Ways to increase the energy efficiency of cutting processes

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MAPAL Fabrik für Präzisionswerkzeuge Dr. Kress KG

The path to lower energy use is give and take

It is necessary, however, to pay sufficient attention to cutting edge stability and process security within the application. Lower cutting forces due to slower feed rates or reduced depths are counterproductive as far as energy efficiency is concerned. They raise the energy requirements because of the longer processing time.

As a rule, the basic power, and thus the energy necessary for operational readiness in the processing machine, has a relatively large share of the total power and energy. The machine up time thus influences the total energy relatively strongly. For an energy-efficient process, therefore, both the main and secondary times should be as low as possible.

Raising productivity has always been an optimisation goal in cutting. In this field, there are a series of effective measures for lowering energy consumption.

By optimising the tool, a reduction of the main time by 70% and a reduction of the energy requirement during the main time by 60% was achieved. The most important change in the tool geometry was a new arrangement of the guiding chamfers to prevent the drill jamming in the existing bore. In addition, the cutting material and coating were changed.

Tool chatter also limits application productivity

A further opening for raising productivity, and thus the energy efficiency, is to raise the feed rate per revolution. With suitable workpiece geometry, it is possible, in fine machining with multiple-blade reamers, to use tools which possess almost no flute space. As a result, the number of blades, and thus the realisable feed rate, can be increased substantially.

In many cases, the productivity of a cutting process is limited by the tool chatter. Without it, the tool, and also the power of the main spindle, would allow higher productivity (a higher feed rate and/or a greater depth). In order to avoid the self-created vibrations during chatter, however, feed-rate and depth must be limited. In such cases, the productivity can be raised substantially if the chatter tendency of the tool is reduced.

Two examples for tools optimised in this respect are shown on the next page. On the right, a tool is represented with its blades at different twist angles. As a result, the individual frequencies are incited less, which leads to a reduced chatter tendency and to a higher cutting volume.

On the left, the tool geometry of special high-feed mills in comparison with conventional torus mills is outlined. These tools are often used in tool and mould construction. The high-feed mills traverse with extremely small axial infeeds and very large feed-rates, since the angle of attack in these tools is very small. This means that the radial force on the tool is relatively small and the axial force greater. The tool is very stiff in the axial direction, making the high axial force unproblematic. In the radial direction, the tool is comparatively pliable. The smaller radial proportion of the total force, with the small adjustment angle, leads to very high feed-rates and to a substantially higher metal removal rate compared with torus and cylindrical mills.

A good possibility for reducing both main and secondary times substantially is offered by combination tools. In large-scale series production on processing centres, they often permit several processes to be carried out with one tool. This means that, on the one hand, tool changing time and the energy consumption associated with this time are reduced. On the other hand, simultaneous processing at several points also reduces main times considerably. For example, one device of this kind unites nine different working steps in a single one tool. In some cases, this type of combination tool also allows for different kinds of process to be carried out.

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