New Tech Powers Productivity Gains in Indexable Milling

Many of the recent advances in the brutally competitive world of indexable tooling may seem incremental. The details of others are shrouded in secrecy. But since cutting tool choices can have outsized impacts on productivity, it’s always worth asking the experts what’s new.

Let’s start with the age-old question of deciding between indexable and solid-carbide milling cutters. The first considerations, as Luke Pollock, product manager at Walter USA, Waukesha, Wis., put it, are “how much material needs to be removed and how much working space do you have?… When you have the room and a large amount of material to remove, I definitely think indexable tooling is the much better way to go. We even see use of indexable tools on small components for the first few operations, just to increase the material removal rate due to the tool’s size and number of teeth.”

Dan Tucker, manager for the product management group and milling specialist for the western U.S. at Sandvik Coromant, Fair Lawn, N.J., quantified the trade off by saying solid-carbide surpasses indexable milling tools for diameters of 5/8" (15.88 mm) and below and “you could even make the argument that ¾" [19.05 mm] diameter solid carbide might surpass indexable as far as productivity. It is often hard to get enough indexable inserts into a smaller tool to compete, not only in terms of how many are put in the cutter, but also the length of the inserts to match up to a solid carbide.” Tucker’s colleague, Joe DeRoss, milling specialist for the eastern U.S., said there are rare cases in which it’s more cost-effective to rough with a small-diameter indexable tool, even though it would have fewer teeth than a solid tool of the same size. That’s because it’s less expensive (at least from a tooling standpoint) to chunk out material with a two-flute indexable mill and then switch out inserts when they wear.

From an applications standpoint, Edwin Tonne, training and technical specialist at Horn USA Inc., Franklin, Tenn., said slot milling or disk milling (face milling) are the best examples in which indexables beat solid tools. “Another example would be face milling in smaller closed pockets. With the Horn 304 mini mill, the bottom of a deeper pocket can be machined without the expense of a longer solid carbide tool,” he said.

For Sandvik Coromant’s DeRoss, the driving factor in picking a solid-carbide tool over an indexable is the need to achieve a superior surface finish. Wiper inserts can be added to an indexable tool to improve the surface finish, but DeRoss said the only way to compete with the solid tool’s higher flute count is to slow the process. Pollock explained that by its nature, an indexable tool will never present a continuous cutting edge as perfectly as a solid tool. “Even though a long-edged indexable tool might line up well, maybe even to the point where you don’t feel a witness line, there is almost always a witness line. … There will always be a little bit of mismatch due to an indexable tool being an assembly. It is [comprised of] inserts put into a steel body. It is never going to have a pocket in the perfect location, and you’re never going to be able to grind the insert to an exact size. There’s always going to be a little bit of a stack up. That is not true with solid tools.”

That’s not to say the tool manufacturers aren’t trying to tighten these tolerances. Tucker said Sandvik Coromant was the first company to machine the bodies in a hardened state, making for a “much more accurate cutter” (as opposed to heat treating them after the pockets were cut). He added that Sandvik Coromant aims for 0.0005" (0.0127 mm) in radial runout for the pockets, and is now looking at new materials and different processes to achieve tighter tolerances. What’s achievable when assembling the cutter on the shop floor “depends,” he said. For example, ground inserts would be needed to come close to maintaining this tolerance. On top of that, they should all come from the same batch. But if it is a truly indexable tool and using randomly selected production inserts, “you’re probably looking at five-tenths to a thou from insert to insert with ground inserts,” said Tucker.

“If the periphery of the insert isn’t ground,” Walter USA’s Pollock added, “the variation will be on the order of ±0.001-0.0015" [0.0254-0.0381 mm]. This could result in a total runout of up to 0.003 [0.0762]" in the most extreme case.”

“Indexable” doesn’t have to mean “simple” or “standard.” For example, “gear milling used to require dedicated hobbing machines,” observed Horn USA’s Tonne. But advances in both CAM software and cutting tool technology have enabled the production of “even the most complex gear shapes and splines on standard lathes with C-axis synchronization. Horn also produces gear skiving tools on an indexable platform due to these advances.” DeRoss mentioned Sandvik Coromant’s indexable milling cutters for turbine root forms. They also make special tulip cutters in both indexable and solid carbide for the corresponding form in the ring the blades lock into. Railroad rails, which have a rounded edge, are another big application for special indexable tools.

