Automobile manufacturers and suppliers are facing new challenges when it comes to components for electric motors. The example of the housing of an electric motor shows how big these challenges are: Compared to a transmission housing, this has to be manufactured within significantly tighter tolerances, since the accuracy has a decisive influence on the efficiency of the motor.
In addition, due to the special structure, such as integrated cooling channels, the electric motor housing is usually significantly thinner than a transmission housing. In addition, bearing bushes made of steel materials are pressed into some of these housings. Special protection shields in the tool ensure that steel chips do not come into contact with the aluminium surfaces during processing and damage them.
Machining requirements and features of different housing types
Description: Highly integrated, complex housing with stator mount, transmission mount and connection for the power electronics. High functional integration saves assembly costs. Compact design. Complex cast housing as a result.
FEATURES
Stator incorporated directly in the housing or via a stator carrier / cooling jacket
Stator bore with stages and flat surfaces as functional surfaces
A bearing bore of the rotor is integrated coaxially with the stator bore in the housing
Positioning of the second bearing cover via dowel pins or fitting surfaces; second bearing bore must be coaxial
Bearing bores of transmission stages integrated in the housing; high concentricity and positional accuracy are required
Cooling channels partially integrated in the housing
Complex cast aluminium housing
MACHINING REQUIREMENTS
Elaborate contour trains with several diameter levels ( high cutting forces and large machining volume)
Mixed processing (-> chip separation / removal)
Interrupted cuts (-> contacting, cooling circuit)
15°-30° flat lead-in chamfers (-> flow chip formation and high radial forces)
Description: To reduce the complexity, in particular to implement a simpler construction of the cooling jacket, pot-shaped or bell-shaped housings or stator supports are used.
FEATURES
As an intermediate housing for integration in the overall system
Stator bore with stages and flat surfaces as functional surfaces
A bearing bore of the rotor is integrated coaxially with the stator bore in the housing
Positioning via fitting surfaces on the outer surface
Cooling channels as ribs on the outside
Thin-walled, susceptible to vibration
Tension problematic
MACHINING REQUIREMENTS
Extremely thin-walled parts (-> ap corresponds to wall strength)
Outer cooling ribs must be machined
Pot or bell shape (-> promotes vibrations, special clamping concepts and vibration dampers)
15°-30° flat lead-in chamfers (-> flow chip formation and high radial forces)
Description: The simplest design of motor housings is tubular. The length of the housing and thus the electrical machine can be varied comparatively easily for different powers. As a result, the installation effort increases due to the low level of functional integration.
FEATURES
No rotor bearing bores integrated in the housing
Two bearing covers to hold the rotor
Positioning of the two bearing covers over fitting surfaces for coaxiality of the bearing points
Minimal complexity
Practically rotationally symmetrical
Thin-walled, susceptible to vibration
Tension problematic
MACHINING REQUIREMENTS
More stable components usually with an internal cooling structure
Extruded profiles also possible (AlSi1 -> flow chips)
Without clamping straps (-> special clamping concepts)
Hybrid transmission housing and hybrid module/intermediate housing
Description: Integration of the electrical machine into the existing transmission architecture using disk-shaped hybrid modules or intermediate housing. Space neutral structures are also implemented with partially pot-shaped housings as slide-in parts.
FEATURES
Hybrid module/intermediate housing
Mainly incorporation of the stator
No rotor bearing with disc shape
Rotor bearing integrated with pot shape
Hybrid transmission housing
Extreme length-diameter ratios
Thin-walled, susceptible to vibration
Elaborate contour trains
Interrupted cut
MACHINING REQUIREMENTS
Hybrid transmission housing
IT6 tolerance
High demands on coaxiality and stage dimensions
Restricted maximum weight and moment of tilt
Basic procedure for machining of stator housings
The machining process as well as the tools are designed individually depending on the measurement situation, machine park and clamping setup. In this way, the cutting forces applied to the component are kept as low as possible. The machining of the stator bore is divided into 3 steps: Pre-machining, Semi-finish machining and Finish machining.
In most cases, the machine tool is the critical factor in the design of the tool for pre-machining the stator bore. In the machining process that MAPAL recommends, the first choice is a boring tool with a cartridge and PCD-tipped indexable inserts. The advantage of this tool is that it achieves a high material removal rate very quickly and thus cost-effectively because it allows work at high cutting speeds and feed rates. However, using this tool requires a machine with high maximum torque and power to match. If that is not available, the alternative is to pre-mill the stator bore. MAPAL offers an ISO helix milling cutter with PCD-tipped indexable inserts for this very purpose. Although this tool can also be used to work with very high cutting speeds and feed rates, the machining time is much longer than it is with boring on account of the longer machining stroke.
In designing the tool for semi-finishing, MAPAL also focused on the torque and power of the machine. This stage of machining involves pre-machining the complex contour definition of the electric motor housing in such a way that the complete contour including chamfers and radial transitions can subsequently be created to the required quality during fine machining. For this stage of machining, MAPAL recommends a precision boring tool in welded design with PCD-tipped ISO indexable inserts.
The final stage involves machining the stator bore to micron precision with a fine boring tool, also a welded design. The PCD-tipped indexable inserts are finely adjustable, which helps to maximise accuracy. The tool is fitted with guide pads to provide the best possible support in the bore.
