Machining processes are used to produce final-shape components that are ready to install; the starting material can be a cast or forged blank or a semi-finished product, for example a length of bar or a shape cut out of plate. One differentiates between five technologies to machine components: turning, milling, sawing, drilling and grinding.

There are differences in the characteristics of machining systems: the resultant chip shape, the surface finish, the wear effect on the tool, the necessary cutting forces and power and the geometric accuracy. Depending on the specific task and the machining process, the focus will be on one or even more of these machining criteria. One uses the term ‘machinability’ to express the sum of them all. This means the machinability is not a physical property of a material that can be defined as a characteristic number. It is more an expression of the technological processability in the specific case and should always be considered in relation to the elements of the machining system.

Overall, the actual machining process has the greatest influence. Strictly speaking, statements relating to the machinability of aluminium are always only applicable to the respective process (turning, drilling, milling, etc.). However, as a result of its more widespread use and because of the simpler kinematic relationship between tool and workpiece, statements regarding machinability are usually based on turning. Such statements cannot be applied unreservedly to other processes.

The relatively large influence of the material testifies to the fact that as a class aluminium-based materials contain a range of different alloy groups with very different machining properties. It has proven useful in practice to differentiate between various groups of materials in order to classify their machinability.

High-performance machining of wrought aluminium alloys

An example of this is the milling of thin-walled integral components from thick plates of high-strength wrought alloys, such as might be considered in aircraft manufacture. The weight of swarf produced per part can be up to 20 times greater than the weight of the finished aluminium component. This already gives an indication of the direction development work should take: important here is that both the tool and the machine tool have to be capable of handling a high machining rate. A target of 10,000 cm³/min is regarded as being achievable by high-speed machining today; this is considerably more than the 6,000 to 7,000 cm³/min that can be achieved using high speed cutting (HSC). It is not only the rotational speed of the spindle that is important but also the size of the forward feed that the tool has to be able to withstand. High speed means, of course, that there is a risk of the tool wearing more rapidly. The wear can, however, be kept within tolerable limits by using special measures such as coating of the cutting edges.

A very successful example of the machining of aluminium is the Coca-Cola bar in the Kölnarena, Cologne’s multifunctional arena: this component was drawn in 3-D, machined and then partly subjected to bending after machining, so that the curved shape of the whole bar could be completely accentuated.

Turning

Typical values for turning have to be geared to the machinability criteria that are of particular importance in a specific case and whether any requirements have to be taken into consideration, for example by the machine performance.

Milling

Milling machinability differs from that for turning in various aspects. The chip shape is only of minor importance in milling because the swarf has to be removed via the chip space. The chip space has to be large enough to accommodate the relatively large chip size. Consequently, milling cutters used to machine aluminium have a smaller number of teeth as those used for steel. In addition, the surface produced by milling is generally somewhat better than that from turning because the surface roughness is partly levelled by the subsequent teeth. One specifically makes use of this peculiarity of the process by using wiper edges in cutter heads to improve the surface finish.

Essentially, the factors affecting wear here are the same as for turning, but overall it is greater with milling because of the percussion-type cutting-edge loading resulting from repeatedly interrupted cuts. In addition, it depends on the contact conditions. Instead of using the cutting forces as the parameter, in milling one uses the specific machining performance Q (in cm³ Al/kW x min). A typical figure for an average cutting speed is Q approx. 40 cm³ Al/kW x min. This value increases markedly with increasing cutting speed.

Striking examples of turned or milled components are highly polished aluminium mirrors whose surfaces have been so finely machined with a diamond tool that they can be used, without any subsequent processing, as light-reflecting mirrors with accuracies in the submicron range.

Drilling

Drilling differs from turning in its kinematic characteristics and the significantly lower cutting speed. To some extent, other evaluation criteria than with turning apply when evaluating machinability by drilling: the deviation (over- or undersize) depends on the material being drilled and the effectiveness of the cooling. It is important to use sharp tools and ample cooling with emulsions. For a high quality finish on the wall of the drilled hole, subsequent operations such as reaming or countersinking are necessary. With soft aluminium alloys there is a risk that the flutes will become blocked and the drill will break. One uses a range of measures to facilitate chip removal to counteract this: cutting oil, polished flutes and a positive cutting angle. Furthermore, when drilling deep holes one should plan to withdraw the drill to clear chips.

Sawing

Aluminium-based materials are processed using circular saws or bandsaws. With bandsaws, one uses flexible tools (bandsaw blades) that are guided over rollers and in certain cases are also twisted. By contrast, rigid tools (saw blades) are used with circular saws. This dissimilarity results in a number of differences in operational characteristics, such as different cutting rates, thicker cutting channels with circular saws and burr formation with bandsaws.

Grinding

When processing aluminium one must not use grinding or burnishing tools that have been previously used on steel, copper or other heavy metals. Such separation or thorough cleaning of the tools and machines, including filtering and/or replacement of the cooling lubricant, is necessary to prevent foreign metallic particles becoming embedded in the surface of the aluminium. If there is then ingress of moisture, such particles can cause contact corrosion as a result of the formation of an electrolytic cell.

It is important to observe the safety regulations issued by the authorities particularly with respect to the risk of fire and explosion due to grinding swarf. In Germany the relevant document is Richtlinien zur Vermeidung der Gefahren von Staubexplosionen beim Schleifen, Bürsten und Polieren von Aluminium und seinen Legierungen (Guidelines on the Avoidance of Risks of Dust Explosions when Grinding, Brushing and Burnishing Aluminium and its Alloys) published by Carl Heymans Verlag KG, Cologne.

To grind aluminium one uses either grinding wheels, made of silicon carbide with synthetic resin bonding or even metal-encased synthetic diamonds, or textile abrasive belts, with synthetic resin or full synthetic resin bonding. Emulsions with a ratio of 1:35 are used as grinding aids, with wetting agents added where necessary. Here, increasingly higher concentrations counter clogging of the disk.