Grinding specialists should be rethinking their plans for the future, now that EMAG Salach Maschinenfabrik GmbH are relying on their VTC 315 DS for the simultaneous, vertical grinding of shaft-type components. In contrast to the conventional process, the VTC315DS uses two opposing grinding spindles for the 4-axis simultaneous grinding of vertically chucked workpieces. The advantage: the axial forces cancel each other out and optimal access to the workpiece is made possible.
EMAG: Vertical machining opens up new potentials
13/01/2009 - Oliver Hagenlocher
To utilise the always best technology for the machining of the workpiece is a tactic that EMAG has implemented since EMO 2007, when they first introduced the VTC 315 DS. At AMB 2008 the company is going to present the next stage of the machine’s development: a version for 4-axis simultaneous grinding. The idea was triggered by the VTC platform. EMAG introduced the first 4-axis vertical turning center of the VTC series as early as 2003. Since then the VTC series has been consistently developed further.
Although there are some common denominators, there is no question at all that the VTC 315 DS is in direct competition with the classical horizontal two- and four-axis grinding machine. However, the idea that the people in charge at Salach are pulling the rug from under their own feet by pitting the new machine against their own classic grinders is not something they agree with. After all, the vertical machine was designed for the machining of smaller component batches, where they offer short setup times and great flexibility. For larger quantities the single-purpose machine still remains first choice. Albeit, the vertical design still offers a number of advantages in the application of grinding technology, especially when it comes to the machining of gearshafts with a number of bearing seats. One of these advantages immediately becomes apparent in the construction of the machine. The two independently travelling compound slides mounted to the inner wall feature X- and Z-axes with integrated grinding spindles. In the centre is the workpiece, which is held from below by a fixed tailstock centre and from above by a moveable work head centre, thus allowing simultaneous grinding from left and right. This design not only ensures excellent access; it also makes use of the naturally developing forces. With the grinding process the main force input is usually generated by the axial forces, which are often three to four times that of the tangential forces. On the VTC 315 DS the axial forces are neutralised by the fact that the grinding wheels are facing each other. The contra-rotational movement of the grinding wheels also cancels out the grip torque created by the tangential forces. This is particularly effective with workpieces that are moved only by the traction created by the centre in the work head. In terms of resetting, it means that no clamping equipment or separate drivers are required. All that needs to be done is to adjust the span and to restart the program.
Precision has top priority
Time saving, ease of access and flexibility are surely factors that should automatically be built into all modern machine tools. For grinding machines, however, precision is usually the top priority. This is no different when it comes to the VTC 315 DS. It is a requirement about which Dr Guido Hegener, senior executive responsible for grinding technology at EMAG Salach Maschinenfabrik GmbH, is pretty cool: „We have tested the machine thoroughly and made a number of comparisons with our horizontal machines. Where precision is concerned the machine is first-rate. The sturdy, rigid machine construction with its base in Mineralit® (a polymer granite that has a much greater damping effect than cast iron) forms an excellent basis for achieving the required precision. Owing to the internal dimensions there are sections where rigidity is even greater than at the equivalent points on a classical grinder of similar size. Simultaneous machining also provided us with some outstanding results.“
Ease of operation is paramount
Although this development has, in some areas, required a rethink of the original idea of combination machining, the changes did, according to Dr Hegener, not pose any great technological difficulties: “The changes that had to be made included the installation of vertical grinding spindles and the accommodation and handling of the grinding wheels. For instance, we had to design a special fixture for the changing of grinding wheels, as the construction of the machine does not allow the use of a crane for the purpose. This example shows that the operator-friendliness of the machine has been of utmost importance to us.“ In fact, a receptor with a taper similar to the HSK is used to change the grinding wheels. As the machine concept favours the use of CBN grinding wheels, the receptor must be of great precision. The CBN wheels can be used for infeed grinding, multiple plunge-cuts and longitudinal grinding as well as for the machining of various shoulders and radii. The precision of the workholding system minimises the dressing effort, which would otherwise make the use of these cost-intensive CBN wheels uneconomical.
A reasonably priced alternative leads us into the future
The people in charge of developing the VTC 315 DS have also closely studied market requirements for on-board measuring equipment, automation and programming. The machine has been equipped accordingly. For instance, the standard equipment includes a length measuring probe for the correction of centre offsets. The machine also features an in-process gauging control. Dr Hegener’s personal favourite, however, is the also available post-process measuring device. For automation EMAG relies on a robot that loads and unloads the workpieces from the side of the machine. And programming? Well, of the effort involved in the control of the developing forces, i.e. the left- and right-hand rotation of the grinding wheels, the man at the machine notices nothing. Quite to the contrary, the Siemens 840D control makes programming and operation very easy. This leaves us to conclude that with the VTC 315 DS EMAG has created a machine that impresses – above all – by the short setup times, outstanding accessibility and a surprising degree of operator-friendliness. That the small footprint and the compact construction of the machine also represent an economically priced alternative to the classic grinding machine is something that will surely convince even the many grinding specialists. Mind you, the machine concept and, in particular, the simultaneous grinding feature will be developed further. First thoughts have already been directed at the less stable workpiece, with additional support features for these more delicate shaft-type components. This would surely make the concept a viable one for mass production too.
By the by …
Dr. Guido Hegener, senior executive responsible for grinding technology at EMAG Salach Maschinenfabrik GmbH comments: „At the beginning some users might still have a few reservations, but in certain areas the new concept will eventually replace the classical machines.”