A Toolpath Like No Other

Featured on www.americanmachinist.com - April 2005

Unconventional paths take cutters to machine speed limits and beyond

Written by Charles Bates, Senior Editor, American Machinist

Thanks to a CAM software engine that generates special toolpaths, operators may never again have to adjust a machine's feed override. That's because these toolpaths let shops machine any part-feature geometry as if every cut made is along a straight-edge wall at a constant depth of cut. Maintaining such a best-case, cutting-condition scenario permits machining at and, in most instances, above recommended cutting parameters along an entire toolpath without adjusting machine speeds and feeds to prevent tool breakage.

The patented TrueMill toolpath engine within Surfcam, according to its manufacturer, Surfware Inc., of Westlake Village, Calif., is not a toolpath-optimization program because that implies modifying an existing toolpath. TrueMill doesn't use any type of standard toolpath. Instead, it generates its own tool motion right from the start of the programming stage, basing movements on maintaining cutter-engagement angles with material. Glenn Coleman, vice president of product design at Surfware, explains.

Typically, shops use one or more toolpaths to machine a part feature, and programmers select geometries for defining that feature. Conventional CAM software then automatically replicates the tool movements a machinist would use to manually cut the feature, meaning part geometry drives the toolpath.

For example, to machine a rectangular-shaped pocket, conventional software makes a series of cuts each offset by the pocket's outer boundaries with a user-provided stepover value (a percentage of the cutter diameter). The resulting cut paths basically start in the middle and mirror the pocket's finished shape.

Every step-over value, says Coleman, equates to one specific tool-engagement angle, which holds true only when cutting in a straight line while maintaining constant radial depth-of-cut (best-case scenario). Holding engagement angles steady generates even chip loads on cutters, allows for increased machining parameters (speeds and feeds) without tool breakage, produces smooth surface finishes, and increases tool life. Unfortunately though, engagement angles fluctuate wildly even with the simplest of toolpaths.

For instance, a shop specifies a 50% stepover, which equates to a 90 engagement angle, for machining a rectangular pocket. At the desired pocket depth and when making the initial cut, the tool is fully engaged, or "buried," and the engagement angle rises to 180.

Once cutting straight, the angle returns to normal, but then increases again in the pocket corners, forcing operators to set speeds and feeds according to these worse-case scenarios for the whole toolpath. This means that tools run slower in places they could be going faster.

Instead of mirroring part-feature geometries and maintaining stepovers, TrueMill moves tools on a path that keeps the first cut and all following cuts at or below tool-engagement thresholds. As these toolpaths near the desired part-feature shape, the toolpath engine completes it with a light finish-cut.

"To most, TrueMill toolpaths look completely foreign," comments Coleman. They have no sharp directional changes, so tools never come to a complete stop. This happens often in traditional toolpaths, a tell-tale sign being circular dwell marks in corners.

Shops program TrueMill the same as conventional CAM systems. NC programmers specify geometries, cutters, axial depths of cut, and stepovers. The engine then determines a corresponding engagement angle and structures a toolpath to maintain it.

"Also, TrueMill gives shops running lights-out operations more confidence to take full advantage of the engine's positive impact on cutting speeds as well as tool life," says Coleman. "Such shops no longer have to run overnight jobs at slower-than-normal speeds to prevent tool breakage."

Surfware's initial release of TrueMill handles 21/2-axis milling, used in most high-volume production environments. However, the company is incorporating the engine into its mold-roughing toolpath product and 3- axis surfacemachining package. According to Coleman, the engine's principle would also apply to 5-axis applications.

In test cuts, TrueMill toolpaths had cutters running at double the recommended feeds/tooth. And since tool-engagement angles are constant, so too is the load on the machine tool. Even at these aggressive cutting rates, the load on the spindle stays well below its limits, says Coleman.

"We are not dictating how fast shops should cut," he explains. "But we are suggesting that with the toolpath engine it's possible to start at factory-suggested spindle speeds and feeds and work up (increasing them) from there." Typically, he adds, shops work down from these rates.

On a test machine with a 10,000-rpm spindle and a 300-ipm feedrate limit, cutters following a TrueMill toolpath immediately ran to these parameters, literally cutting as fast as the machine could. So Surfware switched to a faster machine and further increased cutting speed.