If you had the opportunity to walk the SME-sponsored tradeshow RAPID + TCT this past April in Detroit, Michigan, then you saw a tremendous offering of additive technologies on display. These technologies give the design engineer new tools to create components in shapes and configurations that were previously unobtainable with traditional machining processes. Although molding and casting are variants of additive manufacturing, 3D printing has not yet become a suitable manufacturing method for quantity gear production.
Traditionally, methods such as hobbing, skiving, shaping, and grinding have long been the cornerstones of gear production. Each of these methods involves the removal of material from a prepared blank resulting in a finished product with fully functional gear teeth.
Additive manufacturing, which is the building of objects layer by layer, can be traced back to the early 1980s. One of the earliest forms was stereolithography (SLA) which was developed in 1984. The process involves using an ultraviolet laser to selectively cure layers of liquid photopolymer resin, solidifying them into a three-dimensional object. This marked the birth of 3D printing technology and introduced the fundamental principle of layer-by-layer fabrication.
Around the same time, Fused Deposition Modeling (FDM) was developed. This technique involves extruding a thermoplastic filament through a heated nozzle, depositing material layer by layer to build the part. FDM’s simplicity and versatility made it one of the most widely adopted AM processes, however it does not produce gears with a suitable surface finish.
In the 1990s, AM technologies were developed. These included Selective Laser Sintering (SLS), which uses a laser to fuse powdered materials such as plastics, metals, or ceramics and Laminated Object Manufacturing (LOM), where layers of material are cut and then bonded together. In the early 2000s, continued development of AM techniques included the introduction of Electron Beam Melting (EBM) and Digital Light Processing (DLP). EBM uses an electron beam to melt metal powder in a vacuum, while DLP employs a projector to cure liquid resin.
While the term “additive manufacturing” encompasses a broad range of technologies, several are particularly relevant to the production of gears. It is important to distinguish these from traditional manufacturing methods such as casting, forging, and welding, which, while still valuable, operate on subtractive or formative principles. Interestingly, some additive manufacturing techniques are now being used in conjunction with or as alternatives to these traditional methods.
Powder Bed Fusion (PBF)
Powder bed fusion is a prominent metal additive manufacturing process where a heat source, such as a laser or an electron beam, selectively fuses powder particles layer by layer within a powder bed. After each layer is completed, a new layer of powder is spread, and the process is repeated until the entire part is built. Unused powder provides support for the part during the build and can often be recycled.
Several variations of PBF exist, including:
- Selective Laser Sintering (SLS): Primarily used for polymers but can also process some metals and ceramics. A laser sinters the powder particles together.
- Selective Laser Melting (SLM): Specifically for metals. A high-powered laser fully melts the powder particles, resulting in denser parts compared to SLS.
- Direct Metal Laser Sintering (DMLS): Like SLS but optimized for metal powders. It sinters the powder, and a range of engineering metals are available.
- Electron Beam Melting (EBM): Uses an electron beam as the heat source and is conducted in a vacuum. Suitable for metals such as titanium, cobalt-chrome, and stainless steel, often resulting in fully dense parts with good mechanical properties.
PBF offers significant design freedom, allowing for the creation of complex internal features, optimized tooth profiles, and lightweight lattice structures within gears. It is also valuable for rapid prototyping and customization.
Directed Energy Deposition (DED)
Directed energy deposition is another class of additive manufacturing processes where material, typically in powder or wire form, is melted and deposited layer by layer using a focused heat source such as a laser or an electron beam. Unlike PBF, DED does not use a powder bed. The material is fed directly to the melt pool as it is being created.
Common DED techniques include:
- Laser Metal Deposition (LMD): A versatile process that can use both powder and wire feedstock with a laser as the heat source.
- Wire Arc Additive Manufacturing (WAAM): Uses an electric arc to melt wire feedstock, offering high deposition rates and the ability to produce large parts. Variations include Plasma Arc Deposition and Cold Wire MIG.
- Electron Beam Additive Manufacturing (EBAM): Employs an electron beam to melt wire feedstock in a vacuum, suitable for large-scale metal part production.
DED is particularly useful for repairing and adding features to existing parts, as well as creating large, complex geometries. It offers a wide range of material compatibility and high deposition rates.
