04.12.2020
Elektrifizierte Mobilität
Kleine Gehäuse für Elektromotoren aus Magnesium und Aluminium prozesssicher bearbeiten
Heute erklimmen Hobbyradfahrer Anstiege scheinbar mühelos, die eigentlich selbst Radprofis zum Schwitzen bringen. Die dafür verantwortlichen E-Bikes, Pedelecs, also Fahrräder mit elektrischem Hilfsmotor, sind aktuell allgegenwärtig und finden mehr und mehr Verbreitung. Mit der Beliebtheit der E-Bikes steigen auch die Produktionszahlen ihrer Bauteile.
The production of e-bikes
In Germany alone, manufacturers produced one million electrically powered bicycles in 2019. The motor housings, among other things, pose a challenge during the production phase. After all, they have to be small and light and at the same time highly accurate. The small size of the entire drive is a result of the limited space available on an e-bike. Most of the motors are installed directly in or onto the frame itself as unobtrusively as possible. The entire drives must be particularly light-weight to ensure a long battery life. The less loads have to be moved, the less the motor has to “work” and the longer the battery can go without needing to be charged. After all, the housings must be manufactured with high precision to ensure that the motor runs both quietly and smoothly. As well as this, only a precisely manufactured motor runs smoothly and achieves the highest possible level of efficiency.
As a result of the requirements mentioned above, most manufacturers of small electric motors produce their motor housings from die-cast aluminium, more often from die-cast magnesium. Both workpiece materials are low in weight. Magnesium has a density of 1.7 g/cm3 and so is slightly lighter than aluminium with a density of 2.7 g/cm3. On top of this, magnesium is even easier to cast than aluminium. This allows for designs with even thinner walls and more intricate structures. Whether they’re made from aluminium or magnesium – most motor housings consist of the actual housing plus one or two covers. They have very thin walls and are unstable, so are therefore susceptible to vibrations. Multi-stage contours within the housing provide space for the various functional components of the motors. The geometric and dimensional requirements are high – narrow shape, running and position tolerances are specified.
The motor housing challenge
“For machining the housings, the properties of the material and the thin walls of the part pose the greatest challenges”, says Leander Bolz, Sales Manager at the MAPAL Centre of Competence for PCD tools. Furthermore, the housings are often already coated when they are machined. These coatings must not be damaged during machining. “Our customers in this sector produce very high volumes, so it is equally important that the tools for machining are highly economical to use”, adds Bolz.
Over the past decades, MAPAL has gained extensive experience in the machining of small motor housings in both aluminium and magnesium. “Small housings have always been used for chainsaws, mopeds or lawnmowers, for example, but with electrification the precision requirements have increased even more”, explains Leander Bolz. And so MAPAL has adapted its programme for the complete machining of small housings to the changed conditions. First and foremost, PCD and solid carbide tools are best suited for machining both workpiece materials. In some cases, the tool experts design the process as dry machining. Polished chip spaces and particularly smooth surfaces on the tools stop them from getting dirty. They make the machining process safe even without the need for a cooling lubricant.
“When it comes to designing the tools for machining a magnesium housing, we’re always at the upper tolerance limit in the first step”, explains Bolz. This is because stresses inside the workpiece, different coating thicknesses or the ductility of the material, which contracts after machining due to the heat introduction, cause deviations in some diameters and bearings. “It’s only after a test drilling with a subsequent dimensional check on the part that we determine the required tool diameters, which also apply to the subsequent tools”.
Most economical solution thanks to combination tools
PKD-Werkzeug bearbeitet Lager- und Positionsbohrungen
Ein Beispiel dafür ist das Werkzeug zur Bearbeitung des Lagersitzes eines Magnesiumgehäuses. „Bei dieser Bearbeitung hatten wir mit starken Vibrationen zu kämpfen, da das Bauteil vor allem im Bereich der dritten Lagerbohrung extrem dünnwandig ist“, erinnert sich Leander Bolz. Das Werkzeug muss an den vorgegossenen Bohrungen 0,6-1 mm Material abtragen.
Der Kunde stellte hohe Anforderungen:
- Rundheit < 0,01 mm
- Durchmessertoleranz IT7
- Gemittelte Rautiefe Rz < 10 µm
MAPAL legte dafür ein komplexes, mehrstufiges PKD-Kombinationswerkzeug aus. „Damit bearbeiten wir die drei Lagerbohrungen und die Positionsbohrung des Lagersitzes in einem Schuss – prozesssicher innerhalb der geforderten Toleranzen“, so Bolz.
Das Werkzeug arbeitet mit folgenden Schnittdaten:
- Drehzahl 8.000 min-1
- Vorschubgeschwindigkeit 3.200-4.800 mm/min
- Vorschub 0,1-0,15 mm
Bohren und Fräsen kombiniert in einem Werkzeug
Ein weiteres Werkzeug kombiniert die Fräs- und Bohrbearbeitung. Während Bohrstufen die Lagerbohrung und die Positionsbohrung bearbeiten, kommt eine Frässtufe zum Fertigen der Dichtnut zum Einsatz. „Auch bei diesem Werkzeug war es unsere Hauptaufgabe, Vibrationen zu verhindern und den Schnittdruck zu reduzieren“, erläutert Bolz. Die Werkzeugexperten erreichten dies, indem sie Zähnezahl und Geometrie der Frässtufe optimal aufeinander abstimmten. „Dadurch vermeiden wir auch Späne in der Nut und stellen sicher, dass der Fräsprozess sicher läuft“, sagt Bolz.
Die Frässtufe am Werkzeug arbeitet mit folgenden Schnittdaten:
- Drehzahl 8000 min-1
- Vorschubgeschwindigkeit 7.200 mm/min
- Vorschub 0,15 mm
