Modern crankshaft machining methods
The manufacture of crankshafts is not a straightforward matter; many pitfalls await the adventurous novice attempting to make his own crankshaft, and there are reasons why there are so few reputable race crankshaft manufacturers.
The general trend for trying to reduce engine friction means that modern design practice is often in the direction of reducing bearing diameters. This means greater care needs to be taken in manufacture, especially heat treatment, if serious distortion is to be avoided. Manufacturing stresses, with surfaces commonly stressed in tension by the shearing action of machining, mean that distortion is likely in heat treatment where these are excessive.
The traditional method of rough-machining the crankshaft was to turn each crankpin on a large sturdy lathe with the crankpin on centre and the central axis of the crankshaft offset by half of the crank stroke. The forces involved were considerable, and the machining was time-consuming. The advent of modern machining inserts has improved things, but these types of tools are often not forgiving of the type of intermittent cutting resulting from eccentric turning.
What has markedly improved the machining of crankpins is the modern machining centre, where cranks can be produced on lathes with milling attachments or on milling machines equipped with a fourth axis. In these circumstances the pins are produced by a milling cutter that follows the motion of the pin while the crankshaft blank is slowly rotated. Not only are the machining forces lower, but there are other advantages too. The rough-machining operation is quicker, and management of machining swarf is much improved. Crankshafts thus produced are likely to suffer less from distortion in heat treatment.
Modern machining methods have also improved the accuracy of manufacture and reduced the amount of manual dressing of features. Features that previously required careful manual processing were the edges of bevels on the webs between crankpins and main bearings, as well as the edges of oil holes. Both types of feature are commonly finished now by CNC machining methods, and the surfaces of crank-bevel features are no longer restricted to swept surfaces defined by eccentric machining on a lathe.
The consistency of oil-hole dressing is very important, as these features are often the weak point of a crankshaft as far as fatigue is concerned, and it is likely that incorrect oil-hole preparation - or even forgetting to dress one hole - would render an otherwise optimal crankshaft unfit for use. Where the edges of oil holes are machined, there should be no excuses for not having consistency of form and finish before nitriding.
The use of 3D machined surfaces, courtesy of modern CNC machining processes, opens the door to more complex shapes being used, and for the crankshaft designer to design lighter crankshafts which (with some thought put into the design and analysis process) can be more resistant to fatigue failure.
Race crankshafts are always likely to remain difficult and time-consuming to produce, with specialist producers being able to invest in new machinery. Perhaps the most impressive crankshaft manufacturing machinery is that used for production crankshafts, where the time taken for the entire machining operation is measured in seconds rather than tens of hours.
Fig. 1 - Modern crankshaft manufacture has given us shorter manufacturing times, lower levels of machining stress and the opportunity to design a light, stiff, fatigue-resistant part (Courtesy of Pankl)
Written by Wayne Ward