To implement higher-productivity milling strategies, your cutting equipment must be up to the task.
Do you have what it takes to raise your milling productivity to the next level? In addition to the requisite know-how, you need cutters and machine tools that will allow you to employ milling techniques that exceed what’s normally possible. Aided by the right hardware, you may soon be performing feats like pushing your feed rates to new highs and cutting harder materials than ever before.
Getting a Feel for High Feed
One of the best known and most popular forms of unconventional milling is high-feed milling. As the name suggests, high-feed milling involves running at a higher speed (in sfm or rpm) on the feed rate than that recommended for cutting a particular material with a standard face-milling cutter, explained Joe DeRoss, milling specialist for cutting tool manufacturer Sandvik Coromant (Fair Lawn, NJ). In addition, the depth of cut is shallower than that produced by conventional milling.
High-feed milling is done with cutters with a smaller lead angle (the angle between the leading cutting edge of the insert and the workpiece surface) than the standard 45°. The typical lead angle for a high-feed milling cutter is 15° or less, according to DeRoss.
In addition to a shallower cut depth, the high-feed milling strategy is based on chip thinning. A cutter with a smaller lead angle produces thinner-than-normal chips. To produce chips with the same thickness as those created by a 45° cutter, the feed rate must go higher. “Chip thinning allows advance per tooth to go up exponentially,” said Brian Winterlin, applications engineer for cutting tool maker Horn USA Inc. (Franklin, TN).
Though this sounds like a winning formula for boosting productivity, high-feed milling won’t always yield that result. “A big misconception is that high-feed milling is always more productive,” said Thomas Raun, national milling product manager for tool manufacturer Iscar Metals Inc. (Arlington, TX). “There are times when that approach will be more productive but it’s usually more beneficial if you have a less rigid machining center.”
Why? Reducing the lead angle will also reduce radial cutting forces, lessening vibration and stress on the spindle bearing of the machine tool, explained DeRoss. That makes high-feed milling a good choice for roughing out material with some newer machines that are not very rigid and so cannot handle a large depth of cut.
High-feed milling is also a good option for roughing out shapes that are not very deep, noted Steve Boss, product manager for Horn USA.
On the other hand, “you’re not going to be holding any kind of dimensional tolerances or surface finish tolerances” with high-feed milling, said Raun, so it must always be followed by a semifinish or a finish pass.
And though it may not require a heavy-duty machine tool, high-feed milling does make certain demands on a machine. “You might be running 400 to 700″ a minute with a high-feed mill,” Winterlin said. “Is your machine capable of running a programmable feed rate that high? That will usually dictate whether or not high-feed milling is a viable option.”
As for appropriate cutting tools, one thing to keep in mind is the depth of cut per pass that you want to achieve in a high-feed milling process. Though there are many high-feed tools on the market, Raun pointed out that in many cases the depth of cut they offer may only be 0.060″ or less.
Another important consideration is the stresses the tools can handle. According to Raun, Iscar offers tools with “dovetail” pockets that, along with the attaching screw, help hold inserts in place. Thanks to this sturdier design, these tools are a match for “much more aggressive” feed-per-tooth cutting parameters, Raun said.
Raun also pointed to game-changing new designs that allow more flutes in the cutter. Iscar, for example, has developed tools for high-feed milling that can hold one more flute. That may not sound like much of a change, but one more flute can translate into a much higher metal removal rate. For example, Raun said, Iscar added a flute to small-diameter indexable tools that traditionally had only one flute, creating a two-flute design that can allow users to double their productivity.
Extreme Milling from the Side
While high-feed milling is generally done with the face of the cutter, trochoidal milling uses the side of the tool to make shallow radial cuts with relatively large axial depth. As with high-feed milling, trochiodal milling results in chip thinning that allows higher-than-normal feed rates.
“With high-feed milling and trochoidal milling, you may be taking a smaller depth or width of cut, but your speeds and feeds are so much higher that you can remove more material than you can with conventional milling,” explained Horn USA Applications Engineer Byron Haney.
Normally, trochoidal milling is done with solid-carbide end mills, according to William Fiorenza, die and mold product manager at Ingersoll Cutting Tool Co. (Rockford, IL). The toolpath is an algorithm designed to minimize the amount of radial tool engagement (usually from 5 to 20%). Most higher-end CAM systems have some type of algorithm with this capability, Fiorenza said.
The low radial engagement allows users to maximize the tool’s axial depth of cut. “If it’s a ½” diameter end mill with a ¾” cut length, you can leverage the entire ¾” cut length of that tool,” Fiorenza said.
By going in and out of a cut very quickly with low radial engagement, a trochoidal cutter reduces radial cutting forces and also the amount of heat in the workpiece and tool. As a result, Winterlin of Horn USA said, tool life can be two or even three times greater than that of standard end mills used for traditional roughing or cutting. He added that the technique is particularly well suited for machining high-temperature alloys, a process that normally produces a good deal of heat.
According to Fiorenza, trochoidal milling is a good choice for milling slots and other features into steel grades such as P20, H13 and S7, as well as other materials with hardnesses ranging from 30 to 60 HRC.
If trochoidal milling sounds enticing, make sure your machine is up to it before giving it a try. “The machine controller can dictate what type of [milling] strategy you use,” Winterlin said. “And there are a lot of machines that can’t handle either trochoidal or high-feed milling.”
For trochoidal milling, the key is whether or not the machine is capable of what Winterlin called reading ahead. “If the machine can’t read the programming code fast enough to keep up with what you need the cutter to do, then you have to fall back to a different type of machining,” he said.
