New plunge-milling technologies and strategies increase productivity and extend tool life.
Learn more about plunge milling
View a brief Ingersoll technical presentation on plunge milling on www.ctemag.com by clicking here.
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
A DHU-style (deep hogging utility) plunge-milling cutter from Ingersoll Cutting Tools generates a scalloped wall behind the cutter as a result of the tool’s repetitious vertical cutting path.
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
When plunge milling, radial DOC, or step-over (Ap1), replaces the axial DOC of planar milling. The inserts have a slight lead angle to prevent their edges from rubbing on the wall of the part as the cutter descends.
Courtesy of Kennametal
New features on Kennametal Z-axis cutters include serrated areas on the cutter bodies to improve chip formation and evacuation and coolant ports that enhance thermal management and chip evacuation. Those features and the inserts’ high-positive rake face reportedly reduce cutting forces and power requirements, increasing tool life and reliability.
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
The goal of a CAM program for plunge milling is to optimize the milling cycle so the desired amount of material is removed in the least number of plunges possible. This screen shot from ESPRIT CAM software illustrates rough plunge milling of a centrifuge component.
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
A variety of plunge-milling cutter designs from Seco Tools.
Courtesy of Seco Tools
Plunge-milling cutters take the forces straight back in the Z-axis. The geometry of the tools, illustrated by this Plungemill cutter from Seco Tools, is similar to that of a 90° square-shoulder cutter.
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. CTE
About 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 billk@jwr.com.
High-feed milling is an alternative to plunge milling. Employed at high feed rates and light DOCs to remove metal quickly, tools like this PowerFeed+ cutter from Ingersoll Cutting Tools minimize lateral loads on the machine and tool.
Plunge vs. high-feed milling
Maximizing the benefits of plunge milling requires dedicated cutters and careful CAM programming. In many cases, high-feed milling is a simpler alternative.
A high-feed mill is basically a high-lead-angle, straight-edge cutter. The large lead angle thins the chips, and feed rates must be increased to maintain sufficient chip thickness. At high feed rates and light DOCs, high-feed cutters remove metal quickly while minimizing lateral loads on the machine and tool.
Tom Noble, MAXline product manager for Ingersoll Cutting Tools, said part feature size and configuration can help shops decide between plunge and high-feed milling. “If it is a small pocket, you are probably better off plunge milling,” he said. “You are not going to gain a whole lot with radial milling because the radial motion is short. But if it is a fairly large area, it may make more sense to go in and whack away at it with a high-feed mill.” Lateral loads do exist but are minimized by the use of a light DOC, fast feeds and multiple passes.
Ingersoll’s Bill Fiorenza said extended-reach applications with plunge mills 2 " in diameter and larger can be effective. On the other hand, extended-reach situations with smaller diameter cutters may be more appropriate for high-feed milling. When extensions grow to 4 or 6 diameters deep, he said, “you start to contend with certain types of chatter. You may want to handle those applications with a high-feed mill employed with a light 0.015 " to 0.020 " DOC.” He added that a vibration-resistant tooling configuration, such as a solid-carbide shank and modular head, may be required.
Noble said a key factor in choosing a milling method is the shop’s day-to-day activity. “If you’re doing a lot of 3-D milling and want to do a little bit of plunging, let’s say to relieve corners, I would recommend a high-feed cutter with which you could do limited plunging. But for true pocketing, straight walls and channels and large volumes, you should invest in a dedicated plunge mill.”
—B. Kennedy
Contributors
Doosan Infracore Machine Tools
(973) 618-2500
www.doosaninfracore.com
DP Technology Corp.
(800) 627-8479
www.dptechnology.com
Ingersoll Cutting Tools
(815) 387-6600
www.ingersoll-imc.com
Kennametal Inc.
(800) 446-7738
www.kennametal.com
Seco Tools Inc.
(248) 528-5200
www.secotools.com/us
Courtesy of Doosan Infracore Machine Tools
Plunge-milling techniques can boost metal-removal rates for light-duty or aging machines. However, heavy-duty machines maximize the advantages of plunge milling. This Doosan Mynx series VMC, stripped of its exterior sheet metal, reveals the machine’s heavy single-piece cast base, box ways, massive spindle and large table.
Choosing the right machine for plunge milling
“One size fits all” means the same thing 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.
—B. Kennedy
Related Glossary Terms
- 3-D
3-D
Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.
- alloys
alloys
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
- boring
boring
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- centers
centers
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- centrifuge
centrifuge
Filtering device that uses a spinning bowl and the differences in specific gravities of materials to separate one from another. A centrifuge can be used to separate loosely emulsified and free oils from water-diluted metalworking fluid mixes and to remove metalworking fluids from chips.
- chatter
chatter
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- clearance
clearance
Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.
- computer-aided manufacturing ( CAM)
computer-aided manufacturing ( CAM)
Use of computers to control machining and manufacturing processes.
- conventional milling ( up milling)
conventional milling ( up milling)
Cutter rotation is opposite that of the feed at the point of contact. Chips are cut at minimal thickness at the initial engagement of the cutter’s teeth with the workpiece and increase to a maximum thickness at the end of engagement. See climb milling.
- coolant
coolant
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- depth of cut
depth of cut
Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.
- endmill
endmill
Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.
- facemill
facemill
Milling cutter for cutting flat surfaces.
- feed
feed
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- lead angle
lead angle
Angle between the side-cutting edge and the projected side of the tool shank or holder, which leads the cutting tool into the workpiece.
- metalworking
metalworking
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling cutter
milling cutter
Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- plunge milling
plunge milling
Highly productive method of metal removal in which an axial machining operation is performed in a single tool sequence. The tool makes a series of overlapping, drill-like plunges to remove part of a cylindrical plug of material one after another. Because of the increased rigidity of a Z-axis move, the tool can cover a large cross-section of material.
- rake
rake
Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.
- relief
relief
Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.
- shank
shank
Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.
- stainless steels
stainless steels
Stainless steels possess high strength, heat resistance, excellent workability and erosion resistance. Four general classes have been developed to cover a range of mechanical and physical properties for particular applications. The four classes are: the austenitic types of the chromium-nickel-manganese 200 series and the chromium-nickel 300 series; the martensitic types of the chromium, hardenable 400 series; the chromium, nonhardenable 400-series ferritic types; and the precipitation-hardening type of chromium-nickel alloys with additional elements that are hardenable by solution treating and aging.
- step-over
step-over
Distance between the passes of the toolpath; the path spacing. The distance the tool will move horizontally when making the next pass. Too great of a step-over will cause difficulty machining because there will be too much pressure on the tool as it is trying to cut with too much of its surface area.
- tapping
tapping
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
- toolpath( cutter path)
toolpath( cutter path)
2-D or 3-D path generated by program code or a CAM system and followed by tool when machining a part.
- undercut
undercut
In numerical-control applications, a cut shorter than the programmed cut resulting after a command change in direction. Also a condition in generated gear teeth when any part of the fillet curve lies inside of a line drawn tangent to the working profile at its point of juncture with the fillet. Undercut may be deliberately introduced to facilitate finishing operations, as in preshaving.
- width of cut
width of cut
Width of the milled surface, reflecting a face milling cutter’s radial engagement, and a peripheral milling cutter’s axial engagement, in the cut.
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