Smooth roughing: Turning Performance

When a toolpath is precisely dialed in for high-efficiency roughing (HER), the cutting tool sings a sweet song of potent productivity rather than moaning like it’s struggling for its life. “It’s not that grunting, groaning and grinding,” said Scott Tiehen about HER. “It’s like comparing a motor that is tuned up and humming well to one that has two or three cylinders not firing correctly.”

Tiehen is national sales manager for Helical Solutions Inc., Gorham, Maine, which makes high-performance cutting tools to enhance the productivity-boosting performance of software developed to generate toolpaths for HER. One such software offering is VoluMill from Celeritive Technologies Inc. “We realized the effectiveness of the VoluMill toolpath when we first saw it,” he said.

Moldmaking motion

Glenn Coleman, chief product officer for Celeritive, Moorpark, Calif., explained that CAM-system-independent VoluMill, introduced in 2008, controls the removal rate, or volume, of metal being milled at any given time so it never exceeds a user-controlled upper threshold. “What users experience is a consistent, predictable and known load on the cutting tools, machine tools and workholding components,” he said, adding that the software’s volume-based approach calculates cubic inches of material being removed by multiplying the DOC by the WOC by the ipm. “You can run speeds, feeds and depth of cuts that are considerably more aggressive than you can with traditional toolpaths, but you’re still being kinder and gentler on the cutting tools and machine tools.”

HER is particularly useful for moldmakers because they frequently rough molds with multiple cavities, some of which have deep and angled walls, and complex, freeform shapes.

In addition, moldmakers often perform hard milling and HER can be effective for that. For instance, as shown in the video that accompanies the online presentation of this article (see below), Accede Mold & Tool Co. Inc., Rochester, N.Y., initially began high-efficiency roughing using the Dynamic toolpath motion strategy generated by Mastercam CAD/CAM software, noted Robert Fackelman, CNC programmer for the moldmaker.

He added that Accede needed to produce a mold from H-13 tool steel hardened to 50 to 52 HRC for a pressing job. “We were kind of laughing because we really didn ‘t think it was going to work, but we were going to give it a shot. ” However, no problems arose. “It kind of brought the naysayers to their knees, ” Fackelman said. “It was pretty awesome. “

A complex mold plate, usable for a variety of different molds, made using toolpaths generated by Mastercam ‘s Dynamic Motion high-efficiency roughing module.

VoluMill toolpaths constantly change the motion of the cutting tool to maintain a consistent chip load, such as when moldmaking.

Breaking with Tradition

According to Coleman, the traditional approach uses parallel offset toolpaths where the CAM program repeatedly offsets the boundary of a part feature, such as a deep slot in a mold, to generate the toolpath. “That immediately introduces sharp corners into the path,” he said. “No matter what a programmer might desire as his cut width or step-over, it will always be exceeded when the tool gets into those corners.”

This scenario means cutting tools and machine tools are pushed to their limits in the corners, where cutting conditions are the worst, but are underutilized elsewhere, which reduces roughing productivity, Coleman noted.

He added that all CNC machines operate according to one principle: Do what the toolpath tells them to do. “If the toolpath tells a machine to jam that tool in the corner, that is what it’s going to do,” Coleman said. “The cutting tools are just along for the ride.”

To overcome this offset-toolpath limitation, HER software constantly changes the motion of the cutting tool to maintain a more consistent chip load while applying the entire flute length of a tool rather than just a portion, as the conventional method does, said Ben Mund, marketing for CNC Software Inc., Tolland, Conn. “You take a light-but-deep cut, which distributes heat much more effectively, and tool wear is uniform and greatly reduced.”

The developer of Mastercam CAD/CAM software created the Dynamic toolpath motion strategy to enable HER and is integrating it into its suite of milling and turning products. “A tool manufacturer will give you the maximum efficiency of a tool based on a straight cut,” Mund said. “At its simplest, Dynamic motion tries to take that ideal condition and spread it over the entire toolpath.”

Traditional offset-based toolpaths (left) are governed by the shape of the geometry that drives them and there is little concern for or knowledge of the remaining material, engagement, chip thickness and efficiency. In contrast, high-efficiency roughing toolpaths (right), such as the ones generated by Mastercam’s Dynamic Motion, are based on maintaining the exact same chip thickness regardless of where the tool is or the material the tool is shearing.

Mund added that Dynamic motion evolved from the concept of radial chip thinning, where the goal is to produce chips as close to ideal at all times. This requires engaging as much of the tool as possible while frequently changing the engagement to ensure consistent chip load. The advantage of Dynamic motion is that once end users are satisfied with their feed rate parameters for a given material, tool and tool diameter, they can feel confident that the tools will always be kept in a safe, optimal cutting condition, regardless of part geometry.

Although some might consider HER to be similar to high-speed roughing, Mund emphasized the differences. A high-speed machine is suitable for HER, but it doesn’t require it. “One of the counterintuitive things is that a lot of folks look at Dynamic toolpaths and think they need a high-speed machine to use them; this is completely untrue,” he said, noting much of the efficiency comes from the tool motion and not the specifics of the machine. “Even someone using an old Bridgeport 3-axis machine is going to benefit from Dynamic.”

Coleman concurred that HER can extract high performance from any machine tool, but the smooth, more-controlled cutter load often makes the gains seen in small, low-horsepower, less-rigid machine tools the most impressive. “We have customers with extremely old machines that are able to run much faster on those machines to the point where they are kind of in a state of disbelief.”

The resulting tool marks from those toolpaths can seem chaotic because of the constantly changing motion. “The part is dead nuts on, but the tool marks look strange because the tool motion is so much different than what people are used to,” Mund said. This aesthetic difference means some end users, such as those in the medical industry, may not use if for all their finishing operations, but they still benefit from using it for roughing.

HEM and HER

When a machine tool lacks rigidity, such as from a long tool overhang, small spindle interface or 90° milling head, Iscar Metals Inc. offers High Efficiency Machining (HEM) as an option that correlates with enhanced toolpath-motion technology, noted Tom Raun, OEM specialist for the Arlington, Texas, toolmaker. In Mastercam, HEM integrates the CAM software package’s Dynamic toolpath motion with Iscar’s Chatterfree solid-carbide cutting tools and Iscar’s cutting parameter algorithms, which are built into the software to control cutting conditions such as rpm, feed rate and radial chip thinning.