimensional accuracy in subtractive manufacturing, the environment must be controlled. For example, if you want to design a part from a solid piece of steel, you must bring the material into the room where it will be cut at least 3 days in advance, otherwise it will change the working dimensions based on temperature and humidity.
Subtractive manufacturing processes also benefit from a wider range of compatible materials. Compared to 3D printing, where if you want to use different materials you have to have several printers based on different processes, it is possible to do subtractive processing of metal, plastic or wood parts on the same machine.
Subtractive manufacturing provides high dimensional accuracy
However, subtractive manufacturing processes have significant drawbacks. First, unlike 3D printing, where different subtractive manufacturing production methods leave behind a lot of waste, additive manufacturing is attractive because it only uses the amount of material needed. For example, in machining, debris and other dust must be removed during the manufacturing process to limit the amount of excess material that can interfere with the cutting process. In addition to the dust produced by subtractive manufacturing, the fumes produced by the process also have the potential to be very harmful to operators, which is very common when using laser or EDM.
Additionally, subtractive manufacturing does not offer the same degree of manufacturing freedom as 3D printing. A printable part sometimes requires multiple operations, using different machines, and disassembly into multiple parts for assembly before it can be replicated by subtractive manufacturing. Finally, achieving complex geometries with material removal processes is more difficult. Additive manufacturing allows more freedom in the design process. In other words, the designer will not have to worry about the limitations imposed by the machine tool he is using and will therefore be able to let his imagination run free without being limited by manufacturing capabilities.
How do subtractive and additive manufacturing complement each other?
Since the two technological approaches to subtractive and additive manufacturing are different, they are mostly used in a complementary manner. Since 3D printing technology started growing strongly, it has been most commonly used for prototyping. With the technical advantages of 3D printing itself, it allows the rapid and low-cost manufacturing of multiple parts, thus providing the possibility of different iterations.
For part production requirements where shape and material are defined, it is possible to use subtractive manufacturing methods for mass production. Some processes, such as laser cutting and waterjet cutting, can design large numbers of parts in a very short time. However, other methods, such as CNC machining, can be very time-consuming. This technique requires a significant programming step, as well as necessary labor costs. Today, CNC machining is mainly used in the manufacture of injection molds, a form-forming manufacturing technology.
Additionally, while subtractive manufacturing allows for the repair of objects, 3D printing offers many more possibilities in this area. One example is the directed energy deposition (DED) process, which can be used to repair metal parts without having to alter them. Specifically, the technology adds material to existing parts, eliminating the need for unnecessary assembly or replacement of large parts. On a larger scale, 3D technology allows parts that are faulty or worn by time to be replicated, all on demand and in small batches. Users can repair their parts instead of throwing them away and minimize their impact on the environment.
DED technology allows parts to be repaired
Despite their differences and processes, subtractive manufacturing and additive manufacturing tend to go hand in hand. Due to their respective characteristics, these two types of manufacturing complement each other, allowing for the rapid creation of precise parts never imagined before. To combine these manufacturing technologies, several investment projects have been created, such as hybrid advanced manufacturing. By integrating various technologies, we provide the design freedom of additive manufacturing and the precision of subtractive manufacturing.
