It used to be that if you needed to grind gears, your decision was limited to a slow-form grinder that was better suited for small lots, but not regarded as competitive for larger production runs or, alternatively, a machine considered to be a production-style “threaded wheel” generating grinder. The latter was well suited for 3,000 gears, but it was not competitive for the 30-piece stuff because of the development time required. Now you can have your cake and eat it, too. Figure 1
Since the mid-forties, there were threaded wheel grinders. Some in our industry have had the audacity to call them “stone age” hobbing machines; that is to say machines that used a stone (vitrified grinding wheel) that worked with a thread, like a hobbing machine. This was an oversimplification, but an analogy that most people could understand. In principal, the process of generation was easy to understand, but sometimes difficult to execute. As gear profiles became increasingly complicated, convoluted, and contorted, the wheel form was no longer just a simple straight-sided rack made up of two sides that corresponded to the pressure angle of the gear. It was necessary to have special diamond dressing discs that were lapped and had specific modifications containing the desired profile.
Fast forward nearly 60 years. With the development of “line dressing,” a gear manufacturer now describes the curved, straight, and twisted characteristics of the involute and fills in a prompted blank in one of the set-up screens of the control. Now a single diamond disc does the work of several special sets of diamond discs. Not only can it dress single start wheels, it is suitable for multi-starts–up to seven–as well. The RZ 400 is like no other generating grinder found on the market today. It is a 10-axis CNC machine (14 optional) that has been designed and built to be integrated into today’s factory environment. The machine is configured in such a way as to make it easily attended, whether for set-up of the dressing unit or manual loading of the work spindle. Everything has been carefully engineered from the perspective of the operator. The entire column that carries the grinding spindle rotates into three discrete positions where dressing of the wheel, loading of the work spindle, and wheel change are affected. With the appropriate dressing unit, the RZ 400 can be used to efficiently grind batch sizes as small as five, or as large as 5,000. Figure 2
Set-up and development of a job can be accomplished quickly and efficiently with the use of “line dressing,” a Windows-based software program that coaches the operator through the process. If the profile is not just the way you want it the first time, program in a correction, and in about 20 minutes you’re ready to grind the next gear. Acoustic touch dressing simplifies the dressing disc alignment with the wheel and can prevent damage and premature wear to your diamond tools. Figure 3
It used to be that machines employed single-start wheels to assure the highest accuracy possible. While accuracy was of paramount importance, productivity sometimes suffered due to the number of starts in the wheel. Multi-start hobs have been used for decades in CNC hobbing machines, so why not use multi-start wheels to achieve higher productivity in the grinders as well? A huge hurdle had to be overcome to successfully use multi-start wheels on the new generation of machines. In particular, the stiffness of the work spindle had to be improved to counteract the forces created when using multi-thread wheels. A patented design of the work spindle using a planetary friction drive allowed us to improve the dynamic rigidity of the work spindle sixfold over the previous model, and because we do not use gears to transmit the motion to the work spindle, there are no ghost frequencies, or “fingerprints,” on the gears ground on these machines. The stiffer work spindle design allows grinding wheels with a greater numbers of starts, resulting in higher productivity. “LNS,” or low noise shifting, is another new technology developed by Reishauer specifically for ground gears that must be honed. The implementation of this new technology obviates the need to hone gear teeth after grinding for high-speed applications that are sensitive to noise. Figure 4
Figure 5 Further, RZ 400 productivity enhancements have been made by incorporating automatic fine balancing of the grinding wheel on the grinding spindle. Between grinding cycles, and after all dressing operations, the wheel is rebalanced and made ready for the next part to be ground. A non-contact probe is responsible for aligning the spinning threaded wheel to the workpiece to evenly divide the grinding stock, which further reduces non-productive time. Figure 6
With nearly 90 machines on order and delivered, Reishauer has been able to verify its claim that it is the most productive gear-grinding machine on the market.