While a solid-carbide tool is very hard and resists abrasive wear, “it is also on the brittle side so you are limited as to how hard you can push it,” Pollock said. With indexable tools, however, the toughness of the steel body comes into play. “You usually don’t think about an indexable milling body breaking, but solid-carbide tools break all the time,” he noted, adding that the steel body also makes indexable tools less susceptible to vibrations.

From a cost perspective, indexable tools require a significant upfront investment. According to Seco Tools’ Aydt, however, the investment cost can be paid back within a few months, depending on the application.

While speed always seems to be emphasized and prized in manufacturing operations, Pollock maintains that it shouldn’t be shop personnel’s top priority when they are engaged in tool maintenance activities. “In this area, saving time isn’t necessarily the most important thing,” he said. “Doing a good and accurate job during the process is also important.”

Some machine tool companies, he added, have options to check tool wear on the machine, “helping smooth machining and prevent work hardening. Additionally, by collaborating with our industrial partner Fusion Coolant Systems (which offers a supercritical CO2 minimum quantity lubrication and coolant system), we’re driving more effective cooling, increasing performance and optimizing productivity.”

And for finishing, Kennametal’s HARVI III line of solid-carbide end mills is designed for aerospace materials to provide “excellent surface finishes at very productive feed rates and deliver outstanding tool life,” said Francis. “The carbide grade, KCSM15, provides the toughness and reliability expected in aerospace part roughing and finishing.”

Stainless steel is far from an unknown quantity in machine shops. Yet, particularly in automotive and aerospace applications, tools and cutting methods continually evolve to optimize output—particularly as parts get more complex.

In fact, most parts today are closer to near net shape and are usually accompanied by a model to help with programming. “Newer software seems to account for these features and allows the programming to select the fastest way to remove material,” said Sharma. “This includes dynamic milling areas on a part that would otherwise require cutting air in a standard toolpath. I have seen newer machine control software in aerospace accounts that allows the operator to download a model to a USB, upload it to the machine, and select machining strategies from the floor, using the machine controls.”

What accounts for the popularity of indexable tools? Fundamentally, they are efficient tools for metal removal because of their structure, which typically consists of a number of carbide inserts installed in a steel body. This structure is very strong and capable of taking high-horsepower cuts, resulting in maximum material removal rates, according to Luke Pollock, product manager at toolmaker Walter USA LLC, Waukesha, Wis.

“Kennametal worked with the material manufacturer and the aircraft manufacturer to identify the best insert grades and cutting tools for the job and then defined best practices for machining the components from a forging.”

A powerful spindle is also key, he added. “Machines with adequate horsepower and ample torque will provide much better results when cutting stainless and will also help the machine last longer. Lighter-duty machines can have success when cutting stainless, but if the machine is tasked with cutting it often, then a machine with the right components will provide better results and more longevity. CAT 50 or BIG Plus dual-contact spindles can be helpful as well.”

Hurco’s motion system has dynamic variable lookahead up to 10,000 blocks, Cope continued, “which means the motion system is smart enough to do the adjustments for you, depending upon the toolpath. Hurco made UltiMotion standard on all machining centers sold in North America because motion control is critical to surface finish, reducing cycle times, and longevity of the CNC machine’s key components.”

Machining this component in cast iron would normally have its limitations, he continued, but stainless steel “has added to the demand to hold tolerance and tool life. Mounting surfaces, such as gasket surfaces with higher finish requirements, are the most demanding due to interrupted cutting of cast irregularities. With this current customer, we were able to provide extensive finish requirement testing in our corporate headquarters lab, using our tooling, to ensure we can provide tool life and hold the needed tolerance.”

“When you have multiple teeth in the cut, the forces required to spin the cutter go up significantly,” he said. “So you may run into instances where you will essentially stall the machine.”

For finishing, “employ climb milling and avoid interruptions, if possible. Use a larger lead angle, if possible, and only use cutting fluid if running at lower cutting speeds.” Typical speeds range from 590-1,300 sfm (180-396 m/min).Sandvik Coromant expects to release new ISO S stainless steel grades in the near future. ISO S refers to heat-resistant superalloy materials, “which in some cases we treat just like machining stainless steel,” Pusatera said. “This usually refers to using PVD-coated tools for added sharpness as opposed to using CVD-coated.”

Another toolmaker trying to move replaceable cutting edges into solid-carbide territory is Sandvik Coromant, Fair Lawn, N.J. The company offers an exchangeable head (EH) system consisting of a removable carbide cutting tip that typically screws into a steel body. In addition to reducing the amount and cost of carbide, the EH system speeds up and simplifies the process of exchanging cutting tips because it doesn’t require the removal of the entire tool from the machine, noted Anja Redzepagic, a Sandvik Coromant sales engineer.

However, because of the alloying elements in the material, this stainless steel is more difficult to machine, he continued. Instead of wearing, the material gets harder over time. The material can work harden during machining, which contributes to tool wear and failure.

Whether or not shops choose to upgrade their indexable tool technology, they should never miss a chance to upgrade their insert indexing and changeout practices. Many tool manufacturers now offer customer instruction in this vital area—and it’s often badly needed, according to Sandvik’s Redzepagic.

Another technological advancement extending the life of indexable tools is the modern electrostatic press. Andersson pointed out that these presses tend to be more precise than their older counterparts in insert production. “Any time you have smaller deviation from insert to insert, you reduce runout and get longer milling-tool life,” he said.

