Directing Investments In The Right Way

Directing investments in the right way: Procurement of new machines is not the only way of increasing production capacity. The right tools and suitable software can also generate up to 85 percent gains in productivity.

Expand the existing capacity or start to turn down orders? Many companies find themselves in this dilemma: Every time a company turns down an order, there is the risk the customer may not consider this company for future orders. So there is no alternative but to continue to accept every new order. However, at some point the existing stock of machine tools will reach its limits. What then? Is it justifiable to procure an additional machine when it is uncertain how long the surge in orders will be sustained? Moreover, there is a time lag of several months between ordering and receiving a new machine. So how can the new machine address the current production bottleneck?

Many Production Managers suffer sleepless nights wrestling with such questions. Indeed, new machines represent substantial investment, often a seven-figure sum – they need space, personnel to operate them, and are not immediately ready for productive use. Alternatives by which a relatively small investment and no loss of time can achieve significantly more from existing productive resources are therefore very attractive. PPC milling (Parabolic Performance Cutting) is one such technique. The success story is already well established. For instance, Koller Formenbau GmbH of Dietfurt in the Altmühltal, Germany, has used PPC milling cutters supplied by the Hoffmann Group to reduce the finish machining time for geometrically ruled surfaces from 100 hours to 15 hours.

Massive Reductions In Finish Machining Times

PPC is also called barrel milling. Similarly, to that of a ball-nosed slot drill, the main cutting edge of a PPC milling cutter is curved as an arc of a larger radius circle. Whilst for a classic ball-nosed slot drill the effective radius of the tool is only half the diameter, a PPC milling cutter has a much greater effective radius, up to 1000 millimetres, thus permitting a significantly greater engagement length on the workpiece. A barrel milling cutter achieves a line skip up to nine times greater than that available using a ball-nosed slot drill of the same tool diameter, thus placing less stress on both the tool and the machine. If on the other hand the same line skip is maintained a surface quality up to 80 times better can be achieved. The Hoffmann Group is currently offering a portfolio of tools with effective radii up to 1000 millimetres. Even larger radii are conceivable and could be implemented as tools, but the very long engagement length would demand very high contact pressures. The resulting displacement of the tool would have a negative effect on precision, which of course in a finishing process cannot be compromised. The Hoffmann Group also offers special solutions with effective radii different from those in the catalogue, to allow the customer to gain the optimum benefits from PPC for specific applications. In order to fully exploit the advantages of PPC, a radius in the range of 700 or 800 millimetres has generally been found to offer the best solution. Very often the best solution depends not only on the component but also on the machine tool in use.

Different Profiles And Approach Angles

PPC is particularly effective when the tool profile is exactly suited to the workpiece surface contour. Therefore, depending on the application, up to four different tool profiles are necessary for optimum machining of workpieces and free-form surfaces with highly complex surface geometries. This is because surfaces with interference contours, large areas, deep cavities or flat faces each demand different tool profiles. The Hoffmann Group therefore currently offers the following principal cutting edge profiles: “straight”, “tangential”, “conical” and “stub point conical”; each representing a specialised solution for machining particular types of surface. For all profiles except for the straight profile the tool is also mounted at an oblique angle – and the selected approach angle also makes a difference. Therefore, the Hoffmann Group offers conical and stub point conical PPC milling cutters with three different approach angles. These allow the programmer to use the optimum type of milling cutter for the specific application. Depending on the component, for instance steeper approach angles allow interference contours to be avoided.  Or the use of a machine tool that has a large headstock may require the choice of a tool with a larger approach angle.

CAD/CAM With Tooling Database

In contrast to ball-nosed slot drill cutting, PPC is more dependent on software. For classic ball-nosed slot drill cutting the CAM software requires only a small amount of information to calculate an appropriate milling strategy: It is sufficient to declare the tool diameter, tool length and the information that the relevant tool is a ball-nosed slot drill. These data can also be processed manually. The more complex tool profiles of PPC on the other hand can be calculated only in combination with CAM software that offers “barrel milling” as a strategic option. In addition, the software must have available a tooling database which holds the exact geometries of the PPC milling cutters.

Furthermore, because for PPC the tools are aligned obliquely to the workpiece, this process can be employed only in conjunction with a 5-axis milling machine. As 5-axis machines come increasingly into use, PPC milling is now really taking off. In any case it is not always necessary to control all 5 axes simultaneously. Often, once the tool has been set up, the draft angles can be machined with one or two axes clamped, in order to achieve even better results. At one time there were only a few software programs that offered “barrel milling” functionality. That too has changed in recent years. Koller Formenbau for instance already had a 5-axis machine in use and operated Hypermill software, and only then did it seek suitable barrel milling cutters. At a moderate expense the company has been able to achieve up to 85 percent gains in productivity thereby significantly increasing the productive capacity without having to procure new machine tools.

Increasing Areas Of Application

A classic area of application for PPC is finish machining of complex components and free-form surfaces in machine tool manufacture and in tool and die production. In medical technology, turbine technology and aerospace, PPC is attracting increasing interest. A relatively new area of application for instance is the finish machining of workpieces produced by additive manufacturing. 3D printing permits particularly complex workpieces to be produced in a single production operation. However, workpieces produced on a 3D printer fall well short of the surface quality required for contact faces. Such workpieces must be finish machined. PPC was conceived for exactly such applications. And not least because a variety of different materials are often used for the 3D printing process, the Hoffmann Group has extended its PPC milling cutter family to cater for machining a wide range of materials. PPC milling cutters for machining aluminium were demonstrated for the first time at AMB 2018. The new milling cutters are particularly sharp-edged and can also be used for machining plastics. This significantly extends the existing portfolio over a wide range of applications.

Conclusion

Companies which direct their investment into alternative paths can achieve substantial increases in productive capacity at a relatively small outlay, without adding to their existing stock of machine tools. CAM-controlled production using Parabolic Performance Cutting is such an alternative. PPC permits genuine step function gains in productivity with a relatively modest investment in software and tools.

Article by Dr. Michael Kersting, Director of Milling at the Hoffmann Group

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