The manufacture of crankshafts belongs to the “champion league” of metalworking activities owing to the crankshaft’s complex geometry and its underlying instability. It is the reason why there are so few specialists offering machines for the highly efficient grinding of these components. For many years there have been no important developments in the grinding technology applied in the manufacture of crankshafts. The last major development was the chasing of the pin bearings – called “pin chasing” – with CBN grinding wheels. And that happened some 15 years ago. EMAG is bringing a new impetus to this market with the introduction of a completely new, possibly revolutionary, grinding process. The Synchronous Support-Grinding process is to reduce machining times by up to 70%.
EMAG: New impetus for the crankshaft
23/09/2009 - Oliver Hagenlocher
What is behind this Synchronous Support-Grinding technology?
In synchronous support-grinding two opposing CBN wheels machine the same point on the workpiece. This process, where two grinding wheels simultaneously machine a certain point on a component from opposite sites, eliminates the axial forces generated in feed-in direction. To also eliminate the tangential forces, the process employs an “NC steady rest”. This 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 feedrates; and the use of two grinding wheels drastically reduces machining times. If, in addition, both directional and counter-directional grinding strokes are utilised, the forces generated during machining are also cancelled out. This makes it possible to hold the crankshaft between two centres with just the momentum created by simple friction.
The machine concept as the basis for productivity increases
The machine design is quite different from anything that has gone 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 too is of the latest design. In contrast to what the classic grinding machine operator has been used 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 the VTC 315 DS 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 centres.
The VTC 315 DS 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 VTC 315 DS’s low temperature growth makes the machine 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.
The results speak for themselves
As the crankshaft bearings have to be machined to the highest precision, manufacturers try to complete-machine the shaft in a single set-up, to avoid clamping errors. This includes all centre 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, using an eccentric machining process is the other. 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 the VTC 315 DS the grinding specialists at Salach have succeeded in combining the positive aspects of the two methods. It is the synchronous grinding with two wheels on the one hand and the eccentric clamping of the crankshaft on the other that 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. To also provide flexibility, a new eccentric chuck with fully automated spacing and stroke adjustment has been designed. This ensures 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 the VTC 315 DS is its small footprint, compared to multi-wheel machines. It stands at just about 20 m², including 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% compared to the conventional corundum grinding process, and 50% 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.
A summary of the technological advantages:
- Two grinding wheels that machine the same bearing simultaneously
- No distortion of the workpiece – thus no indeterminate planishing and spark-out times
- No need for in-process diameter measuring
- No backrest action at reduced feedrates
- Centric grinding of the pin bearings on an eccentrically clamped shaft allows for higher workpiece speeds
- Simultaneous grinding of the shoulders on the thrust bearings
Something of interest noted in the margin
The basic VTC 315 DS is batch-produced at the EMAG works in Zerbst (Saxony-Anhalt). This lowers manufacturing costs and has a direct influence on the already very attractive price-performance ratio.