Machining problems and solutions for major parts of electric vehicles

2021/08/15


We analyze the machining problems brought by the new parts of electric vehicles compared to conventional vehicles, and discuss the corresponding machining solutions using MAPAL's advanced tools as an example.


1 Preface
In order to save energy consumption, protect the environment and reduce CO2 emissions, the automotive industry has to change the power drive source and the corresponding mechanisms and reduce its own mass, thus requiring technological innovation and modification in many aspects. Currently, the power drive source has started to change from fuel-fired internal combustion engine to hybrid drive, pure electric drive and hydrogen fuel cell drive. Among them, pure electric vehicles are the most energy-saving, and have simple structure, fast acceleration, low price and low noise. But there are also many shortcomings, such as short range, charging is still inconvenient and long, as well as susceptible to external conditions; oil-electric hybrid energy vehicles still need internal combustion engines, so there are transmission, rotation system, fuel tank and oil circuit, etc., in due time after charging, can be driven electrically to drive driving, driving range than pure electric vehicles. So far, fuel-fired internal combustion engines and hybrid and electric types of cars exist simultaneously, but industry insiders estimate that sooner or later cars powered entirely by electricity will make up the majority of vehicles on the road.

China is now the world's number one producer and seller of new energy vehicles, and by 2020 China will have produced and sold a cumulative total of 5 million pure electric and new plug-in hybrid vehicles. Many of the components used in new energy vehicles are different from the original major components of internal combustion engines, and the processing processes and tools used are also different. MAPAL Group in Germany, for example, as a major supplier of cutting tools to the automotive industry, they have long considered this issue, and gradually transfer the advanced technology experience accumulated in the past decades of processing conventional powertrain to the processing of new energy vehicle parts. A far-reaching technological change is coming, which is causing vibrations and changes in the machine processing industry, and we are prepared and see this change as an opportunity." In addition, for the new energy vehicles themselves, many companies in many countries are constantly developing new system components, improving the introduction of new parts and adopting new materials in order to improve and perfect themselves. In order to cope with these new components and the efficient processing of new materials, manufacturing companies should also make timely preparations accordingly.

2 Electric vehicle typical parts processing problems and solutions
With electricity as all or part of the power source, it must have electric motor system (as the heart), battery system (as the fuel tank) and power electronics (as the nerve system of control) and other related parts; in addition, there are many auxiliary electric devices, such as electric refrigeration compressor, electric auxiliary heater, electric starting and auxiliary steering equipment, etc. The development of various electric electric control equipment will inevitably bring many new parts. This paper introduces several important issues summarized by MAPAL and its new machining solutions according to its main typical parts.

2.1 Problem 1

How to achieve economical and reliable precision machining of electric system shell parts, especially larger diameter shells? Electric system shell is mainly bored machining, according to the general method of processing, to bear a large tool tool bar weight and rotational inertia, cutting torque can reach 50N-m, often should be mounted on the tool shank interface for HSK-A100 spindle, thus requiring the use of large machine equipment, installation and use is very inconvenient, high cost and poor economy. For this reason, MAPAL has adopted a fine boring tool with ultra-light structure and a shank interface of HSK-A63, which can be used on a machine tool with a smaller spindle. At the same time, the cooling channel has been improved to allow reverse flushing, making it easy to discharge chips and avoid scratching the surface of the finished hole, making the machining quality significantly improved and economical and efficient.

The aluminum motor housing in pure electric vehicles has a large center bore, usually >250 mm. To finish such a large diameter deep hole, MAPAL has scientifically designed a lightweight welded structure boring tool that is ideal for machining various types of thin-walled unstable housings. The high accuracy of the machined parts, even with a long tool overhang, is due to the fact that the welded structure is determined computationally using finite element analysis (see Figure 1), a method based on structural mechanics for effective numerical analysis of the various parameters of the structure. A variety of software is available to simulate the cutting forces to be applied, to determine a reasonable cutting edge distribution, to calculate the rotational inertia and mass, to evaluate the weld seam, to calculate the deformation under the axial forces and torques applied, to calculate the inherent frequency of the system, and to calculate the distribution and flow rate of the cutting fluid on the cutting edge and guide bar, etc. A precise knowledge of these parameters allows the design of the most material-efficient and lightest reliable structure. This welded tool holder structure is only 1/2 the mass of a typical boring tool, and it has an additional guide bar support and a well-designed ribbed plate to enhance the bending resistance, making it very stable during machining with minimal chatter. They also use the finite element method in the design of the front face of the insert to produce a special and reasonable structure for chip cutting and chip removal to ensure a suitable chip shape is formed during cutting, so that it can be cut off and discharged in time and the cutting force is reduced to a very low level. In addition, a good clamping system and a suitable cutting amount are used during machining, so that the machining accuracy of mm level can be achieved.
Figure 1 Example of finite element analysis

The structure of various quality-reducing boring tools used in different machining stages is shown in Figure 2. The roughing stage uses an ISO boring tool with a tool holder and PCD ISO indexable insert; the semi-finishing stage uses a precision boring tool with a welded structure design and PCD ISO indexable insert; the finishing stage uses a precision guide tool with a welded structure design and a fine-tuning PCD indexable insert and support guide. All stages of boring tools use ISO standard economic PCD indexable replacement inserts suitable for aluminum alloy machining, with long life and high machining quality.
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Figure 2 Boring tools used in different stages of machining

2.2 Problem 2

Since there are many thin-walled parts for electric vehicles, they are easily deformed by forces during machining, so different tools and processes need to be used according to different requirements. MAPAL's unique and advanced SPM type end mills for milling thin-walled housing parts are shown in Figure 3, which have a large cutting edge with a large front angle in order to reduce cutting forces, a reasonable arrangement of space for chip discharge slots, which are well polished, and a synergy of PCD insert material and oil mist lubrication technology to reduce cutting forces by about 15%. If cutting forces are still too high, they can be combined with cycloidal milling methods to minimize cutting forces in several ways, thus reducing workpiece deformation. After roughing, semi-finishing and finishing, the machining accuracy of mm level can be achieved.