Briefing - Trochoidal milling’s time is now

Q: What is trochoidal milling? Is it new? A: Trochoidal milling, sometimes called wave milling, sees the cutter orbit about the Z-axis as it moves forward into a cut. It is not a new process, but it is not used as widely as it could be. Q: And what is the benefit of this orbiting motion? A: The main advantage is reduced tool contact angle. In conventional techniques, the contact angle is 180º (full cut); in trochoidal milling, this can be limited to 70º, as a smaller cutter is used to machine the same slot width. Q: Okay, the reduced contact angle is good, but why? A: Less heat is developed and the machining forces are also reduced, so spindles with low power and torque can be used successfully. In addition, the chip load is lower and, as the milling cutter does not completely fill the slot or pocket that's being machined, chip congestion and re-cutting of chips are, for all practical purposes, eliminated. This is particularly important for slots that are deep in comparison to their width. In these cases, if conventional milling is used, low feed rates and cutting depth are essential to avoid problems with chips – a maximum milling depth of 1.5xD is typical. With trochoidal milling, by avoiding the full cut, cutting depths of up to 2xD are achievable, even without the use of specially developed milling cutters. Q: So, in the round, what are the full benefits of the process? A: Shorter machining times, better tool utilisation and significantly reduced tool wear, plus lower initial tool prices, because of the smaller diameter of the tools used in trochoidal applications. Furthermore, because a wide range of slot widths or pocket sizes can be produced with the same tool, this frees up space in a machine's tool magazine and also means fewer tool changes are needed. Finally, because radial forces are lower, parts that are more delicate or that have thinner walls can be machined, while greater accuracy can be achieved. Q: Standard cutters can be employed, but are there specific cutters that offer greater benefit? A: Yes. To maximise benefits, tools have been specifically designed for this technique. These are readily available with a 2xD cutting length, but new milling cutters with a cutting length of 3xD and diameters from 6 to 20 mm have recently been introduced. The latter are part of Mapal's OptiMill product family. These have unequal spacing of their five cutting edges, combined with an innovative geometry design that greatly reduces resonance and vibration. Balancing during the manufacturing process reduces vibration further, allowing these cutters to be used at high spindle speeds. And having an integrated chip breaker, chip jamming is avoided. Importantly, no special tool adaptor is needed to use these tools, although chucks with good damping properties and high holding force are recommended. Q: Can you offer any specific machining examples? A: In a trial that was carried out recently, with conventional milling of case-hardened steel (16MnCr5), a maximum cutting speed of around 200 m/min at a feed rate of 0.05 mm per tooth was possible. With trochoidal milling, using a high performance solid carbide cutter developed specifically for trochoidal applications, a cutting speed of 600 m/min at a feed rate 0.1 mm per tooth was easily achieved. Even when machining V2A steel, it proved possible to achieve significant improvements: with trochoidal milling and a feed rate of 0.05 mm per tooth, a cutting speed of 250 m/min was possible, compared with just 60 to 100 m/min for conventional milling. Q: So these special tool developments now make trochoidal milling more attractive? A: Yes. But more than that, these are complemented by powerful CNC controllers and CAM software. That's why the time is now right for trochoidal milling.