There are already an infinite number of different gear geometries. Nevertheless, engineers are constantly coming up with new variants. For example, gears are becoming smaller, more complex, and lighter. At the same time, the demand for performance — quiet, smooth operation, even under highest load — is always higher. The variety of gears knows no limits, but high-precision manufacturing does.
Today, the desired geometry of a gear is often achieved by skiving. This means the precision of the gear is limited by the precision of the tool. The principle of skiving has been known for more than a hundred years, but the technology in mechanical engineering has only been able to perform skiving reliably for a few years. That is, the previously desired precision of the tools and thus of the skiving process was achieved in elaborate development steps.
“Just a few years ago, the quality of gears in class 4 to 6 was the measure of all things,” said Haas Schleifmaschinen software manager Wolfram Hermle. “Today, tolerances of 2μ are not uncommon.”
Hermle said the time was ripe to take a fundamental look at this issue in order to search for a groundbreaking solution.
Exact timing
In a high-precision gearbox, exact timing is everything. The cylindrical gears must mesh 100 percent. Flank shape and helix angle do not allow any tolerances, and this applies to both internal and external geometries. The manufacturing of such complex profiles requires highly sophisticated calculations. In the profile accuracy or the pitch accuracy of the gears, the smallest error continues and leads to inaccuracies in the entire gear unit. Often, when the gearbox is prototyped, it is not even apparent where exactly the error is hiding. Therefore, it was time for the engineers at Haas Schleifmaschinen to come up with a solution.
“On the market, there has been no satisfactory calculation model for ever increasing precision requirements.” Hermle said, describing the situation two years ago when he initiated the project in the development of the Multigrind® software.
The goal was to create a calculation basis that is able to cope with the unbelievable variety of gears. And this with a precision that opened up new horizons for gear manufacturers. Derived from the perfect geometry of a cylindrical gear, the calculation is based on the geometry of the skiving tool. The motion sequences in production and the relative speed of the flanks define the future geometry of the respective generating skiving tool.
“We calculate with the exact path resulting from the movement of the gear,” Hermle said; however, he does not want to be more specific about this because, after all, this calculation model is a new development from Haas Schleifmaschinen that is causing a lot of excitement in the market with its unique position.
The desired grinding paths are created on the calculated surfaces in longitudinal, transverse, and trochoidal paths. The contact point between tool and workpiece can thereby move on the grinding wheel profile. This shortens the grinding paths and, thus, the grinding time. Standard grinding wheels are used for this application. This eliminates the need to purchase expensive form grinding wheels.
However, the new calculation model is only one side of the coin. The other is implementation. Because precise software requires equally precise coordination with precise hardware, the exact transfer of the Multigrind® Horizon software to the machine movement does not allow any deviations. To generate this precision, the alignment of the grinding wheel must be accurate to 1/10,000ths of a degree.
Measuring, Dressing, Grinding, Measuring, Dressing, Grinding
The desired grinding performance cannot be achieved without sophisticated measurement technology and frequent dressing of the grinding tools. The desired accuracy of the skiving tool and subsequently of the gear can only be achieved if all production steps are coordinated with each other in a collaborative process. The results of the permanent re-measurement with subsequent compensation of errors are the basis for the fine correction of the grinding path. This automatic process takes place entirely in the sense of a closed loop. In this way, the quality remains constant and error-free — workpiece for workpiece. The Multigrind® Horizon software handles this production sequence reliably and quickly, even in an unmanned operation.
“As far as I know, this level of precision can only be achieved within the Haas Schleifmaschinen Cosmos,” Hermle said. “Only when software and hardware are perfectly matched to each other can the perfect tool be created for manufacturing a perfect gear. At Haas, we specialize in developing from the desired end product. The question ‘What properties should the gear have?’ becomes the starting point of our manufacturing strategy for the perfect skiving tool.”
Haas Schleifmaschinen: Turnkey solutions since 1934
Haas Schleifmaschinen opened its doors in the southwest of Germany in 1934. In the beginning, the multi-axis high precision grinding machines served for automotive and watch-manufacturing industries. The third-generation, family-owned company offers in the Multigrind® series several models of 5-axis CNC grinding centers that can grind parts from 3 millimeters up to 3 meters; however, the highly flexible grinding machines of the Multigrind® series become high-tech tools thanks to the smart software solutions. The unique combination of perfectly matched hardware and software offers the customers of Haas Schleifmaschinen maximum added value. Last year, the daughter company Haas Multigrind® LLC moved to Charlotte, North Carolina. For more information, go to multigrind.com.
