Planetary drives (or gearboxes) use one sun gear in the middle and planet gears around it, all embedded into a ring. All of the gears, including the ring, have involute profiles. These types of drives are ubiquitous – they are used in a broad diversity of applications, including cars and other road vehicles, and are produced by virtually every gear manufacturer in the world.
When the Taiwanese machine tool company Chien Wei Precise Technology needed CNC grinding machines for manufacturing specialist gears used in robotics applications, they called on CNC high-end application company, NUM to assist.
Based entirely on NUM’s latest-generation Flexium+ 68 CNC platform, Chien Wei’s new machines are designed to speed the production of both involute and cycloidal profile precision gears. These machines are believed to be the first gear grinding machines on the market that are capable of handling both types of gear profile. There are two versions of the grinding machine, one for internal gears, the other for external gears.
Chien Wei Precise Technology initially specialized in precision machine tools such as vertical grinders, jig grinders, and machining centers, together with coordinate measuring machines (CMMs). Over time, it also diversified into robotic automation systems.
Robotic systems typically use either planetary gearboxes equipped with involute gears or cycloid drives based on a combination of a reduced epitrochoid rotor and a cycloid stator. While planetary gearboxes have been around for a long time and are quite common, cycloid drives have far fewer moving parts and offer a more efficient means of achieving high reduction ratios. A speed reducer with a ratio of 200:1 would typically need a chain of three planetary gearboxes with twelve moving involute gears. The same reduction ratio could be achieved with a cycloid drive using one stator and one rotor. However, cycloid gears are notoriously difficult and expensive to manufacture.
In 2015, Chien Wei decided to bring gear manufacturing in-house, by developing its own gear grinding machine. By producing its own gearboxes, the company could control quality, shorten lead times, and reduce costs. It would also enable it to sell gearboxes to machine builders — and possibly the machines themselves to gear manufacturers.
Chien Wei initially based its gear grinding machine on a Fanuc series 0i-MF CNC system, which it uses for other machine tools, together with Mastercam CAD/CAM software and its own CMM. But it soon became apparent that the profile complexity of cycloidal gears meant that CMM data was inadequate for controlling the manufacturing process. Another major disadvantage was that customers wishing to purchase the machines would also need to invest in an expensive CAD/CAM system and additional personnel.
The company decided it needed a CNC system that fully supported gear grinding from the outset so that its customers could input the parameters of the gear they wanted with the CNC then controlling all aspects of the machine’s dressing and grinding processes in real-time.
Gearboxes that use cycloid gears have very few moving parts and are some of the most efficient and reliable speed reducers available today. Single stator/rotor combinations can accommodate ratios as high as 300:1 and can provide efficiencies higher than 93%.
The new generation of Chien Wei’s gear grinders together with NUM’s new profile grinding technology now provides the ability and flexibility to manufacture both cycloid and involute gears on the same machine.
Chien Wei’s internal gear grinder is a 9-axis machine. The work piece table is mounted on a linear axis that moves axially towards the grinding wheel, which is belt-driven (due to space constraints inside the gear) and mounted on a vertical axis driven by a linear motor. As the grinding wheel spins, it is driven up and down by the linear axis, while the work piece table moves in continuously. Both flanks of the gear are ground simultaneously. The machine also handles gear dressing. During the dressing cycle, the entire tool head is moved horizontally to the right, and a symmetrical dressing disk moves along the outer shape of the grinding wheel in three sections, right/left flank and tip. Cycloids and involute shapes can be dressed.
The external gear grinder is an 8-axis machine. Broadly similar to the internal gear grinder in operational terms, it features a direct drive grinding wheel. Again, like the internal gear grinder, it also handles dressing. However, in this case, the shape of the tooth gap can be reduced epitrochoids or involutes.
Manual correction of the gear tooth shapes is possible on either machine. Both machines are controlled by Flexium+ 68 CNC systems, equipped with FS153i touch-sensitive operating panels. The application-specific HMI that has been jointly developed by NUM Taiwan and Chien Wei controls both the grinding and dressing cycles and allows users to fully specify the required gear profile by entering the appropriate parameters. Graphical DXF (Drawing Interchange Format) files can be imported from — and exported to — CAD systems, and the system features a comprehensive database for grinding wheels and work pieces. The NC programs are generated entirely automatically, without any need for operator involvement.
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