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Machining concepts for today’s future mobility

Source:International Metalworking News for Asia- October 2018

Date Published:10/4/2018 01:10:13 PM

In view of the expected dynamic development of e-mobility, the automotive-related machine tool industry is confronted by challenges in terms of machine concepts and production processes, which need to be solved using innovative approaches.  

Efficiently geared by using the power skiving technology

Because in order to achieve the required drive torque, gearboxes in electric vehicles require a very high gear transmission ratio.

The accompanying engine speed rates of up to 17,000 rpm have a negative impact on noise sensitivity and wear behaviour. For example, noises which occur near the powertrain are perceived much more clearly due to the lack of background noise associated with a combustion engine. To solve these problems, enormous requirements are made with regard to the balance quality of moving components in the powertrain of electric vehicles, which results in tighter shape and position tolerances and thus the necessity of machining the corresponding components even more precisely.

Soft machining

Taking these quality premises into particular account, Werkzeugmaschinenbau Ziegenhain (WMZ) has realised a pioneering integrated production solution for the complete machining of hollow shafts for electric motors – from the raw to the finished part – for a renowned German automotive supplier, working in cooperation with its DVS sister companies BUDERUS Schleiftechnik, PRÄWEMA Antriebstechnik and DVS Production.

For this purpose, WMZ has developed an efficient complete machining center for the holistic soft machining of hollow shafts, which is then followed by hard-fine machining processes using a machine from the DVS UGrind series designed by BUDERUS Schleiftechnik and a gearing honing machine from PRÄWEMA. The series production of the first shaft type is currently in the ramp-up phase at DVS Production GmbH in Krauthausen in Thuringia/Germany. The design space available is extremely limited in the electric drives segment, so that motor and gearbox almost merge into one unit by means of function integration. For this reason, the drive gearing is integrated into the hollow shaft of the motor. The electric motor rotor is located between the bearing seats (Figure 1: reference A and B).

In addition, corresponding motor shafts are characterised by a through hole which both serves to reduce the mass inertia and makes the power take-off possible on the opposite drive side. A hardening processing is also necessary due to the running gears. The critical manufacturing tolerances for hollow shaft machining resulting from this, shown in Figure 1, are the tight run-out tolerances of the shaft bore and gearing to the bearing seats and the low tool run-out during gear cutting caused by the design space. With a view to the required manufacturing quality and cycle time of the subsequent hard-fine machining, the soft machining process must produce the complete inner contour, several external surfaces and grooves, the gear cutting as well as the bearing and rotor seats already to the finished dimension or with only a very small allowance. Otherwise the final shape and position tolerances will not be achieved.

Power skiving method

In order to fulfil all requirements and guarantee maximum process and cost efficiency, WMZ has created a multi-technology manufacturing solution that combines turning, milling and drilling operations as well as gear cutting using the highly productive power skiving method. In order to minimise reclamping errors which have a negative impact on manufacturing quality, the option of distributing the machining steps to several individual machines was excluded from the very beginning. As a side effect, retooling, transport and waiting times – and thus the overall processing times – can also be reduced. Nevertheless, the complex variety of technologies used makes the fulfilment of various attributes of a machining center absolutely crucial: while turning and milling work needs maximum flexibility and dynamism, both precision drilling and power skiving require maximum rigidity and geometric accuracy.

Moreover, the drilling of bore holes to the required depth and tolerance makes enormous pressures and volumes necessary with regard to the cooling lubricant system in order to facilitate the fast and safe clearance of chips. For this reason, the successful WMZ H200 was chosen as an adequate machine base, since its modular design permits the flexible implementation of a wide range of different machining operations. Numerous series applications based on this machine concept, which stands out on account of its flatbed construction, confirm its remarkable dynamic and thermal rigidity.

Whereas the turning and milling requirements were able to be realised using tried-and-trusted components, the WMZ design engineers – in close cooperation with the DVS power skiving specialists PITTLER T&S and PRÄWEMA – came up with an innovative combined power skiving module for the high-precision manufacturing of the running gears and the shaft bore. With the development of this module, that can be adapted flexibly to different gearing geometries, WMZ is expanding its range in the field of the complete machining shaft-shaped components and can fall back on this development for applications down the line.

Precision chuck prevents reclamping faults

The complete soft machining of the hollow shafts is carried out completely automatically by an H200 machine and only two clamping processes. A tried-and-trusted WMZ loading module guarantees automated loading and unloading as well as the onward transport of the hollow shafts from OP10 to OP20. Thanks to the optimum interaction between the machine's internal automation and the specially developed precision chuck made by the DVS partner SWS Spannwerkzeuge GmbH, reclamping faults are negligible and thus have no negative impact on the manufacturing quality. The subsequent hard-fine machining can then be carried out using optimally pre-machined workpieces.

After the manufacturing process has been completed, the workpieces are marked with a dot matrix code by an engraving laser station integrated in the finished part conveyor, enabling clear part identification and traceability. Within an extremely short cycle time, a simple cut off piece is turned cost- and process-efficiently into a hollow shaft with more than 150 complex characteristics and meets the most stringent of precision requirements – exactly as required for the components of the increasingly important electric drive.

Deep hole drilling

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