Production Machining

JUN 2018

Production Machining - Your access to the precision machining industrial buyer.

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Rotational Turning Bypasses Grinding By Barbara Schulz F or applications such as radial seals, bearing surfaces and bearing seats on shafts and axles, the workpieces must be machined absolutely cylindrically and scroll- free (without machining marks). In order to meet these close-tolerance specifications, most hardened parts are transferred from the lathe to the grinding machine for final machining. Rotational turning technology elimi- nates grinding because of special kinematics between the rotating workpiece and a rotating tool. Hard turning has long established itself as a machining method to eliminate grinding and its associated high tooling costs, refixturing of the part and relatively long processing times. However, like every technology, hard turning has some disadvantages. One of these disad- vantages is caused by the path of the tool during turning operations. Since the edge of the tool moves one unit in axial direction during one turn of the workpiece, the chip is cut off from the surface helically. e depth of this gener- ated groove or twist depends on the construction of the tool, the cutting parameters chosen and the hardness of the machined surface. Because of this micro-thread or scroll structure on the workpiece, some applications for hard-turned surfaces can be limited. For example, hard turning is not recom- mended, without a final grinding process, on surfaces below sealings or treads of needle roller bearings. ere are several methods available from machine tool manufacturers that accomplish scroll-free hard turning. Rotational turning developed by German builder Weisser Corp., with U.S. operations in Greer, South Carolina, is one of these machining methods. Rotational turning was devel- oped to eliminate the grinding process while creating a twist-free surface on a turning machine. e starting point for development of these alterna- tives to conventional turning processes was the elimina- tion of axial feed of the cutting tool, which is the cause the unwanted micro-threads. In contrast to tangential turning operations, where a slanted blade is only linearly moved along the Y axis while the workpiece is turning, rotational turning involves an additional rotational movement of the cutting tool. is tool is better described as a "blade." Instead of a single-point turning insert shaped like a triangle, diamond or circle, this rotational turning tool uses a special-geometry straight or helix-shaped blade that vaguely resembles the blade of a turbine. e curve of this cutter presents a slanting edge to the spinning workpiece. Pivoting the tool turret in C axis sweeps this edge across the work, so the cutting action proceeds across the width of the blade from one side of the blade to the other. Depth of cut is 0.1 to 0.2 mm. Since the cutting action gets spread across the blade (not concentrated at a tip the way it is on a conventional turning insert), tool wear occurs much more slowly. Weisser has developed the rotational turning kinematics in its machine and works with three exclusive cutting tool suppliers who supply the company with blades built to Weisser's proprietary geometry. According to one of its cutting tool manufacturers, Mapal, the key factor is the large length of the insert. Combined with the contact angle, it gives the Mapal tools an effective machining length of up to 45 mm, with which a surface roughness of less than Rz 2 microns and radial runout accuracies of 3 microns are achieved. e cutter range offers VDI and HSK-T connec- tions in versions as single carrier with one insert or as double carrier with two inserts. e tools can be installed vertically or horizontally, depending on the workpiece. One example is a gear wheel hardened to 58 to 60 HRC, featuring a root length of 25 mm that needs to be machined. A horizontally mounted single carrier with a HSK-T connec- tion is used. e total machining allowance of 0.18 mm is removed in two working passes. e roughing pass (ap = 0.15 mm) is performed with a feed rate of as much as 0.7 :: Hardened workpieces, such as this shaft part, are candidates for rotational turning to eliminate grinding and produce scroll- free, critical surfaces. TECH BRIEF TECH BRIEFS 26 PRODUCTION MACHINING :: JUNE 2018

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