Pollock cited customized insert shapes for crankshaft machining. “It’s one of the most expensive components in an internal combustion engine and machining them consumes a lot of inserts, so this area gets a lot of attention.” Both the shape of the insert and the geometry on the face are critical, explained Pollock, “in order to machine carbon steel that’s going to produce a long chip and be able to control it.” He added that the ability to press unique insert shapes and hold them to tight tolerances in the sintering process is “probably the biggest innovation” in creating these form tools. “The tolerances are good enough right from the sintering furnace. You don’t have to come back and grind them, which makes complex geometries more possible. If it had to be ground then there is more of a limit to the shapes that could be produced.”

Walter USA also makes an indexable threadmill with special inserts. (For 7/8" [22.23 mm] diameter and below it also makes a solid-carbide version of the tool.) According to Pollock, the design cuts the chip and folds it in on itself such that it pulls away from material. The tool features a multi-row arrangement that requires multiple revolutions to cut a complete thread, Pollock explained. “Depending on the thread pitch, it could take as many as four or five times around to make the complete thread. But the lower engagement forces produced by our design allow us to feed the tool faster, and we can actually reduce the cycle time to be even faster than cut taps. Normally, when switching from tapping to thread milling, it is common to expect improved thread quality, improved tool life, and improved cost per part. But that also means knowing you’re going to have to sacrifice cycle time, because thread milling typically takes a little bit longer. Not with this tool.”

When should you consider a tool customized for your unique situation? Sandvik Coromant’s Tucker answered, “when the part count and the ROI makes sense. … Or if there’s a feature on a part that can’t be done with a standard tool.” Tonne said a large percentage of Horn’s sales are made to order or custom solutions and offered the following criteria: “When the part complexity or features have a tighter tolerance than a machine tool can handle. For example, if groove spacing must be within microns and the machine tool has some error due to worn internal parts, a custom solution would be a good choice. If the programming of a feature becomes too complex, an indexable custom solution would be a good option, [as it would] if operations can be done simultaneously, such as chamfering, profile milling, and groove milling.”

Coatings remain a key factor in improving the durability of both the insert and the tool body. The “secret sauce” that the OEMs use to achieve the latest coatings remain just that, proprietary and hidden. Sandvik Coromant has released a new version of its Inveio coating, which uses the CVD process Tucker says can be controlled “at the molecular level,” such that the multiple layers stand up in uniform columns. “We’ve really mastered the consistency of how the width and length of each layer aligns. … And we get a big increase in tool life and even wear across the cutting edge of the insert.” Sandvik Coromant’s website doesn’t tell you what the coating consists of, but Tucker reported that it’s TiAlN.

Sandvik Coromant has also released a PVD coating, Zertivo, that follows the same principle. Again, Tucker said the differentiating factor is Sandvik Coromant’s ability to “add greater strength at the micro level or the nano level of the edge line. We’re able to bring strong particles up to the edge line where we need them, and keep other material behind that where we need toughness.” He also emphasized that 90 percent of the company’s coatings are done in-house, with specialized processes, and include secret edge preparation.

For its part, Walter USA has developed an ultra-low pressure CVD coating process that Pollock said allows it to “put more aluminum into a titanium aluminum nitride coating. We often recommend titanium aluminum nitride for milling, because it’s very tough and it’s very thin, so it handles the interruption of milling very well. But by putting more aluminum in the titanium aluminum nitride you can run a lot faster, because aluminum is the part that resists the heat. It can take a lot more heat like an aluminum oxide insert, but it’s not as fragile as that coating due to the titanium.” He described the approach as a blend that manages to achieve the benefits of both a PVD titanium aluminum nitride coating and a CVD aluminum oxide coating. It required modifications to the furnace, but naturally Walter won’t say much more than that, other than the name: Tiger-tec Gold.

Pollock said Walter USA has also introduced a unique coating for its tool bodies, “to help protect the steel and avoid chip wash.” He said the industry in general has transitioned from simple case hardening and through hardening to a steam oxide process that produced a black surface that controlled rust, to nickel plating, which produced a flat silver look. “Now the industry seems to be going back towards black. But ours is not just the steam oxide process; it’s an actual coating that’s even harder and more wear resistant, called Walter BLAXX.” Tucker teased that Sandvik Coromant will introduce a new milling cutter body in October 2022 that has eight times the life of the materials now in use.

