March 2008 eNews

Challenge: to provide world-class competence for crankshaft machining

The crankshaft is one of the most challenging workpieces to machine. It is highly unsymmetrical, long and relatively slender, made of material that has poor machinability, has close quality limits and is subjected to very competitive manufacturing demands.

A central component of any combustion engine, crankshafts come in many shapes and sizes – from small ones found in two-stroke engines in garden equipment to giant ones found in diesel engines in ships. Even crankshafts in automotive engines vary, each one unique to its engine type and make. As a mass-volume component, the vehicle crankshaft is ruled by cost efficiency and has undergone considerable evolution as regards design, material, and machining.

This evolution will likely continue as demands grow for greater environmental friendliness, better performance and lower manufacturing costs. Environmental pressures are making the manufacture of crankshafts more difficult. Engines are being downsized and combined with turbo- and superchargers, and manufacturers have to reduce unwanted emissions while at the same time minimizing loss of power. The evolution of the crankshaft has and will continue to contribute to this.

Turn-turn broaching of a crankshaft.

Today’s truck engines already run cleaner than small car engines from the early 1990s, and power efficiency is markedly higher. Moreover, developments in the machining of crankshafts have helped to achieve precision crankshafts while maintaining the manufacturing cost per piece.

A few industrial standards illustrate the level of expectation: The run-out of the milling tool that machines an automotive crankshaft is typically within 0.02mm. Tool life must be long and predictable to suit production schedules, and cutting edges must last reliably up to as much as 8,000 crankshafts between tool changes. Cycle times are optimized to within seconds to guarantee shift volumes, and workpiece materials are ever tougher – steel forgings with higher tensile strength and, to a growing extent, austempered ductile iron, which is lighter and stronger. Both materials are, needless to say, more demanding to machine.

For optimizing performance and results, individual machining solutions have to be established for each crankshaft model. This results from close cooperation between the crankshaft manufacturer, the machine tool builder and the cutting tool supplier.

At the center of Competence for Crankshaft and Camshaft Machining in Germany, Günter Wermeister and his team at Sandvik Coromant have a proven track record, with nearly 900 tools installed worldwide.

Crankshaft machining is not a new field for Sandvik Coromant; the company introduced special milling tools during the 1970s. But, it was not until the early 1990s that a substantial buildup of capacity and competence took place in response to requests from the automotive industry. Turn-milling has also become a speciality of the center.

Today, with the global auto production above 60 million vehicles per year and demands of engine modifications to improve emissions, weight, combustion and friction values for fuel and power efficiency, solutions for crankshaft machining are more sought after than ever.

Tools for crankshaft machining may contain 350 inserts on a cutter diameter of 700mm. The indexable inserts are held in segments that can be easily and quickly changed and very accurately set.

“Crankshaft evolution is continual,” says Wermeister. “Successful solutions are based on careful, exhaustive analysis of factors such as production volume, cycle time, equipment requirements, extent of machining, quality levels, tool changing intervals, and more,” he says. “There is no single solution that fits as a standard. All solutions have to be tailored to their application. The crankshaft needs several different operations to be completed and forgings or castings vary as blanks. The manufacturer bases his process satisfactionon his assessment of the resulting production cost per crankshaft, throughput and investment cost.

“A crankshaft in a position that is supported by bearings in the engine is a strong, stable mechanical element, but it is highly unsuitable as a workpiece,” he continues. “This has contributed to the fact that there are various ways to machine crankshafts, especially in the automotive industry. Whatever the requirements, cutter designs, insert geometries and grades have been developed extensively by Sandvik Coromant over many years to optimize applications individually.

“The crankshaft itself is the weakest link in the machine-component-tool system,” says Wermeister. “The instability of the part affects minimal cycle times and quality consistency. This makes huge demands on the development and design of cutters and indexable inserts as well as the application know-how. It means combining the distribution of cutting edges to achieve balanced cutting forces and to establish high cutting data optimized for each type of surface, resulting in smooth, chatter-free cuts.”

Wermeister says that to be accepted as a serious player in crankshaft machining, you have to have the right background, capability and resources. “This was part of the thinking behind the Competence Center and its central European position in Germany,” he says. “We have close, trusted collaborations internationally with both manufacturers of crankshafts and the relatively small number of machine tool manufacturers for this area.”

Turning is a flexible method used mainly to machine the concentric parts of a crankshaft. In some cases and in some parts of the world, it is also used for the eccentric pins. When this is the case, the unbalanced, out-of-center clamping positions require special chucks with mass compensation. Turning is performed on special crankshaft machines as well as on standard lathes with tailor-made turrets. Demands on the tools are severe because of overhang and vibration tendencies.

Turn-broaching has been used to a more limited extent in fewer installations. This is a broaching process, either through linear turn-broaching with a conventional straight broach fed tangentially against the rotating crankshaft tool or as the broaching process has evolved using circular- or spiral-formed tools. Although efficient, the lack of flexibility to suit various crankshaft configurations has held the method back from becoming more widespread.

Turn-turn broaching is a combination of turning and turn-broaching where the turning and turn-broaching tools are mounted radially on a disk turret that moves into the crankshaft and along the bearings and pins, machining as the crankshaft rotates. It is a method used for most types of crankshafts. The turning operations are mainly roughing cuts, with a stationary turret and the cutting edge on the centre line of the workpiece. The turn-broaching tools perform finishing cuts, moving tangentially through a slowly rotating turret against the bearing surfaces of the rotating crankshaft. An advantage of this method is the use of the most efficient number of sister tools on the turret for the application.

Internal milling is also known as whirling or planetary milling and can stand up to heavy machining over longer cutting times. The milling cutter forms a ring through which the crankshaft is rotated. The indexable inserts are positioned closely pitched on the internal circumference to machine the bearing, pin and cheek surfaces of the crankshaft. This is a very stable setup, used mostly for larger crankshafts and when a lot of stock has to be removed. External milling is used mainly for large-volume machining of small to medium-size automotive crankshafts. Basically a fast-rotating, close-pitch side- and facemilling type of milling cutter, it machines the surfaces as the crankshaft rotates in combined motion. High metal removal rates and rapid positioning for cuts characterize the method. Typically a cutter can have up to 350 inserts in segments on a cutter diameter of 700mm and machines bearings, pins and the tops of the cheeks of the crankshafts. The method is capable of short cycle times and fast tool-handling and tool-setting times. Courtesy of Metalworking World 2008 No. 1.

Want more information? Click below.
Sandvik Coromant