SYNCHRONOUS SUPPORT-GRINDING

The manufacture of crankshafts belongs to the ?champion league? of metalworking processes because of the complex geometry of the crankshaft itself and its underlying instability. This is the reason so few specialists offer machines that can efficiently grind crankshaft components and also why no important innovations have occurred in crankshaft grinding technology for many years.

The last major development was the chasing of the pin bearings (?pin chasing?) with CBN grinding wheels that happened some 15 years ago. Now a completely new, possibly revolutionary, grinding process called synchronous support-grinding is being introduced into the market to reduce machining times by up to 70 percent.

In synchronous support-grinding, two opposing CBN grinding wheels simultaneously machine the same focal point on the workpiece from opposite sides, eliminating the axial forces generated in feed-in direction.

To further eliminate the tangential forces, the process employs an ?NC steady rest,? which is basically a single-sided steady that pushes against the workpiece from the direction of the machine base. It ensures that the workpiece is firmly clamped and does not give way under pressure.

The elimination of both axial and tangential forces permits the use of extremely high federates. The use of two grinding wheels drastically reduces machining times. If both directional and counter-directional grinding strokes are additionally utilized, the forces generated during machining are also cancelled out. This makes it possible to hold the crankshaft between two centers with just the momentum created by simple friction.

The machine configuration is designed quite differently from anything seen before: work headstock at the top, tailstock below, two compound slides, to the left and right a grinding spindle each with a CBN-compatible power rating of 30 kW and cutting speeds of 150 m/s. Also integrated are functions for gap control and automatic balancing.

The wheel receptor in the system is newly designed. In contrast to what the classic grinding machine operator is accustomed to, the wheel is not changed horizontally but loaded into the work spindle vertically, with the help of a simple wheel changer. The new receptor ? with short taper and flanged contact surface ? makes wheel changing easier.

Another distinctive feature of synchronous support-grinding is that, in most cases, the work headstock does not have to be equipped with a special driver. This makes setting the machine much easier ? a positive effect, brought about by the cutting forces cancelling each other out. A rotating quill in the tailstock guarantees that the frictional torque remains low and the crankshaft can be safely clamped between two centers.

A synchronous support-grinding machine is equipped with two measuring sensors. The longitudinal alignment of the crankshaft is carried out with the aid of a pivoting length measuring device, whereas the diameters are checked with a type of in-process gauging head.

However, in-process measuring is usually only used for set-up purposes. In production, this is replaced by post-process measuring, as the machine?s low temperature growth makes the system thermally stable and ensures that the required machining tolerances are held. Two integral dressing spindles with solid-borne sound sensors and a maximum speed of 15,000 rpm allow for simultaneous wheel dressing, thus noticeably reducing idle times.

Since crankshaft bearings must be machined to the highest precision, manufacturers try to machine the shaft complete, in a single set-up, to avoid clamping errors. This includes all center bearings (which is relatively easy to accomplish) and the flange bearings, the low ends and the main bearings.

The real challenge, however, lies in the machining of the pin bearings. There are two generally accepted ways to do this: the so-called pin chasing process is one, the other uses an eccentric machining process. The advantage of the pin chasing process lies in its flexibility. However, the feedrates that can be used are lower than those for eccentric machining. This marginally reduces productivity levels.

The disadvantage of eccentric machining lies in the resetting, as every new crankshaft variant also calls for a change of chuck. With a synchronous support-grinding machine, grinding specialists have succeeded in combining the positive aspects of the two methods.

The synchronous grinding with two wheels on the one hand and the eccentric clamping of the crankshaft on the other combine to increase productivity levels in the machining of pin bearings. This approach permits the application of the cylindrical grinding method with its high feedrates for the machining of these bearings.

A new eccentric chuck with fully automated spacing and stroke adjustment has been designed to provide flexibility and ensure that a crankshaft can be finish ground in a single set-up. The thrust bearings are machined simultaneously, using the ?kiss grinding? method. It is also possible to machine the flange and lower ends in the vertical position.

With respect to automation, a number of options are available, including the use of one or two robots, or a shuttle. Another advantage of a synchronous support-grinding machine is its small footprint when compared against multi-wheel machines. It stands at just about 20 sq m, including the electrical cabinet.

Synchronous Support-Grinding is the first process to combine flexibility with high output rates in the grinding of crankshafts, bringing about noticeable grinding time reductions of up to 70 percent compared to the conventional corundum grinding process, and 50 percent compared to the machining with CBN.

This innovative manufacturing system will lend new impetus to the machining of crankshafts and change forever the grinding world as we know it.

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Dr. Guido Hegener is the managing director of EMAG Salach Maschinenfabrik GmbH, Austraße 24, D-73084 Salach, +49(0)7162/17-267, Fax: +49(0)7162/17-199, www.emag.com .