Making the most of tired CNC machining centers

You’ve probably heard the news: Milling has gone high performance. You’ve read the magazine articles about trochoidal milling, watched endless YouTube videos about high-feed and high-speed machining techniques and chatted with your cutting tools salesperson about plunge milling. Unfortunately, the newest machining centers at your shop were installed when Bill Clinton was president, and the oldest one still uses paper tape.

So what difference do these and other high-tech milling strategies make to you?

Image courtesy of Greenleaf.

Wouldn’t it be cool, though, to toss out those HSS corncob roughers with their 5-ipm (127mm/min.) feed rates and leverage some of these more productive milling methods?

According to Tomas Roman, sales and applications manager for GWS Tool Group, Tavares, Fla., nothing is stopping you

Bye-Bye Corncobs

“We get calls every day from shops that either can’t afford or aren’t yet ready to buy a new machining center but still want to get better performance out of their existing equipment,” he said. “Most of the time, it’s just a matter of adjusting a high-performance milling strategy to match the capabilities of your machine tool.”

One example comes from an aluminum shop in the Dakotas that Roman worked with. A vintage machining center could barely rapid-traverse at the feed rates specified by the CAM system’s trochoidal toolpaths, let alone interpolate at those values. Another example is a power generation company machining titanium. The company’s older equipment was unable to meet a tool’s recommended speeds and feeds.

Despite the apples-and-oranges difference between machining aluminum and titanium, the answer in each case was to increase step-over amounts and reduce feed rates, even though the shop’s CAM software didn’t agree.

Unless special plumbing is installed, some carousel-style toolchangers can limit cutting fluids to the right side of the spindle, inhibiting chip flow. Image courtesy of GWS Tool Group.

“You can program any job at 500 ipm, but a lot of times the machine can’t accelerate and decelerate that quickly, something that’s true even on some newer equipment,” Roman said. “One indicator that this problem is occurring is when the actual machining cycle takes longer than what the programming software says it should. But by taking a more aggressive step-over and easing back on the feed, we were able to actually increase metal-removal rates compared to the original toolpaths.”

However, the less-than-optimal cutting conditions generated greater heat, which caused premature tool wear. The toolmaker countered by adjusting the tool geometry to make the endmills “a little bit beefier.” This meant tweaking the relief and rake angles, changing the edge prep to match the application and switching to a more heat-resistant coating.

According to Roman, this beefed-up approach is effective in many “old machine” situations, whether or not a newfangled toolpath is used. That’s because the backlash and lack of rigidity common with older CNCs require a cutting tool able to take abuse without complaint.

“Often with poor machining conditions,” Roman said, “heat’s going to be an issue. Aside from that, tool performance comes down to a delicate balance of aggressiveness and strength, while at the same time applying a carbide grade that’s stout enough for less-than-optimal cutting conditions.”

Happy With High Feed

One milling strategy that works especially well on older machines is high-feed milling. Most often, a special resembling a flattened ballnose endmill is applied with a shallow Z-axis, or axial, DOC, relatively heavy radial engagement (upward of 50 percent) and high feed rates, all of which tend to thin the chip while directing cutting forces into the spindle. Roman said one customer was able to replace seven separate tools with a pair of high-feed milling cutters, reducing cycle time and tool costs alike.

Vic Dodd, rotating-product manager for Tungaloy America Inc., Arlington Heights, Ill., saw similar results from his own days in the shop. “I once had a couple of 1997 Bridgeports that I used for mold work and had great results with high-feed tools,” he said, adding that with older equipment like that, the spindle isn’t strong and the servomotors are weaker than those found on newer machines. “The programmers need to understand those limitations if they’re to utilize what’s available.”

Indexable high-feed milling cutters are an effective machining method for old and new machines alike. Image courtesy of Tungaloy America.