Whatever the industry, in virtually all machining disciplines, companies are searching for efficient ways to streamline processes.

Straightaway to a perfect finish and perfect quality: What in the past would have been hardly conceivable in milling is now becoming a regular trend due to modern machine concepts and tools. The number of precision applications in which milling is no longer followed by grinding, polishing or eroding is growing continuously. In these applications the toolholding systems perform several tasks: they ensure exact run-out accuracy of the tools, compensate vibrations, and guarantee both high geometric precision and perfect surface quality.

Whether in the watch and clock industry, machine making or medical technology: in virtually all machining disciplines, companies are searching for efficient ways to streamline processes. Especially manufacturers who use ultra-precision machining processes, in which the engravers and finishers put the final touch on surfaces by hand, are pioneers in this respect. In view of machining times of up to 100 hours for a single workpiece, investments in innovative machine concepts, air bearing spindles, and modern tools pay off quickly – assuming the high precision of the machine is retained all the way to the cutting edge. The toolholding systems play a decisive role in this connection.

Mirror finishes with precise geometry

The levels of quality that can be achieved with precision machining nowadays fascinate even experienced users. Often the quality that can be achieved is equal to the results of eroding, grinding, polishing or laser-beam machining, yet much faster and therefore more economical. Test series with an aerostatically surface guided ultra-precision machining centre at the ETH Zürich show that a consistent surface quality of Ra < 25 nm can be achieved with line-by-line milling, and Ra < 3 nm with surface milling. These levels of quality correspond to polished surfaces, and also exhibit high-precision geometry. While injection moulds for high-gloss plastic parts in the past were first milled and then finished using an intricate polishing process, today it is possible to produce objects with extremely flat and smooth surfaces during the precision machining process. The effect is even more pronounced in the case of non-ferrous metals: through milling alone with diamond tools, it is possible to achieve geometrically precise mirror-finish surfaces that are suitable for use in laser optics, for example. The result is a combination of several effects: the time-consuming process of finishing is reduced significantly, while reducing the risk of convexities occurring or corners being rounded during grinding and polishing.

Wear-free clamping

Conventional toolholding systems, such as collet chucks or heat-shrink toolholders, generally are not capable of such demanding machining tasks. Users repeatedly complain of chatter marks, damaged tools, imprecisions in the workpiece and concentricity errors, which are caused by minute contamination of the clamping faces. However, Schunk Tribos polygonal clamping technology features special properties: even the standard version of the patented technology from Schunk, the competence leader for clamping technology and gripping systems, achieves run-out and repeat accuracy of < 0.003 mm with an un-clamped length of 2.5 x D, and a balancing grade of G 2.5 at 25,000 rpm. Since Tribos polygonal toolholders have no moving parts, they are not mechanically sensitive, and there-fore ensure virtually maintenance-free and wear-free clamping. Even after several thousand clamping set-ups there is no material fatigue. In addition, they feature excellent vibration damping. With a hydraulic toolholder, tool change is achieved with minimal cleaning within a few seconds, to ensure a stable process. Depending on the type, the toolholders, which are suitable for all tool shanks in h6 quality, have been tested at speeds up to 205,000 rpm. Even tools with very small shank diameters starting at 1 mm can be clamped and changed while maintaining process stability.

Due to the high demand from companies in micro mould making, the optical industry, medical technology, and the coin, clock and watch, and jewellery industries, Schunk has recently expanded its standard product line to include ultra-fine balanced Schunk Tribos polygonal toolholders. Since then the Tribos-Mini and Tribos-RM series with the interfaces HSK-E 25, HSK-E 32, and HSK-F 32 starting with clamping diameters of 0.5 mm are also available with a balancing grade of G 0.3 at 60,000 rpm. The ultra-precision toolholders offer outstanding features for implementing the most demanding tasks with respect to dimensional stability and surface quality. Compared to conventionally balanced toolholders for micro machining, the tool life is also improved.

Reduction of chatter marks in volume machining

That toolholders also have a significant effect on the surface quality in volume machining was confirmed in a study by the wbk Institute for Production Technology in Karlsruhe, which was conducted under the supervision of Prof Dr-Ing Jürgen Fleischer and presented in the year 2014. Different toolholders were tested in full slot and half slot milling on several machines. It was demonstrated that the Schunk high-performance hydraulic expansion toolholder TENDO E compact achieves up to 300% longer tool life than a com-parable heat shrink holder. Especially noteworthy: with only one exception the Schunk Tendo E compact hydraulic expansion toolholder always achieved better surface qualities than heat shrink toolholders. The deeper the grooves, the stronger was the effect of chatter marks. The damping properties of hydraulic expansion technology results in longer tool life as well as significantly better surface quality. Even at an identical surface quality it is possible to achieve higher cutting and feed rates.

Effective double for 5-axis machining

For high-precision 5-axis machining Schunk Tendo E compact hydraulic expansion toolholders, and Schunk extensions can be combined to produce extremely effective clamping units. In the case of minimal interfering contours the hydraulic expansion toolholders provide strong support for the extensions while damping the vibrations that occur during machining. The combination of run-out accuracy, and vibration damping protects the tool cutting edge, lengthens the tool life and provides for brilliant workpiece surfaces. For stable tool change, Tendo hydraulic expansion toolholders require only a conventional Allen key; for Tribos SVL, a simple, manually actuated clamping device is sufficient. The entire clamping process is completed within a few seconds. Both clamping devices are maintenance-free and insensitive to impurities. -HEINOLD KOSTNER

The author is Head of Product & Portfolio Management for Clamping Technology, Schunk GmbH & Co KG, Lauffen/Neckar.

Schunk Intec India Pvt Ltd, Bangalore. Tel: 080-40538999. Fax: 91-80-40538998. Email: info@in.schunk.com