Martin Dillaman, manager of applications engineering/project manager for Greenleaf Corp., Saegertown, Pa., observed that improved CAM software has done a lot to improve tool life. “The increased focus on maintaining consistent chip thickness helps promote longer tool life and controlled wear over the length of the tool. That helps whether it’s indexable or solid tooling.”

Dillaman said Greenleaf has also concentrated on developing insert tools with coolant lines in order to extend life. “Whether it’s used for liquid or air coolant, you need a controlled flow out to the cutting edge to help prevent chips from building up and also for evacuating the chips out of the area where the next insert will be coming through to cut. That prevents possible recutting or packing of chips into the flutes and helps overall performance and finish.”

Ceramic tooling is a large part of Greenleaf’s business. For these tools it does not recommend liquid cooling, owing to the thermal shock and the resulting loss of tool life. Interestingly, in its testing Greenleaf has found no need to design toolholders any differently for air coolant than it would for liquid coolant. Many of its holders can use either ceramic or carbide inserts.

But not everyone thinks coolant is always, or even often, needed and high-pressure coolant (700-1,000 psi) appears to be controversial. In Pollock’s opinion, high-pressure coolant is useful for grinding and turning, but is not needed in milling. High-pressure pumps “bring a lot of vibration into the cut,” not to mention “screaming like a banshee” whenever they’re running. In fact, Pollock said, “there are materials where we prefer not to use coolant. Carbon steel is one. We much prefer to cut it dry. If we need chip evacuation, we might try to get some air on it.” And if the material calls for a liquid coolant, such as titanium, Inconel, stainless steel, or aluminum, a low-pressure coolant is sufficient.

On the other hand, Tonne said “high-pressure coolant has become more prevalent as more and more insert seats are added to each cutter. If you have a cutter with 20 pockets, you need the inlet pressure and coolant volume to be high enough to supply each pocket. Tangential milling systems like Horn’s 610, 406, and 409 systems are also a reason high-pressure coolant is beneficial. The strong core, high feed rates, and smaller chip flutes of tangential cutters require volume and pressure to evacuate chips. When slotting and profiling difficult-to-machine alloys, the cooling and chip-removal capabilities of high-pressure coolant are required.

“A standard coolant system on a typical machine might be in the range of 80 to 200 psi,” he continued. “For each coolant outlet at the pocket pressure is divided. For a 100-psi system and a 20-pocket cutter, that means the outlet pressure for each coolant output is approximately 5 psi. There is no magic number for each application, but more is definitely better.”

Speeding setup adds efficiency, and there have been improvements on the margins. DeRoss said Sandvik Coromant is now using bigger screws on some of its milling cutters, because “sometimes the smaller screws are hard to handle. After they’re used, they can start to lock up in the cutter. So, the bigger screws are a little easier to remove.”

Dillaman pointed to the fact that Greenleaf typically uses a round geometry, “because of the strength of the insert and the chip thinning capabilities of that shape in milling. It also lends itself to quicker and easier indexing of the inserts, because you can loosen the screw and index to the next good edge by simply rotating the insert. Whether you’re rotating by 20˚ or up to 90˚, it’s just a simple rotation, lock it back down, and you’re ready to go.” And as mentioned earlier, Greenleaf offers both ceramic and carbide inserts and toolholders that handle either one. “That way, you could use the same cutter, especially in job shops or places where they’re moving between multiple materials, multiple hardnesses. One cutter could be used with a number of different grades, whether it’s carbide or ceramic.”

Greenleaf also offers a tooling family with indexable nests for the inserts. Called C4 and CP4 cutters, Dillaman explained that they enable the user to easily switch between completely different insert geometries. “Whether it is a round insert, or a square, or an 80° diamond, you can change out the nests, depending on the geometry of the part being machined.”

Tucker and DeRoss emphasized the advantage of quick-change toolholders, such as the Coromant Capto, which integrate with the spindle securely and with high repeatability. The Capto interface is now off-patent and various suppliers offer many extension and reduction adapters, plus adaptations to other machine interfaces. DeRoss reported that roughly three-quarters of all new multi-tasking, multi-functional machines with a B-axis head now come equipped with a Capto spindle.

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