According to Fiorenza, tool motion should be fluid, but that will not be the case if the machine controller cannot process the trochoidal milling toolpath properly at higher feed rates. The adverse consequences in this case could include reduced tool life and workpiece damage.
For those who determine that their machines make the grade for trochoidal milling, the next step is selecting cutting tools for the job. One option that has become fairly common, Fiorenza noted, is “chatter free” end mill designs. Chatter-combating designs include a helix angle that varies slightly between flutes, as well as non-constant flute spacing that helps break up harmonics. The result can be improved quality of cut and longer tool life.
As is the case with high-feed milling, trochoidal milling can benefit from a trend toward cutters with more flutes. Raun pointed out that traditional solid-carbide end mills might have three to five flutes, but now solid-carbide end mills with seven- and nine-flute configurations have hit the market. In fact, he added, Iscar now offers cutters essentially equipped with one flute for every millimeter in diameter. So, for example, a 25 mm end mill would have 25 flutes for trochoidal cutting.
Milling on a Grander Scale
Today, what can be characterized as “extreme” milling isn’t limited to high-feed and trochoidal processes. For example, Raun points to aerospace companies that are investing in large machining centers capable of things that no other machine tools have been able to do before. “These machines are moving 20 cubic inches of titanium a minute,” he said, thanks to what he calls “extreme” coolant flow and pressure rates.
Call it extreme rough milling, which can be defined as conventional milling on a grander scale, according to Brian Wilshire, technical center manager for Kyocera Precision Tools Inc. (Hendersonville, NC). Wilshire and his Kyocera col-leagues have seen a good bit of extreme rough milling in the machine tool industry, where large machines are being built. Extreme rough milling is also being done in the shipbuilding and heavy equipment industries to create big castings and parts that require the removal of large amounts of material.
Not surprisingly, the main requirement for extreme rough milling is heavy-duty equipment. “It’s not something your typical job shop would be able to do,” Wilshire said. “Most of them are running smaller 40-taper machines that can’t use the heavy cutters. The spindles are not rigid enough and many times they don’t have enough horsepower.” By contrast, he said, extreme rough milling usually requires a 50-taper spindle or one even more rigid.
In addition, Wilshire said, extreme rough milling requires large-diameter milling cutters with big carbide inserts that can take very large depths of cut. When it comes to standard rectangular milling inserts, the largest normally seen on the market have a longer edge measuring 17 mm, he noted.
Kyocera, however, offers the MSR “monster” cutter, which uses 25 mm inserts. These extra-large inserts go into “a bigger, stronger cutter body that can withstand the higher cutting forces it will experience with heavier depths of cut and higher feed rates,” he said.
Once an extreme rough milling operation is completed, will a secondary machining operation be required? The answer depends on the surface finish requirements for a particular part, Wilshire said. “We have customers that rough with one cutter and finish with another. And we do have some big-diameter finishing face mills. But many bigger parts don’t have the tight tolerances that the smaller parts do,” so a secondary operation that follows extreme rough milling may not be necessary.
The Hard Problem in Milling
Another type of milling that can be labeled “extreme” is milling hard materials. Any material over 40 HRC is considered hard, according to Roger Goble, national accounts manager for tool maker OSG USA Inc. (Glendale Heights, IL). Among these materials are different grades of steel that can be heat treated, including H13, S7, A2 and D2.
“For soft steel, which is more about big cuts and big chips, light-duty, weak machines are priced great, kind of like the Kia of the automotive world,” Goble said. “But when you’re cutting hardened steel, it’s a really tiny, light depth of cut but you move fast.” To handle this task, he said, shops normally want a rigid, heavy-duty machine with a spindle strong enough to take on hardened steel and also capable of fast movement.
Even with the right machine tool, milling hardened steel can be difficult, so shops undertaking the task must pay careful attention to their cutting tool designs as well.
For easy-to-cut soft steel, you want plenty of chip room in the pockets between the flutes of the tool, and you can get away with a small core, Goble explained. For hardened steel, however, the core of the tools should be big and thick (making the tools stronger and more rigid) but you don’t need a lot of room between the flutes because the chips are very small.
In addition, Goble said, a tapered cutting tool design provides more rigidity than a straight shank. With a stronger tapered tool, he said, “you’re able to run faster and get better surface finishes and higher accuracy because the tool is not deflecting as you are cutting.”
Cutting geometries and edges should also be tailored to cutting hardened steel. For example, Goble said, the flat-face cutting edge on a normal cutting tool can be replaced by a spiral cutting edge that gradually engages the material, thereby reducing the vibration caused by cutting forces.
Another key consideration is the sharpness of the cutting edge. “For softer materials, you want sharp cutting edges because they cut easily and that lets you go faster,” Goble said. “But the first time a sharp edge tries to cut hardened steel, it will get chipped right off. So you want geometries tailored towards adding strength to the cutting edge.” That means blunter edges, which hold up much better than sharp ones when cutting hardened steel, he noted.
In general, Goble believes shops tend to shrink from the task of cutting hardened steel. “Most people rough [a part] out soft, send it to a heat-treat furnace, bring it back after it has hardened and semifinish and finish it,” he said. “But I’m trying to teach people that they can machine the whole thing out of a hard, heat-treated chunk of steel.”
By doing so, Goble said, shops can save the time normally spent on multiple setups and transporting the workpiece. “I’m saying, put it on the machine, do it once and you’re done.”