March 2012 / Volume 64 / Issue 3|
Taking the Plunge
By Bill Kennedy, Contributing Editor
| New plunge-milling technologies and strategies increase productivity and extend tool life.
Significant progress in metalworking productivity most often results from the combined efforts of toolmakers, machine tool builders and software developers. Plunge, or Z-axis, milling is a good example.
In plunge milling, the rotating cutting tool moves straight into the workpiece in the Z-axis, retreats in the same direction and then steps over in the X- or Y-axis to make an overlapping vertical cut and remove more material.
Plunge milling provides a range of benefits. Especially in long-reach applications, such as deep mold features, traditional side-to-side, or planar, milling is slowed by the need to minimize the lateral forces that cause chatter. When plunge milling, cutting forces are directed into the spindle and the machine table, and metal-removal rates can be much higher than in traditional milling.
Courtesy of Ingersoll Cutting Tools
According to AMT Software LLC, Troy, Mich., whose Prospector CAM package includes plunge-milling capability, the removal rates for plunge milling are at least 50 percent higher than those obtained by conventional planar roughing with button-type facemills.
Because plunge milling minimizes lateral loads on machine tool components, it can boost productivity on an old, less-rigid or light-duty machine tool. John Ross, marketing manager for Doosan Infracore Machine Tools, Pine Brook, N.J., agreed that plunge milling reduces stresses on less-capable machines, but added that new machines with features that facilitate the process can maximize its benefits (see sidebar on page 56). Ross pointed out that plunge milling can minimize fixturing issues because cutting forces are directed into the machine’s spindle and worktable.
Bill Fiorenza, die and mold line product manager for Ingersoll Cutting Tools, Rockford, Ill., said plunge milling helps reduce heat transmitted to the cutting tool and workpiece. Plunge milling, he said, “doesn’t introduce much heat into the part because the cutter is in and out so quickly as it rotates. The only part that is engaged is the little bit of step-over.”
That is particularly helpful when cutting difficult-to-machine materials, such as stainless steels, high-temperature alloys and titanium. When he runs a plunge-milling demonstration, Fiorenza said, “The metal chips are so hot you could toast a sandwich in them. Yet when the plunge-milling routine is finished, you can put your hand on top of the workpiece and it is relatively cool to the touch.” In addition to extending tool life, reduced heat minimizes part distortion.Plunge-Worthy Applications
Plunge milling expedites the production of high-end, complex components. “Where you see plunge milling the most is in the mold and die and aerospace industries because of the types of parts they machine,” said Gary Meyers, milling product manager for Seco Tools Inc., Troy, Mich. Mold and die makers mill solid workpieces into complex shapes for mold cavities, and many aerospace parts also are machined from solid. “The stock removal on those parts is incredible,” he said, “in some cases machining 50 to 60 percent or more from the original workpiece.”
Courtesy of Kennametal
Courtesy of Kennametal
In addition to makers of complex parts, general-purpose machine shops can benefit from plunge milling, according to Kenyon Whetsell, product manager for DP Technology Corp., Camarillo, Calif., developer of ESPRIT CAM software. He cited “2½-axis shops, many with older, less-rigid machines they are trying to get high output from, with unstable fixturing. They just want 2½-axis plunge milling.”
Juan Seculi, global product manager, indexable milling for Kennametal Inc., Latrobe, Pa., sees plunge milling widely applied in “complex shapes and cavities in big and medium-size parts, where the length-to-diameter ratio is critical and conventional milling strategies generate chatter, vibration and poor tool life.” He said Kennametal has recently renewed its Z-axis platform in response to growing customer demand. “Time has verified that by showing an ongoing ramp-up in sales of the Z-axis milling tools, growing 40 percent year-over-year,” he said.Designed to Plunge
Toolmakers have designed milling cutters to take advantage of the technique. According to Meyers, the cutters essentially “take the forces straight back in the Z-axis. The way the tool is presented is very similar, geometry wise, to a 90° square-shoulder cutter.”
