Patented pockets

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.

It’s this cutting path inconsistency that’s the problem, Coleman explained. “If you don’t have uniform chip formation, it’s difficult to control heat evacuation,” he said. “And if you don’t get the heat out with the chip, tool life becomes problematic.”

To counter this problem, Coleman added, Celeritive developed VoluMill, where the WOC never changes. This allows users to take advantage of the chip-thinning phenomenon, even during bulk-material roughing modes, by dynamically adjusting the feed rate to compensate for changes in the engagement angle and part geometry.

That’s No Yoke

As proof, Coleman pointed to AFC Tool Co. Inc., Dayton, Ohio. The company implemented VoluMill in hopes of improving production rates on a variety of aircraft yokes machined from 1.5"-thick, D-2 tool steel. Prior to the programming makeover, AFC removed material at a rate of 1.4 in.³/min., with cycle times in the low teens of minutes. Post-VoluMill, the mrr quadrupled and cycle time dropped to under 3 minutes, without increasing tool wear, noted Barry Davidson, AFC Tool’s CAM programmer/designer.

Optimization is more than just effective toolpaths, however. According to Bryan Jacobs, marketing communications manager for Irvine, Calif.-based CGTech Inc., CAM makers have a tough time defining the term optimization. “It largely depends on who you talk to,” he said. “Some vendors refer to optimization as the ability to automatically determine how much stock each cutter can machine without gouging the part. Others refer to optimization as improving the trajectory of cutting motion by using continuous tangent motion rather than sharp, interrupted movements. And, some define optimization very loosely, thinking of it as visualizing the machining process and imagining how to improve it.”

CGTech, known for decades as a simulation software developer, now offers OptiPath, its own version of the optimization story. By having the software adjust feed rates on existing CNC programs, Jacobs claims that manufacturers can cut machining time 15 to 50 percent, as well as extend tool life and impart finer surface finishes, without needing to reprogram the part or implement new hardware.

“OptiPath was developed because determining optimal feed rates presents NC programmers and machinists with a number of problems,” Jacobs said. “Typically, the selected feed rate represents a compromise between tool life, cycle times and the worst-case cutting condition encountered.”

By breaking existing toolpaths into smaller segments, users can obtain the best feed rate for each of these “subtoolpaths.” This avoids the situation seen in some CAM systems, in which the feed rate specified to handle worst-case cutting conditions is used throughout the entire toolpath.

Some companies use CGTech’s solution in a way that might even seem counterintuitive. “At Tell Tool, located in Westfield, Mass., the programmers take a radically different approach by programming a very aggressive feed rate, then using OptiPath to slow down their machines,” Jacobs said. “They have reported that it eliminates most of the chatter, gives a much better surface finish and results in fewer broken tools.”

Courtesy of CGTech

Varying the feed rate based on the cutting conditions is the essence of CGTech’s optimization solution.

If you’re skeptical about slowing the cutter to achieve higher productivity, join the club. Most machinists tend to do just the opposite. Yet Jacobs argues that “giving ‘er hell” isn’t always the right answer.

“Running at maximum feed rates and taking multiple shallow passes is often less efficient than taking fewer passes at slightly greater depths,” he said. “Achieving the shortest cutting time is related to feed rate, but the relationship is not necessarily that the fastest feed rate equals the most efficient cutting. High-efficiency machining—cutting a part in the least amount of time—is the real goal, and the best way to achieve this is to vary the feed to achieve an optimal rate for each cutting condition encountered.”

Bury the Tool

All this speeding up and slowing down the spindle is a little like driving in the Grand Prix. Don’t you need a racecar to compete? Mark Ohlfest thinks so. As an applications engineer for MC Machinery Systems Inc., a subsidiary of Mitsubishi Corp., Wood Dale, Ill., Ohlfest has plenty of opportunity to test the high-performance machining waters. His conclusion is that a rigid, powerful machine is always your best bet. “Higher horsepower means higher chip load,” he said. “That’s true whether you’re machining Inconel or aluminum. It’s all about how deep you can go and how fast you can rip the material out of there.”

Ohlfest works with a number of shops, especially in the die and mold arena, and opinions vary when it comes to CAM systems. “Talk to 10 different guys and you’ll get 10 different answers,” he said. “But basically, CAM systems have come a long way, no matter whose system you’re running.

“Most everyone agrees, however, that with the power available in both hardware and software, it only makes sense to cut deep,” Ohlfest added. “It used to be that CAM systems weren’t really dependable enough that you could bury the tool to full depth. You’d end up plowing into a corner and breaking the tool. That’s changed.”

Ohlfest made a simple statement with far-reaching implications for tool life: “Chip load is chip load, whether it’s axial or radial. Using a light depth of cut with a high feed rate takes the same horsepower as burying the tool and doing a shallow step-over with a trochoidal or similar toolpath. The difference is you’re going to wear out the cutter much faster with a shallow Z-axis cut, because you’re only using the tip of the tool.”

Not Drinking the Kool-Aid

Chris Tate, lead engineer at Mitsubishi Power Systems Americas Inc., Savannah (Ga.) Machinery Works and a CTE columnist, performs “hybrid programming” to optimize toolpath development. “Sometimes, I’ll do a complete program from Gibbs [CAM software], and then other times I’ll punch out a quick toolpath for the profile and copy it into a subroutine,” he said. “On the other hand, I might just program it by hand. It all depends on what I’m doing.”

This reluctance to embrace the full functionality of CAM systems is purely pragmatic. While Tate admits that CAM systems might do everything but sweep the floor, it’s often faster to tweak the program by hand.

“Take most any system out there, and, with a lot of time and effort, shops can generate good, usable code right out of the gate,” Tate said. “Unless you have a dedicated programmer, someone who can build all the models and get the post-processors just right, you’ll end up running back to the office to change the toolpath, repost it and send it back to the machine. Most shops are too busy making parts to spend the time necessary to fully develop their CAM systems.”

Tate’s not entirely onboard with the machine builders, either, and said a commodity machine is nearly as capable of high-performance milling as a “supermachine.” Because today’s optimized toolpaths generate less cutting force than their old-fashioned counterparts, even a light-duty machining center can machine Kryptonite. “Approach it the right way—which means using chip-thinning techniques and removing the causes of high radial forces—and even Inconel cuts like butter,” he said.

Just Say Go!

Want to get some of that “cuts-like-butter” performance? If you’re serious about fully utilizing your machining center, take heed. Tate is right. CAM systems are more powerful and easier to learn than before, but getting to the point of “post and go” still takes substantial effort—just reviewing all the sales demos can be a Herculean effort. To this last point, it makes sense to develop a test case, one that best represents an “average” part for your shop. Have each CAM vendor generate code. Then make some chips and compare. Only then should you sign on the dotted line. Happy milling. CTE

About the Author: Kip Hanson is a contributing editor for CTE. Contact him at (520) 548-7328 or khanson@jwr.com.

---

Contributors

AFC Tool Inc.
(937) 275-8700
www.afctool.com

CGTech Ltd.
(949) 753-1050
www.cgtech.com

DP Technology Corp.
(800) 627-8479
www.dptechnology.com

Celeritive Technologies Inc.
(888) 253-6701
www.celeritive.com

MC Machinery Systems Inc.
(630) 860-4210
www.mitsubishi-world.com

Mitsubishi Power Systems Americas Inc.,
Savannah Machinery Works
(912) 629-7187
www.mpshq.com