While there are reciprocating engines that don't have a crankshaft, these are rare and have generally done little to convince people that we ought to abandon it. The crankshaft, with its eccentric crankpins, is not a straightforward part to make. Evidence for this lies in the small number of manufacturers capable of making top-quality examples.
Traditionally, the work involved in producing the basic functional shape of such parts has been done on the lathe. We can imagine taking a billet, casting or forging, and machining the main bearing journals, as these lie 'on centre', the axis of the rough part as it is mounted in the machine. However, when we come to machine the crankpins, each of which lie off-centre, life becomes a little more complicated. The billet has to be accurately offset within the lathe, and the cuts taken at this stage are intermittent, which means that the lathe, its foundations and tooling all need to be very stiff to withstand the 'impact' of the cut starting on each revolution.
A single-cylinder crankshaft or a 360° twin inline twin-cylinder crankshaft is relatively simple, as it has a single off-centre machining operation in the lathe. For a typical inline four-cylinder or flat-plane V8 crankshaft, there are only two pin positions to consider, but these need to be in the correct position relative both to the crankshaft axis and each other. Other configurations are yet more complicated, with a cruciform or crossplane V8 crankshaft requiring four offset positions, and an inline five or a V10 requiring five crankpin positions, all of which have to be accurately positioned relative to the crankpin and in the correct angular relationship to each other.
The continuing trend towards smaller main bearings and smaller journals makes the manufacture of such crankshafts more difficult. While it is possible to use a 'steady' when turning on-centre, as is the case when machining the main bearings, the same can't be said when turning crankpins. The crankpins need to be accurately machined before grinding so that the amount of material removed during the grinding process does not leave one area with a very thin hardened case; this is especially important for crankshafts that have only a thin nitrided case.
A more modern method of accurately, consistently and quickly producing a crankshaft is to take advantage of CNC machining. The crankpins can be produced with the billet always rotating between centres. The method here is to move the tool continuously when machining the crankpin, and this kind of work is well suited to a 'mill-turn' type of machine, where a powered milling cutter replaces the manually controlled tool post of the lathe mentioned before. The spinning tool 'chases' the crankpin around as the crankshaft is slowly turned on-centre.
The advantages of this method of manufacture are numerous. An important one is that the work can be done while using lathe steadies, minimising workpiece deflection. The cuts are also light and continuous, again minimising workpiece movement, and only one set-up is required; with the number of set-ups reduced to a minimum, the chances of errors creeping in are therefore mitigated.
Written by Wayne Ward