The difference is that the lead angle of the inserts is a few degrees off vertical, perhaps 87° instead of 90°. “If you have a true 90° cutter and machine down a side wall, the edges of the inserts are going to rub all the way down,” he said, “where if you present the cutting edge at 87°, there is relief where it cuts.” According to Meyers, plunge-milling tools cut on the bottom edges of the inserts and avoid cutting with the side of the inserts because the transition from bottom to side is an insert’s weakest area, and machining with the inserts’ sides can induce radial forces that lead to vibration.
Courtesy of DP Technology
Meyers added that although cutting with the sides of inserts is uncommon, there are applications, called “up- or down-copy milling,” for plunge milling complex shapes. The tool cuts on the upstroke and downstroke.
He presented a simple example of milling a straight side wall with an undercut at the bottom. “You could plunge down and then move in and mill the undercut.”
A limitation of the technique, according to Meyers, is the clearance between the tool’s actual cutting diameter and the diameter of the tool body. To supply as much support as possible to the cutting edge, the body of a standard facemill extends as close to the tool’s full cutting diameter as possible. For plunge mills used to copy mill, Meyers said, the insert’s cutting diameter extends beyond the tool body. “It is somewhat limited because you can’t have those inserts hanging out there too far,” he said.
While plunge milling is typically a roughing operation, the technique and tooling are suitable for semifinishing and finishing, Meyers said. He recommends decreasing the step-over on the radial engagement to create a finer finish, in the same way that a smaller step-over is used with a ballnose endmill when 3-D finishing.
Essentially, step-over is based on insert width and the amount of cutting edge being presented to the material, he explained, adding that catalog recommendations provide step-over dimensions to produce a certain scallop height, which determines surface roughness, for a particular cutter.
Kennametal’s Seculi said plunge-milling cutter designs are continually being improved and fine-tuned. New features on Kennametal Z-axis cutters, for example, include serrated areas on the cutter bodies to improve chip formation and evacuation and coolant nozzles that enhance thermal management and chip evacuation. “The use of high-positive rake face inserts in combination with the features incorporated in the cutter bodies lowers cutting forces and reduces power requirements, resulting in longer tool life and higher reliability,” he said.CAM Considerations
Although plunge milling has been in use for at least 15 years, Fiorenza said shops more recently are realizing that it can provide high mrr and are more apt to apply it because toolpaths have become easier to program and verify. A growing number of CAM systems have algorithms dedicated to plunge milling. In addition, simulation software enables shops to prove out their plunging cycles before running them. “You really have to qualify the moves in the toolpath because dedicated plunge mills typically are not center-cutting tools,” Fiorenza said. An improper step-over or ignorance of the amount of material remaining on the workpiece can cause a crash when applying a noncenter-cutting tool.
According to Meyers, some shops employ an NC’s G81 drilling cycle to perform plunge milling. In those cases, however, the milling cutter’s inserts can drag on the part wall when returning from the downward plunge.
To overcome that problem, dedicated CAM plunge-milling cycles move the tool 0.001 " to 0.002 " in the X- or Y-axis at the bottom of the plunge, retracting it from the cut before it returns to the top of its travel. The move prevents the inserts from rubbing on the machined surface.
Plunge cycles can also be programmed manually. Meyers said: “In some cases, if it is a simple plunge where you are going to the same depth, you could write a subroutine and just put an X or Y move in there. But that is a lot of work. It is one of those things where it is a lot more conducive if you can just program it that way and not have to go in and edit the code.”
“We try to optimize the plunge-milling cycle so the customer gets all of his material removed and makes the least number of plunges possible, taking maximum advantage of the tool with every single cut,” said DP Technology’s Whetsell.
“Determining programming parameters,” he continued, “involves calculating dynamically how much of the tool will be engaged in the workpiece, from the axial point of view.” The goal is to use as much of the inserts’ cutting capability as possible in each plunge. “That requires we know what the original stock and final part look like.”
