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GEAR CUTTING Grinding special gearings: Shifting the boundaries of what’s possible

Dec 12, 2019

The steadily increasing requirement for transmissions and other drive components are not just simply posing tremendous challenges for the design engineers who develop them, but are also confronting manufacturing departments with a continuous stream of new tasks. The case study elaborates on how Zörkler procured a new multi-functioning machine, and how it has helped in achieving convenient, precise machining of complex special gearings. - Martin Schweizer, Head of Software Development, Cylindrical Gear Division, Klingelnberg GmbH

Zörkler’s product range offers several complex special applications for the aviation, railway, automotive and industrial sectors – a spectrum requiring a production line that delivers flexibility and high precision in equal measures. To meet these requirements, Zörkler acquired a new multi-functional machine for small and medium-sized workpieces. What makes this machine so special is its control software that can be adapted specifically for each machining task. It is suitable not just for standard tasks, but also for a variety of special tasks, making it extremely economical. Moritz Wurm, Head of Gear Grinding at Zörkler, mentions, “We rely on the Höfler brand’s many years of experience and development. Its list of options covers just about everything that can be manufactured using the profile grinding method — starting with countless variants of involute and non-involute cylindrical gearings to cylindrical worms, clutch gearings and splines to side faces and circular faces.” Support from the control software makes the 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 the setup are always an obstacle to achieve shorter machining times and higher quality. Also, when users create machining programs entirely on their own, they are unable to benefit from the advances made by the machine tool manufacturers. Besides, 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 played a key role in Zorkler’s purchasing decision of the new RAPID 800 K. For years in fact, all (further) developments of Klingelnberg’s GearPro software has centred on this user-friendly interface and an 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 have 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 the ground as well as sophisticated grinding and dressing strategies. For Hirth gearings, 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.

Special gearings with free profile option

An additional application field is opened 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 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 one of the most important applications of such special gearings. This is 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 with 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 within a few parameters. With the help of wizard functions programmed for this purpose, this makes it possible, to present the inherent complexity and associated design know-how only to the extent necessary to machine the workpiece.

When programming machining cycles, a range of variants are available for centring, 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.

The free programming variant

Free programming is the ultimate, in terms of integrating application-specific machining sequences into the userfriendliness of the GearPro interface. With these, customers can themselves program and manage any machining sequences and combine them with functions provided by the machine manufacturer.

Even reusable subfunctions 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 the management, operator guidance and provision of basic machine functions, as well as control of individual sequences such as dressing.

Latest applications

The most recent applications of the 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.

Asymmetric profiles a challenge

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 quite 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.

Because of two tooth flanks with differing slopes, the removal conditions can differ widely, making the standard machining process uneconomical or even impossible. This is reflected not least in the limited values of the performance-related chip parameters, such as the specific stock removal rate Q’W. If the unequal conditions are not considered, 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, too. In both singleflank 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 touchgrinds 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 that is ‘listening in’. 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.

Short and to the point

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.

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  • Martin Schweizer

    Head of Software Development

    Cylindrical Gear Division

    Klingelnberg GmbH

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