Maximum-performance milling outperforms traditional milling by a long shot

Within the last decade, multiple endmilling developments have occurred. These include bold, new cutter designs, extremely tough but wear-resistant carbide grades and toolpaths that reduce mechanical shock to machine tool and cutter alike while reducing cycle times by 50 percent or more and extending tool life.

This new paradigm of metal removal makes it possible to successfully machine difficult-to-cut materials such as titanium and Inconel using even light-duty machine tools. Moreover, it can be done by anyone with a solid grasp of a few basic machining principles and a CAM system that generates the necessary toolpaths.

Straight-line peel milling on the edge of a workpiece. All Images courtesy M.A. Ford Manufacturing.

Old School

Before maximum-performance milling (MPM), shops took a more-traditional milling approach. This was not necessarily climb milling or conventional milling—although both play a factor in any milling operation—but rather the type of milling where a cutter is applied at low to moderate feeds and speeds to take heavy cuts, typically engaging 80 percent or more of the tool width and up to 1 diameter deep axially.

This approach can remove a large amount of material per pass, but it often causes minute chipping of the cutting edges, leading to unpredictable results and premature tool failure. The large amount of heat produced by such hogging cuts requires special tool coatings. Endmills require radial rake angles and core thicknesses specifically designed for this type of machining. Specifically, truncated, profile-style roughers, or corncob cutters, are needed to segment chips and reduce deflection during heavy cutting.

On manual equipment, such as manual knee or horizontal mills, this style of old-school milling was performed conventionally, meaning the cutter rotates opposite the direction of feed. This causes rubbing, deflection and heat. With the advent of NC machining centers and their enhanced rigidity, climb milling became possible. As a rule, “climbing” into a workpiece improves tool life and surface finish, but can cause chatter, which generally happens when taking a radial cut that equals 60 to 90 percent of the cutter diameter. This causes the rake face to slap the material rather than curling and severing it, and this technique is hard on the cutter and machine tool.

Trochoidal toolpaths are suitable in tight areas, such as slots and small pockets.

Machine builders responded by offering rigid machines with ample horsepower and torque, and duty cycles able to take long, continuous cuts under heavy loads. Machine spindles were likewise improved to endure the constant stresses involved.

Toolholders that eliminate tool pullout caused by high cutting forces were also required, but because machines of that vintage were anything but “high speed,” toolholder runout was not the concern it is today.

Peeling Out

Aerospace suppliers were perhaps the first to recognize the need for more-efficient machining processes. By optimizing stock removal and reducing the amount of power required to machine airfoils and other large surfaces, they realized significant energy savings. That optimization came in the form of radically new toolpaths, ones that remove relatively small amounts of material per pass but at much higher feed rates, while maintaining consistent cutter loads to prevent chatter and tool damage.

One of these is straight-line peel milling. It is defined as any toolpath that has a small radial DOC and large axial DOC that travels in a straight line along the inner or outer part of the workpiece. As with all types of peel milling, substantial increases in the feed rate and cutting speed are possible because of chip thinning, the formation of thick-to-thin chips that occurs while climb milling and taking light radial cuts. This reduces heat generation and cutting forces.