Knowing the part’s final dimensions determines how deep the milling cutter must plunge, and knowing the dimensions of the original stock determines where the plunge begins. “Basically, that is knowledge of the current stage of the stock with regard to machining operations that have happened previously. In ESPRIT, we call it ‘stock automation,’ ” Whetsell said.
Programming the X or Y retraction from the cut “becomes a bit delicate,” Whetsell said, “because you can’t just back out into material behind the cutter, and you don’t want to retract back into that scallop you just created.”
CAM software enables plunge milling to be programmed in various ways. “Instead of defining a step-over or a radial width of cut, for example, you can define a scallop height, say 0.010 ", and the software calculates the plunges to accomplish that,” Whetsell said. DP Technology is researching a dedicated plunge cycle for ESPRIT, and some users already program plunge milling via the package’s advanced programming interfaces.
According to Kennametal’s Seculi, some of the cutting parameters and nomenclature employed in plunge milling are different than those of other milling methods. Cutting speeds, for example, will be lower at longer tool overhangs to prevent chatter.
Courtesy of Seco Tools
Courtesy of Seco Tools
In describing a plunge-milling application, the Ap designation used for axial DOC in planar milling “changes its meaning since it is located in the radial view of the cutter and not in the vertical axis of the cutter,” Seculi said. (See graphic on page 50.) “In a Z-axis application, we do not have an axial DOC dimension; we have radial depth of cut (or step-over) and a radial engagement dimension.”
DOC is normally related to insert size, and Kennametal recommends maintaining a DOC greater than 15 percent of the insert cutting length when plunge milling. If cutting depth becomes close to or below the insert’s nose radius value, radial cutting forces increase, negating some of the technique’s benefits.
Plunge milling can be a highly productive metal-removal strategy. Choosing to use the technique or perhaps another depends on a number of factors (see sidebar on page 52). Influencing that choice will be upgrades and evolution of the process via continuing contributions from toolmakers, machine tool builders and software developers. CTEAbout the Author: Bill Kennedy, based in Latrobe, Pa., is a contributing editor for CTE. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or firstname.lastname@example.org.
Courtesy of Ingersoll Cutting Tools
Plunge vs. high-feed milling
Doosan Infracore Machine Tools
DP Technology Corp.
Ingersoll Cutting Tools
Seco Tools Inc.
Courtesy of Doosan Infracore Machine Tools
Choosing the right machine for plunge milling
in machine tools as it does in hats. An all-purpose machine will do many things well, but to maximize productivity (and minimize frostbite), a custom approach is better. Doosan Infracore Machine Tools provides vertical machining centers and horizontal boring mills, among other machines. Its VMCs range from fast, light-duty tapping centers to high-speed, 5-axis mold and die machines and units aimed at heavy-duty metal removal.
Marketing Manager John Ross said the company provides machines tailored to different applications and even different geographic areas. For example, some machine way systems feature linear guides, while others have massive box ways. “As we get into areas of the California market, where they tend to cut a little bit lighter weight material, linear guides are just fine,” Ross said. “When we get into some places in the Midwest, they want to rip off aerospace and high-temperature alloys and need a sturdier box-way machine to be able to handle the cutting forces.”
High-speed mold and die machines with linear guides are excellent for removing small amounts of material quickly, and using plunge-milling techniques may boost their roughing capability to a degree, but, Ross said, “you are not taking the chip load that you could take on a box-way machine.”
He cited Doosan’s Mynx series of VMCs as the type of platform that can maximize the advantages of plunge milling. “It is the most rigid of our vertical machines,” he said, adding that the machine’s base is a single-piece casting and its 60 "×30 " table can handle large molds or aerospace castings.
“The bigger the machine spindle, the more you can hog off,” said Steve Sigg, application engineer for Doosan. At the truly heavy-duty end of the scale, he said, plunge milling has helped customers rough materials like Inconel and stainless steel where radial machining with a hog mill was unproductive. And where tool overhang results in excessive vibration when milling laterally, plunge milling overcomes the problem.
Incidentally, he added that one driver for interest in plunge milling is the return of some mold and die work from China.
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