Binder Jetting
Binder jetting involves depositing a liquid binding agent onto a powder bed to selectively bind the powder particles together, layer by layer. After the build is complete, the unbound powder is removed, and the resulting “green part” is typically sintered in a furnace to achieve the final density and mechanical properties. This process can be used with various materials, including metals, ceramics, and polymers.
Binder jetting offers the potential for high build volumes and relatively low material costs. It is often used for producing complex geometries and can be more scalable for certain applications compared to other powder-based AM methods.
Material Extrusion
Material extrusion, exemplified by Fused Deposition Modeling (FDM), involves extruding a filament of thermoplastic material through a heated nozzle and depositing it layer by layer. While primarily used for polymers, metal and ceramic filaments bound in a polymer matrix are also available. Parts made from these filaments require debinding (removing the polymer binder) and sintering to achieve a dense metallic or ceramic structure.
FDM offers a relatively low-cost entry point into AM and is widely used for prototyping and producing functional parts. While perhaps not the primary choice for high-performance, critical gearing applications, it can be suitable for certain lower-load or specialized gear designs.
Vat Photopolymerization
Vat photopolymerization, which includes Stereolithography (SLA) and Digital Light Processing (DLP), uses a light source (UV laser in SLA, projector in DLP) to selectively cure layers of liquid photopolymer resin in a vat. After each layer is cured, the build platform moves, and a new layer of liquid resin is exposed.
While offering high precision and smooth surface finishes, vat photopolymerization is typically limited to polymer materials. These polymers may find applications in specialized gearing scenarios where high precision and specific material properties are required, but they generally do not match the strength and durability of metals for high-power transmission.
It is crucial to understand that additive manufacturing is not necessarily intended to replace all traditional manufacturing methods for gears. Instead, it often complements them, offering unique advantages in specific areas. When casting or forging gears, additive manufacturing can be used to create complex patterns for casting or near-net-shape preforms, potentially reducing tooling costs and lead times for complex gear geometries. In place of welding, DED techniques can be employed for repairing worn or damaged gear teeth or for adding specific features to cast or forged gears. Additively manufactured gears often require some post-processing, such as machining, to achieve the final dimensional accuracy and surface finish required for optimal performance. As such, hybrid machines that combine additive and subtractive capabilities are also emerging.
For gear engineers, additive manufacturing presents a compelling array of advantages. Additive manufacturing allows for the creation of complex gear geometries that are difficult or impossible to achieve with traditional methods. This includes optimized tooth profiles, internal cooling channels, lightweighting features, and integrated functionalities. Additive manufacturing significantly reduces the time and cost associated with producing gear prototypes, enabling faster design iteration and validation. Additive manufacturing is well-suited for producing customized gears or small batches of specialized gears without the need for expensive tooling. It can significantly reduce material waste by only using the material needed to build the part. Additive manufacturing enables the design of gears with features that can improve performance, such as optimized stress distribution, reduced weight, and enhanced lubrication through integrated channels. It also permits the use of novel materials, and the creation of custom alloys tailored for specific gear applications.
Despite its numerous advantages, gearing produced using additive manufacturing also presents some challenges. These include mimicking the material properties of machined gearing. Achieving the same level of material density and mechanical properties as traditionally manufactured gears can be challenging, although noteworthy progress is being made. Additively manufactured parts often have a rougher surface finish and lower dimensional accuracy compared to machined parts, requiring post-processing. Another issue is that for large volumes, traditional manufacturing methods are typically more cost-effective. One of the nagging issues limiting additive manufacture of gearing is that industry standards and certification processes for additively manufactured gears are still evolving, thus limiting their use to research projects instead of commercial applications.
For mechanical engineers with a focus on gear design, understanding additive manufacturing is no longer optional — it is essential. Additive manufacturing offers a powerful toolkit for innovation, enabling the creation of gears with unprecedented complexity, customization, and performance potential. While challenges remain, the rapid advancements in additive manufacturing technologies and materials promise a future where 3D-printed gears could play a significant role in power transmission across a wide spectrum of industries. By embracing this transformative technology, gear engineers can unlock new possibilities and shape the next generation of mechanical systems.
