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hp tooling 2021 #3

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The journal of hp tooling is the new global publication on all aspects of high precision tools, accessories and their applications.

machining center CNC

machining center CNC machining for electromobility written by Dr. Manfred Berger The challenge of production planning for the vehicle market has never been more difficult due to the unclear parameters from the market and politics. The majority of car manufacturers will not be able to commit to one technology and the end of the combustion engine is still a long way off. The production volume of vehicles with combustion engines (singular or integrated in hybrid drive systems) is still such that in the coming years a complete phase-out of the technology seems impossible for most manufacturers. On the other hand, electric drive volumes are increasing quite slowly and in this scenario the risks and manufacturing are usually left with the suppliers. There are also strategic considerations as to whether the electric drive will remain a core competence of the vehicle manufacturer in the future. Is it not already possible to find an answer to this question in TESLA’s development lead? The considerable development expenditure of vehicle manufacturers for attractive product offerings in each vehicle segment influences the objectives in production planning. Reuse or reconfiguration are often the current tasks. More honing instead of grinding According to an analysis by the German Engineering Federation (VDMA), the production of the powertrain in an electric vehicle contributes about two-thirds less to the value added than in a vehicle with an internal combustion engine. Due to the trend of integrating the electric motor and the inverter as well as the transmission into a common housing, designs are available that require > 1,000 s of total machining compared to a cylinder crankcase (approx. 900 s of total machining; see figure 1). The classic electric drive as shown in figure 2 consists of the stator housing with cast-on front bearing plate, a separate gear housing and an equally separate inverter housing. The end shield is fitted and screwed onto the gear housing via a fitting diameter on the outside of the end shield. With this design, the bearing seat and the outside diameter must be machined in a single clamping operation and from one side in order to maintain the required tolerances. Ideally the bearing bore, outside diameter and stator plate seat are machined in sequence in this setup. The aim is to achieve the smallest possible air gap between the stator and rotor. The gap width between the components determines the magnetic resistance, the level of induced current (asynchronous machines) and thus the efficiency of the motor. For many reasons, including the tolerance chain of the components, the air gap is only 0.5 to 0.6 mm. Another significant difference to the classic combustion engines is the motor speed of > 10,000 1/min. Both characteristics: plugged drive shaft and high speed are responsible for the noise behavior of the drive train. In addition to the high requirement of the position tolerances of the gearbox bearing points, the surface qualities of the gearwheel teeth (honing instead of grinding) are also important for optimum smoothness of the drivetrain. figure 1: agile manufacturing system for cylinder crankcases (turnkey) 34 no. 3, September 2021

machining center power electronics electric motor gearbox housing figure 2: e-axis from BOSCH (photo: Robert Bosch GmbH) figure 3: MAG as a system partner for the powertrain of electromobility with more than 25 years of manufacturing experience of integrated high-performance electric motors Many years of experience with electric motors Within the FFG Group, MAG, as the system supplier of small manufacturing cells up to complete turn-key systems for high production volumes, can take on the role of general contractor at the customer’s request with its knowhow in planning and processing. The FFG Group’s range of technologies covers almost the entire spectrum of components to be machined for electromobility. Whether new or reconstructed machines, the Eislingenbased machine builder has many years of experience. For example, MAG has been manufacturing its own motor spindles (figure 3) and rotary tables for over 25 years, in which electric motors – analogous to the drive of the electric vehicle – are installed. With this experience in manufacturing, assembly and test engineering, as well as the partner company’s broad know-how in insulation, wind - ing technology and impregnation for stator and rotor, plan - ning for a turnkey system and its execution are implemented with professional project management. Although a vehicle drive is dynamically less stressed during operation than the direct drive of machine tools and the running times (duty cycle) are also shorter, the knowledge gained from assembly must be incorporated into the production process in order to control the sensitive side of the electric drive (running noise in the gearbox; housing expansion with frictional installation of the stator). For machining the sensitive thin-walled components (figure 4), direct clamping in the fixture (alternative adapter plate) and multiple clamping (rough and finish machining) are recommended. During roughing, machining should always be performed in the direction of the workpiece support. The extensive chip removal on the inner diameter of the stator housing releases stresses which affect the geometry and therefore reclamping is necessary before finish machining. A fine machining of the inner diameter (butterfly tool) without slide bars to avoid burn marks would also be optimal. figure 4: machining of a stator housing on the SPECHT 600 A/B machining center in three process steps with emulsion and HSK 100 tool interface no. 3, September 2021 35

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