How to Get High-precision Gear Machining?
Three primary categories have historically been used to categorize gear fabrication techniques: generation, forming, and form cutting. While there are still three popular gear machining techniques to mill a gear, there will be more methods added to the list when new technologies for gear machining are created.
Gear generation creates the gear by forming cutting instruments (such as rack cutters, gear shaping, and gear hobbing) into the shape of the desired gear profile.
The tools used to construct the gear profile—gear machining milling, shaping, slotting, planing, and EDM—are involved in gear form cutting.
Without the use of cutting tools, gears can be made using gear formation (i.e. rolling, casting, powder metallurgy, 3D printing)
Methods of Gear Generation
There are three gear machining ways of gear generation, including sunderland method, gear shaping, gear hobbing.
One of the primary techniques for manufacturing gears involves rack-type cutters and is commonly referred to as the Sunderland Method or the Sunderland System. This approach employs a gear machine equipped with a rack cutter featuring rake and clearance angles, which is used to shape the teeth profile on a gear blank. The process relies on the specific relative movement between the workpiece and the cutter during gear machining, resembling the operation of a rack and pinion system.
The resulting teeth profiles exhibit an involute of a circle, essentially a spiraling curve created as an imaginary string unwinds from a stationary circle. This gear machining method ensures the production of teeth with uniform shapes, allowing gears cut by the same cutter to mesh correctly with one another. The Sunderland Method is highly suitable for manufacturing precision gears, including double helix gears. Moreover, it is versatile and cost-effective, particularly for medium to high-volume production runs. Furthermore, since the Sunderland system has been consistently maintained since its inception, it boasts a wealth of available manuals and documentation, even relevant to modern machine designs.
Gear shaping involves the use of a cutter and a gear blank connected through gears, allowing them to move independently as the cutter reciprocates. The cutter begins to carve its way to the desired depth, and both the cutter and the gear blank rotate slowly during the cutting of the gear teeth.
This gear machining method is frequently employed for producing spur gears, herringbone gears, and ratchet gears. While it can be adapted for other gear types, gear shaping is primarily associated with these specific gear varieties. Since it does not necessitate complex programming (the process is essentially cutting in a manner similar to the rack cutter), it simplifies the setup during production. Although other gear machining methods can be used for manufacturing these types of gears, gear shaping offers advantages in terms of speed, design, and setup efficiency during mass production. However, it may not be the most suitable choice for internal gears and worm gears due to the position of the cutter on the outside and its cutting direction.
Gear hobbing is a method for producing gear teeth that involves the rotation of a cylindrically shaped cutter known as a “hob.” This process is particularly well-suited for creating spur gears, although it can also be applied to various other gear types, including cycloid gears, helical gears, worm gears, ratchets, and sprockets. The design of the hob cutter is crucial, especially when dealing with complex geometries, and this method is generally not employed for internal gears. Similar to gear shaping, gear hobbing offers setup advantages but is primarily suited for external gear cuts since the hob cutter works on the exterior of the gear blanks.
Methods of Gear Form Cutting
Form cutting methods are generally not the preferred choice due to their inherent limitations, which include low productivity and compromised quality. Nevertheless, these form-cutting techniques can be valuable alternatives, especially for repair and maintenance purposes.
Shaping, Planing, and Slotting
Shaping involves securing the workpiece while moving the tool back and forth along the workpiece.
Planing features a stationary tool with the workpiece moving back and forth beneath it.
Slotting maintains a stationary workpiece while moving the tool up and down across it.
Both shaper and planer gear machining tools operate in straight lines, with shapers suited for small-scale geometries and planers designed for larger applications. Shapers are adept at creating slots, grooves, and keyways, while slotting essentially serves as a vertical shaper, ideal for cutting internal gears and grooves.
Form cutting through milling is limited in its application but excels in crafting intricate gear geometries. CNC milling is employed for helical and spur gear machining used in various industries, including automobile transmissions and hob cutters.
The milling process is relatively slow due to the heat transfer generated during gear machining, necessitating intervals between milling successive teeth to avoid overheating.
Electrical Discharge Machining (EDM)
EDM is an electromechanical manufacturing technique that removes material from a workpiece by applying electrical discharges between two electrodes separated by a dielectric bath liquid. In gear machining method, electrical sparks serve as a ‘cutting tool’ that erodes the material.
EDM is proficient in cutting complex geometries of various sizes, including gears. However, it has its limitations, particularly in terms of surface damage if control and programming precision are not maintained. This is especially relevant for intricate, curved tooth profiles that may pose challenges for CNC programs. Advanced 3D modeling and Computer-Aided Manufacturing (CAM) software, such as Feature CAM, Autodesk Fusion, Master CAM, and others, can ensure the smooth motion required for cutting curved teeth.
Recent advancements in EDM machines have mitigated issues related to surface finishes, enhancing cutting precision and the resulting material properties, including microstructure and mechanical properties. This process can achieve tight tolerances as precise as thousandths of an inch and is applicable for crafting gears of both small diameters (fraction of an inch) and larger gears (over 20 inches in diameter). EDM is used in delicate applications, such as watches and clocks, as well as for more robust gears, including those used in high-performance race cars.
Methods of Gear Forming
There are four gear machining methods of gear forming, including rolling, casting, powder metallurgy, additive manufacturing.
Rolling is one of the earliest gear formation processes where a blank workpiece is passed through two or three dies, either hot or cold, as depicted below. This gear machining method is advantageous when material conservation is a primary concern during production since it does not generate chips. However, to achieve an efficient process, careful consideration of rolling parameters, deformations, and microstructure effects is essential before scaling up production.
Casting is a forming process involving the pouring of molten metal into a mold cavity to create shapes. Gear casting is employed to produce gear blanks (which are subsequently machined) and full gears with cast tooth profiles. Maintaining tolerances and accuracy is critical in gear casting, and the creation of casting molds involves significant initial costs. However, once the mold and process parameters are established, the economies of scale justify the investment.
Sand casting is primarily used to manufacture gear blanks that are employed in various subsequent processes. Fully functional spur, helical, worm, cluster, and bevel gears are all crafted through gear casting and find applications in washing machines, small appliances, hand tools, toys, and cameras.
Powder metallurgy is a high-precision forming method that provides a cost-effective alternative to conventionally machined steel and cast iron gears. While not suitable for larger gear sizes, it excels in creating small, high-quality spur, bevel, and spiral gears. The porosity of the formed material in larger gears results in reduced fatigue and impact resistance, but a sintering process can be employed to enhance their mechanical properties.
Powder metallurgy is particularly beneficial when gear designs incorporate features like holes, recesses, varying surface levels, or projections. Gears manufactured through this gear machining method find applications in appliances, farm equipment, lawn and garden machinery, automobiles, trucks, and military vehicles.
Also known as 3D printing, additive manufacturing constructs three-dimensional objects layer by layer, following a CAD 3D model. Due to the nature of this process, additive machines can produce intricate designs with lattice structures, facilitating mass reduction that is challenging to achieve through conventional means. Such geometric intricacies are often realized through 3D topology optimization and generative computer design.
Additive manufacturing can be used to create conventional as well as non-circular gears, and high-quality 3D printers are readily accessible and relatively affordable. This availability has made it a preferred choice for repairs and mechanical projects, including educational toys and gadgets that require fully functional gears. Additionally, it allows for the incorporation of extra features and the fusion of geometry with gear shapes to add customized shafts, keys, or grooves to the same solid.
Now, you are acquainted with a range of methods for gear machining, whether for generation, formation, or cutting. For assistance with your CNC design projects, including gear design, consider exploring our helpful guide, the CNC Design Guide.
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