Feature Tools & tooling
Stress free machining of gear base bodies
InoZet pendulum bridges, a clamping system by German company HWR is helping gear specialist Stelter Zahnradfabrik GmbH to machine gear base bodies from solid ball bearing quality steel accurately
T
he challenge faced by Stelter Zahnradfabrik GmbH of Bassum, Germany, when machining gear base bodies was compounded by small production runs of these cus- tomer-specific components.
It was difficult to achieve the required mean roundness error of less than 10 micron without additional grinding and honing operations – and this was not necessarily an economi- cally viable proposition.
The workpiece needed to be held securely to cope with the high rate of material removal during rough machining. However, the less material that remained, the greater the risk of deformation of the machined part due to the high clamping forces involved. Despite using every trick in the book, it was proving impossible to achieve a mean roundness error better than 26 micron when using conventional three-jaw chucks. According to Karl- Heinz Wilkens, Stelter’s technologist responsible for some 200 CNC machines and associated automation equipment, what was needed was a high quality accurate and repeatable manufacturing process, especially given the nature of the products – safety-relevant components destined for car gearboxes and wind turbines. The answer came with the introduc- tion of InoZet pendulum bridges by another German company, HWR based in Oyten. This clamping system, now marketed in the UK by Thame Workholding, consists of three pendu- lum segments that can be mounted to any conventional three-jaw chuck, whether with tongue and groove or fine serration interfaces. A three-jaw chuck is thus easily converted into a compensating six-jaw chuck with the clamping force at each point of contact reduced accordingly.
The clever feature is that the mounted bridges float, which reduces the deformation forces to a minimum while still providing safe and secure clamping. The impact this has on the roundness of the machined compo- nent is best described as significant.
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With the previous clamping method and pressure, using a conventional three-jaw chuck resulted in a round- ness error of 41 micron on the clamp- ing diameter and 26 micron on the internal diameter. However, when a workpiece is clamped with the InoZet pendulum bridges, the error is reduced to five micron on the internal diameter and almost down to 10 micron on the clamping side. And this is achieved without any adjustment of the machining parameters. Karl-Heinz Wilkens is understand- ably happy with the outcome. “We have made a significant step forward and are now carrying out automated production runs of previously difficult workpieces,” he says. Looking at the converted chuck explains why this is now possible: The pendulum seg- ments are positioned on the existing chuck such as to ensure the effective force of all six jaws is always pointing towards the centre. To achieve this, HWR calculates the exact mounting points for each chuck. The segments each have a pair of top jaws which are mounted via two screws each. When the chuck is closed – hydraulically in this case – one of the jaws of each pair pushes itself against the workpiece. On contact the second jaw oscillates into position and all six jaws are then clamping simultaneously and – most importantly – uniformly.
The concept of pendulum jaws is not new, although the InoZet design provides a high level of flexibility and hence cost-effectiveness. Each jaw pair can be moved radially and this means that the entire clamping range of the chuck is covered with one set of grip- per jaws mounted onto the pendulum bridges. In terms of cost the total investment in converting an existing three-jaw chuck is said to be around 20 per cent of the cost of a new six-jaw chuck. In addition to the large diame- ter range, the InoZet chuck can be used for internal and external machining, and both hard and soft jaws as well as special jaws can be used.
The use of soft jaws should not be
Clamping an OD machined component using the InoZet system fitted with soft jaws and (top left) InoZet pendulum jaws convert a standard three-jaw chuck into a compensat- ing six-jaw chuck
underestimated, according to HWR’s Matthias Meier. “If a workpiece is machined to size on the OD, we are able to lock the InoZet chuck in place, bore out the soft jaws and clamp the component accurately via six points – in both a floating or fixed manner.” This approach has been adopted by Stelter Zahnradfabrik to machine large planet gears destined for wind power stations. “Although the quality of these 650mm diameter by 380mm high workpieces was pretty good already, we have since been able to halve the accuracy values thanks to the InoZet chuck,” says Karl-Heinz Wilkens. As a consequence the achieved quality level is now in the middle of the permissible tolerance zone, and just half of the measuring steps previ- ously required to ensure conformity are needed.
Thame Workholding T: 01844 208050
www.thameworkholding.com Enter 311
NOVEMBER/DECEMBER 2011 Factory Equipment
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