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In most production plants, grinding of commonplace Hirth gearings is done on a CNC machine suitable for this purpose, but generally without specific software support

Image: Klingelnberg

Metal Cutting Grinding special gearings

Feb 22, 2018

An application story on the usage of Klingelnberg’s GearPro software by Zörkler that enhances flexibility and high precision in the production line

Zörkler is a high-end gear manufacturer, based in Jois, Austria, whose product range includes a number of complex special applications from the aviation, railway, automotive, and industrial sectors – a spectrum requiring a production line that delivers flexibility and high precision in equal measure.

To meet these demands, Zörkler acquired a new multifunction machine for small and medium-sized workpieces. The specialty of these machines is that the control software is adapted specifically for each machining task and is suitable not just for standard tasks but also for a variety of special tasks, making it economical. Speaking on this, Moritz Wurm, Head of Gear Grinding at Zörkler, shares, “We rely on the Höfler brand’s experience and development. Its list of options covers everything that can be manufactured using the profile grinding method – starting with countless variants of involute and noninvolute cylindrical gearings to cylindrical worms, clutch gearings & splines to side faces & circular faces.”

Support from control software makes a difference

In most production plants, grinding of commonplace Hirth gearings is done on a CNC machine suitable for this purpose, but generally without specific software support. The time required for manual DIN programming and the finesse needed for setup are always an obstacle to achieving shorter machining times and higher quality. Further, when users create machining programs entirely on their own, they are unable to benefit from advances made by the machine tool manufacturer. Dependency on specialist knowledge, typically of a single employee, is a significant risk in terms of staffing.

What a modern machine tool has to offer for convenient, precise machining of complex special gearings, in contrast, is demonstrated by a uniform, intuitive operating concept harmoniously combined with application-specific adaptations for data input, operator guidance, and machining sequences. This feature plays a key role in Zörkler’s purchasing decision. “For us, a simple user interface, particularly for the module used to grind Hirth gearings, was an important reason behind our decision in favour of the new RAPID 800 K,” explains Wurm.

For years, further development of Klingelnberg’s GearPro software has centered on this user-friendly interface and the intuitive operating concept. GearPro is also used in the RAPID 800 K cylindrical gear grinding machine, which combines the familiar user interface for standard gearings with a range of functionalities that has grown over the years—from visualised, three-dimensional gearing and machine models to calculating and generating appropriate tool profiles to determining the precise position of surfaces to be ground as well as sophisticated grinding and dressing strategies.

For Hirth gearings, in particular, due to their sometimes large number of teeth, optimised indexing methods such as the sector indexing method established in gear cutting are indispensable to achieve high-quality indexing.

The free profile option

An additional application field is opened up by the ‘free profile’ option, which enables the transverse profile of cylindrical gearings to be defined by any number of straight line and circular arc sections. Here, the usually data-intensive representation of the profile can be generated externally and then imported, or it can be calculated from just a few input values if a parameterised description is provided.

Cycloid speed reducers

The cycloid speed reducers frequently used in robotics are currently one of the most important applications of such special gearings. This a special type of cycloidal drive, in which one or more cam disks with a cycloidal outer contour roll over cylindrical pins. The cams are supported in appropriately shaped concave slots of a surrounding ring housing.

On the housing side, the profiles to be ground consist of simple circular arc sections and can therefore be described in the definition table with just a few parameters specifying their position and radius. The cycloidal profiles of the cam disks, by contrast, follow a more complex mathematical relationship. However, they can also be described with just a few parameters. This makes it possible, with the help of wizard functions programmed for this purpose, to present the inherent complexity and associated design know-how only to the extent necessary to machine the workpiece. When programming the machining cycles, a range of variants and options are therefore available for centering, grinding, measuring, and correcting. Among these is a data interface for the Klingelnberg P-series measuring machines, which spares users the need to generate appropriate measured data – a process that is time-consuming and prone to errors.

