Patented pockets: General Industry Coverage

Courtesy of Kennametal

CAM systems generate innovative toolpaths to quickly remove large amounts of material when endmilling.

Whether hogging out a prototype engine block for Chrysler’s next-generation K-car, or machining a mold cavity for the newest Barbie doll, staying competitive often equates to fast chipmaking.

High-quality cutting tools and advanced CNC equipment help in this endeavor, but equally important is a machining strategy that best fits a shop’s processes. With unprecedented ease of use and highly efficient toolpaths, today’s CAM systems give shops the ability to maximize tool life, part quality and metal-removal rates.

ABCs of Endmilling

There seem to be as many milling techniques these days as there are endmills. Which technique is best depends on a number of factors, including workpiece geometry and material, machine rigidity and cutter selection.

One such technique is trochoidal milling, which uses a continuous circular motion to remove material. Old-timers may remember the Spirograph, a classic child’s toy in which a ballpoint pen is inserted into a plastic gear-like disc and guided around a piece of paper to generate geometric drawings. Replace the pen with an endmill and the cutter will follow a trochoidal toolpath. This technique excels at milling deep slots, where a high percentage of cutter engagement and challenging chip evacuation is the rule.

Helical ramping, a process similar to thread milling, utilizes center-cutting endmills to corkscrew into a workpiece when beginning a pocketing operation, saving the time needed to drill a starter hole.

Other milling techniques include roll-in milling, which avoids the hammering seen with a conventional head-on facemilling approach and uses an arced entry path to climb into the workpiece. This method creates a thick-to-thin chip, which efficiently transfers heat and extends tool life. In addition, slice milling cleans the corner leftovers that remain after roughing tools have done their work, and spiral morph milling takes the roll-in technique a step further, generating a spiral-shaped toolpath to quickly machine pockets and avoid the sharp corners that can cause chatter and tool breakage.

Courtesy of DP Technology

CAM programs utilize a variety of metal-removal techniques, including trochoidal slotting and corner “picking.”

Courtesy of DP Technology

Traditional milling techniques do not avoid abrupt directional changes and high engagement angles, shortening tool life.

CAM developers do all this and more, resulting in a number of copyrighted and even patented milling engines. For example, Mastercam touts its Dynamic Milling, calling it “an active toolpath that delivers more consistent cutting conditions and allows the use of the entire tool flute length while reducing machining time.”

The latest version of Surfcam Inc.’s TRUEmill offers rest roughing and contour ramping as part of its toolpath-generating bag of tricks, which promises to extend tool life 10 times and reduce machining time by up to 80 percent.

SolidCAM Ltd. released its patented iMachining technology, which uses morphed circles and D-shaped toolpaths to cut cycle times in half, according to the company.

These are just a few of the claims made by CAM developers. Are they true? Dave Bartholomew thinks so. As technical marketing specialist for DP Technology Corp., Camarillo, Calif., Bartholomew points to ProfitMilling, the high-speed milling cycle developed as part of DP’s Esprit CAM package. Using a patent-pending toolpath algorithm, ProfitMilling is said to simultaneously control multiple cutting parameters. “Our approach is a little different,” Bartholomew said. “Where some systems focus on a single parameter—engagement angle, for example—we also look at chip load, feed rate and other considerations that impact cutting forces.”

Cutting Corners

By dynamically adjusting the feed rate based on where tools are positioned in the cut, ProfitMilling follows the laws of physics. Bartholomew said: “Traditional toolpaths, with a single feed rate, work fine if cutting in a straight line. But if you ever rode on a merry-go-round as a kid, you know that the closer you are to the edge, the faster you spin. This is similar to a morphing spiral toolpath used to rough out a pocket. You need to constantly adjust the feed rate to maintain consistent cutting conditions.”

Even more important to an efficient toolpath is avoiding sharp corners. By eliminating the abrupt directional changes often seen in traditional toolpaths, Bartholomew said ProfitMilling never buries the tool. “Corners are bad,” he said. “Using a straight line again as an example, you can see it’s easy to maintain a constant chip load and engagement angle. But once you start heading into a corner, that all changes. You’re going to engage the entire tool. There are a lot of problems with that.”

Those problems include poor chip evacuation, high cutting forces and excessive heat, any one of which can mean a bad day for a cutting tool and substandard milling performance.

“In a traditional step-over-style toolpath, the width of cut is inconsistent. You go into a corner and you’re buried,” said Glenn Coleman, chief product officer for Celeritive Technologies Inc., Cave Creek, Ariz., developers of VoluMill. “You come to the end of a cut and you’re typically not engaging the material. Then, you make a sharp right-hand turn and you’re fully engaged again. This is not only inefficient; it’s also tough on the cutter.”

Keep it Constant

Coleman paints a vision of machining perfection: “If you know the tool is never going to be buried in a corner, that it will never have to accelerate or decelerate to negotiate sharp, abrupt turns, you can machine using parameters that are much more aggressive, with deeper depths of cut, faster spindle speeds and much higher feed rates.” Better yet, Coleman explained, high-performance toolpaths are actually kinder and gentler on milling cutters as well as the machine tool, even though it drives them much faster. “It’s all about eliminating undesirable machining conditions.”

CAM developers introduced trochoidal milling in an early attempt at improving milling conditions. Unfortunately, according to Coleman, the technique comes up short and is limited in terms of substantial material removal. “In the long run, it’s questionable how beneficial trochoidal milling actually is.” He explained that there are dozens of high-performance milling permutations, but all fall within a few basic categories.

So-called “stock gobblers” work by taking a cut, analyzing what’s left and then attempting to remove that material within a bounded cut width, an engagement angle or another parameter that tells the cutter how to best proceed. Other products emphasize chip thinning as the end game of high-performance milling.

Courtesy of CGTech

Side-by-side comparisons of ballnose endmill wear when following an optimized toolpath (right) and a non-optimized toolpath.

No one would argue that chip thinning isn’t a valuable technique, but it’s not without its own challenges. “You can do your chip-thinning calculations all you want, but as soon as the width of cut changes, your calculations are no longer accurate,” Coleman said.

Lastly, morphing spirals are great, but after you pass beyond a circular shape, you’re negating all the advantages of what that spiral offers. “As you get into narrow areas, your cuts get closer together,” Coleman said. “In the wider areas, you get cuts that are farther apart, ensuring that machining loads and chip formation are inconsistent.”

Courtesy of Celeritive Technologies

Celeritive’s VoluMill roughing engine provides the toolpaths for an endmill in Siemen’s NX CAM software.