In many applications, trochoidal milling – where the cutter follows a spiral path by moving radially as it rotates – provides important benefits that include faster machining times, lower tooling costs and reduced loads on machine components. Mapal UK's general manager, Wayne Whitehouse explains more. While trochoidal milling, which is sometimes also known as wave milling, is not a new technique, it is by no means as widely used as it deserves to be, yet it takes only a simple comparison of cutting data for conventional and trochoidal milling techniques to begin to reveal the benefits of the latter.
In a trial undertaken recently, with conventional milling of case-hardened steel (16MnCr5), a maximum cutting speed of around 200m/min at a feed rate of 0.05mm per tooth was possible. With trochoidal milling using a high-performance solid carbide cutter, however, a cutting speed of 600m/min at a feed rate 0.1mm 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 0.05mm per tooth, a cutting speed of 250m/min was possible, compared with just 60 to 100m/min for conventional milling.
Take the right path
This increase in machining performance is made possible by using milling cutting paths developed specifically for trochoidal applications as well as by the process itself.
The main advantage during the machining operation is reduced tool contact. When a slot is milled using conventional techniques, the wrap angle is 180º (full cut), but with trochoidal milling the wrap angle can be selected as a function of the cutting depth and limited to a maximum of 70º. This is possible because a milling cutter with a smaller diameter is used, which moves trochoidally at a high spindle speed with each trochoid penetrating deeper into the slot.
The reduced wrap angle means that less heat is developed. The machining forces are also reduced, which means that even spindles with low power and torque can be used successfully for trochoidal milling. In addition, the chip load is lower. 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 with 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, the situation is very different: by avoiding the full cut, cutting depths of up to 2xD are achievable even without the use of specially developed milling cutters.
A short cut to benefits
Trochoidal milling brings a whole range of cost benefits, since it offers shorter machining times, better tool utilisation and significantly reduced tool wear. Rapidly increasing carbide prices mean that reduced tool wear is now a particularly important benefit, especially as the resulting extended tool life is complemented by lower initial tool prices because of the smaller diameter of the tools used in trochoidal applications.
Further cost savings are possible when trochoidal milling is used in machining centres, because a wide range of slot widths or pocket sizes can be produced with the same tool. This frees up space in the tool magazine, and also means that fewer tool changes are needed. Trochoidal milling also offers benefits in terms of component design. Because radial forces are lower, parts that are more delicate or that have thinner walls can be machined, and greater accuracy can be achieved.
To maximise the undoubted benefits of trochoidal milling, tools that have been specifically designed for this technique should be used. These are readily available with a 2xD cutting length but, for applications with even more demanding requirements, new milling cutters with a cutting length of 3xD and diameters from 6mm to 20mm have recently been introduced.
These new cutters, which are part of the Mapal OptiMill product family, have unequal spacing of their five cutting edges combined with an innovative geometrical design that greatly reduces resonance and vibration. Balancing during the manufacturing process reduces vibration even further, allowing the new cutters to be used at extremely high spindle speeds.
Cutting the chips
To ensure that there are no problems due to the production of long chips that might cause jamming, even with the greater cutting depths now possible, the new tools have an integrated chip breaker that cuts the chips in half. A further benefit of these innovative tools is that no special tool adaptor is needed, although the use of chucks with good damping properties and high holding force, such as those in the Mapal HTC range, is recommended.
The growing demand for smaller and more delicate parts and the constant pressure for reductions in tool, operating and maintenance costs are making it more than ever essential to find ways to improve the efficiency of milling operations. Thanks to the availability of specially optimised tooling, complemented by the increasing use of powerful CNC controllers and CAM software, the time is now right for trochoidal milling to make a major contribution.