by Andrei Petrilin
How turn-milling can increase cutting efficiency
Turn-milling is a process whereby a milling cutter machines a rotating workpiece. This method combines milling and turning techniques and has many advantages, but only relatively recently the introduction of multi-tasking machines has allowed turn-milling to display its benefits.
For years, even under mass adoption of CNC technology, development of metal cutting machine tools was traditional enough, when progress of specific machines like turning, milling or drilling moved towards a separate direction. If machining centres already successfully integrate machining by rotating tools–milling and drilling–turning CNC machines continued with their own progress. Looking for new ways to make the manufacturing process more efficient by reducing settings of a machined part and its transfer from one machine to another led to adding a tool head with a rotary drive to CNC turning machines and allowed realization of turn-milling. Today, modern multi-tasking machines feature additional axes of the head movement, advanced control systems and upgraded software that provide the opportunity to perform the majority of machining operations with only one setting per workpiece.
In turn-milling, there are two kinds of machining: peripheral when axes of a workpiece and a cutter are parallel; and face, for which these axes cross. Peripheral turn-milling is similar to milling by helical interpolation and may apply to external and internal surfaces of the revolution, while with the use of face turn-milling only, the external surfaces can be machined. Despite that turn-milling seems to be very similar to turning (“turning by rotating mill”), there is a substantial difference between these two machining processes. The cutting speed in turn-milling is defined by the peripheral speed of the milling cutter and not by the rotary velocity of the workpiece like in turning. The workpiece rotation relates to feed.
What are the advantages of turn-milling and where is its application practical? First of all, machining of non-continuous surfaces may cause interrupted cutting (various grooves, undercuts, etc.). In classical turning, this operation results in unwanted impact load, poor surface finish and early tool wear. In turn-milling, the tool is a milling cutter that is intended exactly for interrupted cuts with cyclic load.
Machining materials produces long chips. In turning, chip disposal is difficult, and finding a proper chipbreaking geometry of a cutting tool is not a simple task. The milling cutter used in turn-milling generates a short chip that considerably improves swarf handling.
Take for example, machining eccentric areas of rotating components such as crankshafts or camshafts. In turning, off-centre masses of these components (crank journal, eccentric cam, etc.) cause unbalanced forces that adversely affect performance. Turn-milling with its low rotary velocity of a workpiece gives the possibility of preventing this negative effect.
Also, consider machining heavy weight parts. Their rotation, which defines cutting speed in turning, is connected with limitations of the main drive of a machine tool. If the drive does not allow rotation of large masses with required velocity, cutting speed is far from the optimal range, and turning performance will be low. Turn-milling provides a way to overcome the above difficulties effectively.
However, productive machining with the use of the turn-milling method demands right cutter positioning with respect to the workpiece, correct choice of insert geometry and tool path. Cutter positioning, for instance, influences form errors, and insert geometry, surface finish. Usually the final shape is produced by a wiper insert, which is mounted on the cutter. The questions of applying turn-milling, tool choice and defining cutting data deserve fuller consideration and should be examined specifically.
Introducing turn-milling in a manufacturing process can solve serious problems and substantially improve your output. Productivity using this relatively new and promising machining method is possible when you have a suitable modern machine and correctly chosen cutting tools. SMT
Andrei Petrilin is technical manager, indexable milling with Iscar, based in Tefen, Israel.