Manufacturers never stop seeking ways to increase the speed, quality and cost-efficiency of their machining operations. To fill that demand, suppliers of machine tools, CAM software and tooling must continually develop new products and application strategies. By Teun van Asten, Engineer Marketing Services Solid Milling, Seco Tools.

Combined products and strategies address specific machining situations to provide the highest productivity. Current progress in rough milling operations, for instance, clearly illustrates the benefits of applying advanced metalworking technologies in an integrated way.

Elements of the process

The machine tool is the foundation of the milling process. To rough mill effectively, a machine must possess enough rigidity to resist cutting forces, as well as the capability to accelerate and decelerate axes at rates that maximise metal removal and minimise wasted time between cutting passes. Also, a powerful spindle facilitates high cutting speeds and aggressive application parameters. And finally, a machine’s CNC system must have computing power to sufficiently look ahead and meet rapidly changing demands for machining power and movement of linear and rotary axes.

CAM software determines what those demands will be. In generating milling toolpaths, software developers must consider milling processes ranging from simple to complex. A fundamental milling operation is side milling, in which an end mill engages a workpiece with the side of the tool at certain axial (ap) and radial (ae) depths of cut. A simple side milling operation involves minimal change in the tool’s radial engagement, or arc of contact, during each pass. Consequently, an operator can adjust the arc of contact to take full advantage of productive end mill design features, including multiple flutes and thick reinforced cores. Increasing the number of flutes permits higher feed rates and strong tool cores resist heavy machining loads.

Slot milling, however, poses a different situation. In slot milling, the full diameter of the end mill is engaged in the cut, creating a 180-degree arc of contact. The load on the cutter is much greater than that generated in side milling, and chips are more difficult to clear, which means they may be recut and eventually jam the cutter and break it.

Milling workpiece materials with poor thermal conductivity such as stainless steel, titanium and nickel-base alloys exaggerate the problem, concentrating heat in the cutting tool and accelerating tool failure. As a result, it may be impossible to utilise the tool’s full axial depth of cut capability. Completing a part will require multiple steps at increasing axial depths that will lengthen the machining time. Slot milling typically also requires the use of reduced cutting speeds and feed rates, further diminishing productivity.

Advanced CAM software facilitates producing slots by enabling users to carry out complex cutting strategies such as trochoidal cutting. In a trochoidal toolpath, the software guides a tool of a smaller diameter than the desired slot in repeated circular movements in the X and Y axes. Radial engagement of the tool is less than half its diameter, and the circular toolpath effectively changes slot milling into side milling. The tool’s arc of contact is greatly reduced, thus enabling the use of multi-flute end mills and longer axial lengths of cut to increase metal removal rates and reduce cycle times.

Between the extremes of simple side milling and full-engagement slotting are milling operations that involve machining of varying concave and convex contours. These situations are increasingly common as end-user part requirements become more complex, magnifying the need to efficiently cope with changing contours as well.

The key issue is control of the tool’s arc of contact with the workpiece. When milling a concave feature, the tool’s arc of contact grows, increasing loads on it and the machine tool. The converse is true when milling a convex feature; arc of contact decreases and cutting efficiency suffers.

Software suppliers have developed and refined toolpath algorithms that control tool engagement in real time, enabling highly productive and reliable roughing of both simple and complex contours. These toolpaths typically combine large axial depths of cut (ap) with small radial depths of cut (ae), high feeds per tooth (fz) and high cutting speeds (vc) that significantly shorten cutting times and increase metal removal rates. In addition to providing greater productivity, these advanced roughing strategies feature smooth cutting paths without rapid changes in direction or cutting parameters, thus reducing the load on the tool to greatly improve tool life.

The latest software strategies for rough milling toolpaths take two basic approaches. One approach applies the tool at a constant feed rate and arc of contact when milling a concave or convex part feature and varies the step over between passes to achieve maximum metal removal rates. The second approach maintains a constant step over but varies the feed rate and the tool’s arc of contact, with the intent of maintaining constant chip thickness. In the second approach, the tool’s arc of contact can be as large as 80 degrees or even 140 degrees, depending on the brand of software in use.

Tools for advanced roughing

The differences in the two roughing approaches dictate application of different tool designs. In many cases, the first approach permits the use of cutting tools with multiple flutes and double/reinforced cores where desired. And, users can employ high ratios of axial to radial depth of cut.

The second approach can potentially remove metal at a greater rate, but cutting tools must be able to evacuate greater volumes of chips. Tools engineered for use with the second approach typically feature fewer flutes and have open core designs that promote chip flow or are applied with defined flute cavity shapes (high performance tooling).

In general, tools designed for large depths and widths of cut and heavy loads will be effective in either approach. In the second approach, however, multi-flute tools and tools with reinforced core designs will face chip control problems when encountering large arcs of contact given large axial depths. Usable ratios of axial depth of cut to radial depth of cut are more limited.

In some applications, adding certain tool geometry features will enhance tool performance. For example, Seco added chip splitters to the cutting edges of its Jabro JS554-3C end mills. These chip splitters are a series of small notches or grooves on the flutes spaced at a distance of one times the tool diameter.

Regardless of the axial depth of cut, the chip splitters break the chips before they become long enough to interfere with smooth evacuation flow. As a result, the end mills can operate at axial depths of cut up to 3.5 or four times tool diameter. Applied with advanced roughing CAM software programs, these tools can reduce cycle times with 60% to 70% as compared with traditional methods. This cycle time saving is due to both higher metal removal rates as well as more efficient toolpath cycles.

In tools designed for use in the second advanced roughing approach – which employs varying arcs of contact and feed rates – different geometry details are required for top performance. The tools must allow adequate room for chips to form and evacuate. An example is Seco’s Jabro JS554-2C end mill, which features a core that tapers from front to back – a design that provides more space in the flute cavities for chip flow. Tapering the core does somewhat reduce tool strength, and as a result the tools are not suited for full-diameter slotting. While on the other hand, the end mills feature modified tips that increase performance applying helical interpolation ramping. This selection of tools, for instance, can rough mill at up to 2.5 times tool diameter.

Advanced roughing software, used with tools engineered to maximise its benefits, boosts stock removal per each milling pass and results in faster cycle times. A case in point is a mould/die application that involved machining of an aluminium motorcycle engine cover. A manufacturer sought to reduce the 15-hour cycle time of the operation when it was performed with a high-feed roughing approach alone.

To improve the process, a Seco Jabro JS554-3C cutter was applied using advanced roughing software that maintained a steady arc of contact by varying step over. The method permitted the bulk of the roughing to be accomplished in a little over 2.5 hours. Sufficient material was removed so that a following high-feed roughing operation required only four hours to complete. Overall, roughing time was reduced by 55%.


As in any machining operation, specific tools and strategies should be applied with all aspects of the manufacturing situation in mind. For example, when tool cutting length exceeds four times tool diameter or when tools are applied with long overhangs or on unstable machines or workpieces, high-feed milling is possibly a better alternative to the advanced roughing approaches described in this article. By combining shallow depths of cut with high feeds per tooth, high-feed milling methods direct cutting forces axially into the machine spindle to help stabilise the machining process.

Different application strategies suit other manufacturing situations. In some cases, high-speed machining, which focuses on light passes at high spindle speeds and feed rates, or high performance machining that employs conventional feeds and speeds in combination with very high axial and radial depths of cut, will be the best choice. In all examples, combining advanced milling strategies and software with tools engineered for maximum performance at those parameters will produce optimum results in terms of metal removal rates, tool life and cost efficiency.