Wildhaber-Novikov gearings

In response to a special customer request for grinding Wildhaber-Novikov gearings, Klingelnberg went a step further. Here, parameters were specified not for the transverse profile itself, but rather for the circular arc profile of the basic rack tooth profile to be generated. A customer-specific solution was developed in this case – a solution that uses a removal simulation to reproduce the special geometry of the transverse section and, with this as a special case of the free profile option, it makes accessible the proven features of the standard grinding processes.

Tooth trace modifications were also possible here. The necessary profiles were checked for grindability and dressability and the real allowances, which in the case of special gearings are often only imprecisely known in advance, were determined in several tooth spaces where necessary.

The most flexible variant: Free programming

The ultimate, in terms of integrating application-specific machining sequences into the user-friendliness of the GearPro interface, however, are the freely programmable processes. With these, customers themselves can program and manage any machining sequences and combine them with functions provided by the machine manufacturer. Even re-usable sub-functions and diameter-dependent tool paths can be simulated using this flexible concept.

In this way, the design know-how remains entirely in the customer’s hands whereas the control software continues to handle, where necessary, the management, operator guidance and the provision of basic machine functions as well as control of individual sequences such as dressing.

Latest applications

The most recent applications of this most flexible of all GearPro operating variants include grinding of clutch gearings with typical, extremely pronounced crown teeth as well as grinding of freely defined profiles on circular faces.

Challenge: Asymmetric profiles

Particular challenges always arise in addition when the profiles to be generated are asymmetric. For involute gearings, too, this property is increasingly used wherever significant outputs are transmitted via a transmission operated preferably or exclusively in one direction only. The advantage of the often significantly different pressure angles of the left and right tooth flank in these gearings is that a greater load bearing capacity is achieved on the load-bearing flank and the tooth root strength is improved considerably due to the larger possible radii of the root rounding.

As a consequence of two tooth flanks with differing slopes, however, the removal conditions can differ widely, making the standard machining process uneconomical or even impossible. This is reflected not least in the limit values of the performance related chip parameters, such as the specific stock removal rate Q’W. If the unequal conditions are not taken into account, this will inevitably result either in a great deal of wasted time or an excessive thermal load on a tooth flank. The difference becomes clear by comparing the geometric conditions when grinding an even surface and a sloped surface. Although the same radial infeed produces the same cross section (AQ) and thus the same machined volume (VW) per unit of time Δt, the swept areas (AK) differ significantly.

But GearPro’s intelligent removal control automatically ensures an optimised grinding strategy here. In both the single-flank machining and the more economical two-flank machining, the effective chip thicknesses are optimised while at the same time, the specified tooth trace modifications are precisely generated.

Grinding cylindrical worms

When grinding cylindrical worms, in contrast, fundamentally different geometric conditions arise due to the extreme helix angle alone—not infrequently, the transverse profile of one thread encompasses more than 180°, and the axes of the tool and workpiece are nearly parallel to one another. GearPro consequently touch grinds the two tooth flanks in the axial direction, contrary to the usual practice, and in this way, generates significantly clearer signal edges in the solid-borne noise sensor. The respective grinding wheel profile, which can only be dressed in some instances with special dressing rolls, gives rise to additional special requirements for the five supported flank profiles: ZA, ZN, ZI, ZK, and ZC. Due to the axially defined measurands, both the testing and correction of worm-type gearings also differ.

Thus, grinding and measuring special gearings with GearPro combines a high degree of variability with proven user friendliness. Users can expect customised, high-end solutions for future tasks as well.

The article is authored by Martin Schweizer, Head of Software—Machine Tools, Klingelnberg GmbH

Image Gallery

  • Spline shaft, splined hub, free profiles, asymmetric involutes, and cylindrical worm (top left to bottom right)

    Image: Klingelnberg

  • Hirth gearing modeling with Klingelnberg software

    Image: Klingelnberg

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