BMW

CVD diamond tools machining aluminium in the new 5-Series BMWs

The new 5-Series BMW introduced to the German market on 5 July, 2003 featured not just an aluminium/steel body, but also the same solid aluminium chassis that was first introduced in 1995.

No coolant is used in the manufacture of the new aluminium rear axles, partly due to environmental considerations and partly to cost considerations (disposal, cleaning of components, etc). The dry machining of the rear-axle support that consists of a total of four AlSi7Mg aluminium cast alloys threw up the question of a tool material that would be suitable for the machining operation. The tool material that gave the best results in the end was tungsten carbide coated with CVD diamond.

As well as the boring of the supporting bearing bush, the machining operation included producing the weld connection for the tubular cross member and also the milling of various chamfers and plane faces. The main machining criterion is adherence to dimensional tolerance, with a surface quality a Rz value of > 40 µm being required.

The boring bar used is fitted with eight indexable carbide inserts (K10 carbide, less than 6% cobalt), coated with a 4 µm thick CVD diamond layer. This is a nanocrystalline diamond layer with a grain size of between 20 and 200 nm. Due to the small grain size an extremely smooth tool face is obtained, which contributes to the ready removal of chips from the machining zone. The weld connection for the tubular cross member is machined with a bell-shaped tool which has two R9 radius inserts with a positive rake angle. As with the boring bars, the tool inserts are coated with a 4 µm thick nanocrystalline CVD diamond layer in order to minimize the forming of a built-up edge.

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