Here in greater detail is how the process applies to the
EMAG VTC 315 DS: Two opposing CBN wheels machine the same point on the
workpiece. Tis eliminates the axial forces generated in the feed- in direction. To also eliminate the tangential forces, the process employs an “NC steadyrest.” Tis 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. Te elimination of both axial and tangential forces permits the use of extremely high feed rates; and the use of two grinding wheels drastically reduces machining times. If, in addition, both directional and counter-directional grinding strokes are utilized, the forces generated during machin- ing are also cancelled out. Tis makes it possible to hold the crankshaſt between two centers with just the momentum created by simple friction.
Productivity Increases Te design of the VTC 315 DS is un-
usual: work headstock at the top, tailstock below, two compound slides, to the leſt and right a grinding spindle each with a CBN-compatible power rating of 30 kW and cutting speeds of 150 m/s. Also inte- grated are functions for gap control and automatic balancing. Te wheel receptor also 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. Te new receptor— with short taper and flanged contact sur- face—helps make wheel changing easier. Another distinctive feature is that, in
speed of 15,000 rpm allow for simultaneous wheel dressing, thus noticeably reducing idle times. As the crankshaſt bearings have to be machined to a very
high level of precision, manufacturers try to complete-machine the shaſt in a single setup, to avoid clamping errors. Tis includes all center bearings—which is relatively easy to accomplish—and the flange bearings, the low ends and the main bearings.
Pinning Down the Pin Bearings Te real challenge, however, lies in the machining of the
pin bearings. Tere are two generally accepted ways to do this: the so-called pin chasing process is one, using an eccentric machining process is the other. Te advantage of the pin chasing process lies in its flexibility. However, the feed rates that can be used are lower than those for eccentric machining. Tis marginally reduces productivity levels. Te disad- vantage of eccentric machining lies in the resetting, as every new crankshaſt variant also calls for a change of chuck. Te VTC 315 DS combines 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 crank- shaſt on the other that combine to increase productivity levels in the machining of pin bearings. Tis approach permits the appli- cation of the cylindrical grinding method, with its high feed rates, for the machining of these bearings. To also provide flexibil- ity, an eccentric chuck with fully automat- ed spacing and stroke adjustment has been designed. Tis ensures that a crankshaſt
most cases, the work headstock does not have to be equipped with a special driver. Tis helps make 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 crankshaſt can be safely clamped between two centers. Te VTC 315 DS is equipped with two measuring sensors.
Te longitudinal alignment of the crankshaſt is carried out with the aid of a pivoting length measuring device, whereas the diameters are checked with a type of in-process gaging head. However, in-process measuring is usually only used for setup purposes. In production, this is replaced by postprocess 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 dress- ing spindles with solid-borne sound sensors and a maximum
The process makes it possible to hold the crankshaft between two centers with just the momentum created by simple friction.
can be finish-ground in a single setup. Te thrust bearings are machined simul-
taneously, 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 num-
ber 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 foot-
print—about 20 m², including electrical cabinet—compared to multiwheel machines. Synchronous support grinding is the first process to com-
bine flexibility with high output rates in the grinding of crank- shaſts—bringing about noticeable grinding time reductions of up to 70% compared to the conventional corundum grinding process, and 50% compared to machining with CBN.
Edited by Yearbook Editor James D. Sawyer from information provided by EMAG GMBH.
Motorized Vehicle Manufacturing 75
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