Polished Up!

Tool systems for grinding and honing today’s most challenging gears

The trend towards hard fine finishing of gears for automotive transmissions is accelerating. In recent years, the focus has been on increasing the efficiency of gears in order to make optimum use of the narrow gear ratio range, gear by gear, and to reduce fuel consumption. Surface finish, in particular, is under increased scrutiny since it plays such a decisive role in achieving the noise and efficiency requirements of gear units for today’s EV applications.

These higher gear quality standards are driving significant innovation in the grinding and honing manufacturing processes required to achieve them. Fine finishing and, in particular, polishing machining, offer a tailor-made approach to meet these objectives. Inherent in these processes is the fact that two machining steps (roughing and fine finishing) are performed sequentially, in a single setup. Alternatively, different processes can be combined to achieve the same end result, such as grinding with gear honing. The disadvantage of this approach, however, is that the workpiece must be loaded into a machine tool twice. What all these processes have in common is that they offer a wide range of tool specifications, including variations in grit size and bond type, to achieve the desired result in the finished workpiece.

To reliably maintain a maximum form error of nm, manufacturers of dressing tools have to consider many factors. Shape and position tolerances of the basic body must be maintained with extreme precision. The smallest deviations in the axial runout of the tool can cause the µm limit to be exceeded when checking the gear quality. Analyses of noise, vibrations (NVH test), and the surface structure help to check the requirements in all areas with great precision. A homogeneous grain distribution and a clean coating must be ensured before the grinding/finishing of the electroplated coating can take place. In this process two tasks have to be performed equally well: grinding the exact shape of the rotational dressing disc shape as well as the correct treatment of the diamond grits to ensure their shape retention over repeated use and thus the longest possible service life.

Additionally, the dressing of different grain materials and sizes as well as different bonding systems are critically important. Ideally, these tasks should be performed using the same tool. In generating grinding, two-zone grinding wheels with Gleason dressing discs can be used without compromise. No adjustment of dressing parameters or speeds is required. Gleason dressing discs are designed to provide the required cutting capability for roughing as well as to fully meet the requirements for finishing or polishing.

In gear honing, the approach is congruent; only the degrees of freedom are greater since there are two separate honing tools without a connection. In this case, if necessary, a special diamond dressing gear per honing ring can also be executed to utilize the full potential in fine and polishing machining. In addition, for gear honing it is of course also possible to design a dressing tool (diamond dressing gear) for both honing tools, e.g., for Combi Honing.

Through the use of this new generation of dressing tools, the latest grinding wheels and honing tools are well-positioned to meet the increased demands placed on them. The “splitting of micrometers” is a great challenge, one that meshes well with our area of expertise and passion for the work we do.

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