Profile grinding in gear manufacturing
Generating grinding with tools that can be dressed is an established technology in gear production. ZF Steyr Präzisionstechnik GmbH uses this process for a wide variety of applications, from prototyping to series production. Flexible machines manufactured by KAPP NILES and available in the UK from the Engineering Technology Group (ETG) can master all applications with the shortest set-up time.
ZF is a world-leading technology group in driveline and chassis technology with around 230 sites in nearly 40 countries. One of these is ZF Steyr Präzisionstechnik GmbH in Austria. Approximately 500 employees produce components and gears for agricultural machines such as tractors, combine harvesters, forklift trucks and special machines as well as steering gear components for the automotive industry. The company rarely produces a large series for this portfolio, but rather focuses on small to medium series with high part variance. Typical batch sizes are 200-500 pieces, but for some products, it can also be as large as 5,000 to 6,000. Capacity utilisation is high with 17 shifts usually scheduled per week – three from Monday to Friday and two on Saturdays.
Precision technology for large series production and prototyping
The major issue for ZF Steyr is the extraordinarily high precision. The noise emission requirements require immense accuracy for some components. The steering gears, which are located near the passenger compartment, are a typical example.
ZF Steyr relies entirely on the products of KAPP NILES from the Engineering Technology Group (ETG) for grinding purposes. Fifteen different machines are in production; some are brand new while others are from the 1980’s.
Otmar Schlachter, authorised signatory and Head of the Production Process and Tools Management at ZF Steyr, explains the long-standing cooperation: “With KAPP NILES, the combination of tool and machine works perfectly; the machines are always running. You do not always find this interaction between machine and tool with manufacturers from whom you only buy one part.” Lukas Aigner, Deputy Head of the Profit Centre Räder, adds: “The user interfaces are mostly the same, so an employee who is trained on one machine can also operate the others. In addition, many components are interchangeable. This facilitates the maintenance work.” In general, KAPP NILES tries only to use as many standardised parts in a machine as necessary to simplify the spare parts maintenance of various machines for the customer. The service also includes remote diagnosis which allows a technician from KAPP NILES to get an impression of the machine in question before travelling to the site. At times, this might even completely save a journey to the customer. Even service works such as installing new software updates can be done online.
‘DYNAMIC’ machine concept
The latest acquisition at ZF Steyr is the KX 260 DYNAMIC generating grinding machine (Fig. 1). A smaller version, the KX 100 DYNAMIC, is already in use. Both are further developments of the multi-spindle design already realised with the KX 160 TWIN. As a pick-up machine with integrated automation, it is characterised by a very short set-up and process times. The optional automated clamping device makes these machines equally suitable for series production with large and small batches.
The KX 100 DYNAMIC has two separate, rotatably mounted columns whereas the KX 260 DYNAMIC has only one. These are fitted with a vertically moveable pick-up axis, each equipped with a workpiece spindle. While a workpiece is being ground, the other pick-up axis removes the finished workpiece and loads a blank part on the workpiece spindle (Fig. 2). The workpiece is aligned outside the working space so that the workpiece spindle can be swivelled to the processing speed and synchronised into the workspace. This reduces the non-productive times. For the KX 260, this can be reduced to 3.8 seconds. The multi-functional axis is used for discharging measurement and test parts (Fig. 3).
Only continuous grinding with adjustable grinding tools is used as the processing method. Depending on the application, both dressing tools with an integrated head dresser and flexible tools with an independent head dresser can be used on the dressing unit. The ‘Topological generating grinding’ option makes it possible to produce gear grinding with or without targeted entanglement. The advantage of the machine concept is the full integration of automation functions since the parts can be loaded and unloaded from a belt without further handling devices. Optionally, a measuring unit is available for measuring and evaluating all relevant gearing features.
Automation even during the setup process
The set-up operations on the DYNAMIC machines are also partially automated, such as the automatic screw change. For this purpose, only the interchangeable prism is manually swivelled out of the park position (Fig. 4). The machine then places the tool securely at the push of a button. The operator swivels the tool into an ergonomic position and changes the screw manually or with a hoist. Lukas Aigner: “The change can also be easily managed by new employees, as the machine shows the setup sequence on the display and provides point-by-point instructions on what to do. In addition, the employee must acknowledge every step, so no mistakes occur.”
Machinery for all tasks
Otmar Schlachter says; “Quick machining times are important, especially for small components in large quantities. The pick-up procedure is a valuable tool for this purpose. For us, however, the fact that the diamond dressing rolls are interchangeable on all machines is also very important. When a machine is occupied, we can move to another machine for an order. It is only thanks to this flexibility that we can manage our portfolio.” (Fig. 5)
Prototyping: Batch size 1 integrated into day-to-day business
Small and medium batch sizes are the core business at ZF Steyr. From time to time, however, prototypes are also required. In case of large series production of passenger cars, it is very difficult to organise such special orders. Nevertheless, the corporate group relies on these components. Otmar Schlachter explains: “Gear manufacturers want new features every 3-4 years. In the case of upper-class vehicles, the demands on performance and gearing keep increasing while the installation space remains the same. One has to produce prototypes quickly during operation.”
“For this purpose, the production with adjustable discs is unbeatably fast. You take a grinding wheel, pull the profile on it and you can grind within one day. You would have to wait 8-10 weeks for a new tool. It is then ground out of the full material. In the case of a prototype, it is irrelevant if the machine runs for two hours.”
Ulrich Roos, Regional Sales Manager for Automotive Sales at KAPP NILES, describes how the machines support this process: “Some machines must be programmed block by block. Our controls can independently generate programs. First, you simply enter the gearing and technology data. The latter can even be left to the machine. It then makes suggestions for the number of cuts and strokes, speed, infeed depth, etc. Therefore, you do not need to know all the complex inter-relationships. If needed, you can of course make corrections based on your own experience. Once the input is complete, the machine generates the program at the push of a button.”
Integration into the overall process
The described, varied tasks require individual solutions. Ulrich Roos says: “Our advantage is the broad product range: We are proficient in profile and generating grinding, combined with single-spindle, double-spindle and pick-up machines. Moreover, our machines are universal. We can also perform internal grinding on the large KX 500 FLEX. Hardly any other manufacturer has this wide range of processes and machine configurations in its portfolio.”