With a chrome Duratomic coating, however, insert edges that have made any contact with a workpiece turn dark, making it easy to spot used edges. The idea is to reduce the number of insert edges that shops waste.

According to Scott Lawrence, an aerospace specialist with Seco Tools LLC, Troy, Mich., “we have had success with lighter-side milling cutter paths, such as dynamic milling,” when optimizing toolpaths for stainless steels. “Best results are achieved by maximizing the tool’s flute length combined with the correct radial engagement. This eases spindle load, as well as fixturing, with these types of cutter paths; that seems to work well in extending tool life.” He also advised “picking the right-size tool to ensure chip evacuation, employing radius compensation in corners to avoid chatter and adjusting stepover based on axial length of cut.”

Though not right for every cutting application, indexable tools have a number of inherent advantages over solid tools that account for their competitive edge in the tool market. In recent years, moreover, the competitive position of indexable tools has been strengthened by advancements that have increased tool life, improved cutting performance and sped up changeout.

To extend the life of its indexable tool bodies, Sandvik Coromant prehardens the steel using a heat treatment process just before the pockets are machined. This makes it more difficult to damage the pockets when the tool is being used, according to Redzepagic.

To prevent this from happening, Dillaman added, users must know the torque capability of their machine and, if necessary, adjust their cutting operation to make sure the machine isn’t overloaded. Such adjustments could include reducing the feed rate or depth of cut.

Kennametal offers tough carbide grades like KCSM40 and KCPM40 for roughing operations to resist thermal cracking and prevent premature chipping. Pairing those with the company’s KSRM face mills with round inserts allows for scale removal and complex feature machining. Meanwhile, Kennametal’s HARVI Ultra 8X indexable helical cutter with eight cutting edges per insert provides high metal removal rates, insert edge life economy, and reliability, Francis said.

A Seco Tools project illustrates how stainless steels are gaining ground in automotive applications under the right circumstances.

Meanwhile, Inveio’s “tightly packed, uni-directional crystals create a strong barrier towards the cutting zone and chip. This greatly improves crater wear and flank wear resistance.” Furthermore, “heat is more rapidly led away from the cutting zone, helping the cutting edge stay in shape for longer times in the cut.” GC2220 grades optimize stainless steel turning in stable conditions.

“For a good surface finish, you need smooth motion,” Cope explained. “Features such as toolpath tolerance, smoothing, and NC block lookahead are very important. NC block lookahead will determine how far into the upcoming moves the control will begin to prepare itself for smooth motion, and toolpath tolerance and data smoothing options can be controlled within the NC program to affect speed and surface finish. These settings can be opened up to allow for faster motion when roughing or semi-finishing, and then tightened for finishing. Mixing the settings will help cut cycle times when roughing and still provide the control necessary to produce good surface finishes and tolerances.”

Much larger sizes are also possible. “You couldn’t create a 10" (254-mm) diameter cutter out of solid carbide because it wouldn’t be economically feasible,” Pollock said. “But with a steel body, it is very easy and cost-effective to do.”

Over the past few years, explained Matt Gifford, aerospace structures product specialist at Mitsui Seiki USA Inc., Franklin Lakes, N.J., “you’ve seen what the industry calls high-efficiency milling. Instead of large stepover cuts, they’re taking lighter radial depth of cuts and larger axial cuts and going much faster.”

As toolpath sizes generated by CAM systems increase, the ability to process large amounts of data on CNCs is vital. “FANUC will soon introduce the 0i-MF Plus control, with larger memory and high-speed capabilities now standard instead of optional,” Gilmore said. “This upgrade will increase the throughput of their basic control package, along with keeping costs low. Known for its reliability, the FANUC 0i-MF Plus control will unlock the potential of many CNC milling machines.”

“Kennametal flat-bottom drills are suited to a variety of applications with pockets or hard-to-reach areas and enable the user to create a hole to provide access for other tools to complete the machining process,” he said.

In addition to the coating itself, the method of applying the coating can have a major impact on inserts. “Today, coatings are being applied so that the peaks and valleys are much smaller when you look at the coatings under a microscope,” Greenleaf’s Dillaman said.

Sales figures don’t lie: indexable cutting tools—featuring removable cutting tips called inserts—are a smash hit with machine shops. According to a McKinsey & Co. report, indexable tools accounted for about 63 percent of U.S. milling tool sales in 2017, versus 26 percent for solid-carbide tools and 11 percent for solid non-carbide tools.

Why do solid-carbide tools have an edge in these applications? Compared to indexable tools, “you’re going to be a lot more accurate with a solid-carbide round tool,” said Tim Aydt, indexable milling product manager at Seco Tools LLC, Troy, Mich. “Your wall finish will be better and your tolerance band will be a lot easier to maintain.” Indexables, he added, “are mainly just for metal removal and usually for larger machines making larger parts.”

Tools with more cutting flutes of course allow higher feed rates and more metal removal. “However, chip evacuation is also an important consideration,” Cope added. “Traditionally, we see five- to seven-flute cutters for roughing, and a much higher number of flutes for finishing. These are often solid-carbide cutters, but there are many suitable selections of inserted cutters.”

The composition of stainless grades like the 15 percent chromium and 5 percent nickel content in 15-5 PH is what makes machining a challenge, said Mark Francis, staff engineer for toolmaker Kennametal Inc., Pittsburgh. “The aerospace industry continually seeks to make lighter, stronger, better performing parts—faster and more efficiently—and machining operations must continually advance to support this drive,” Francis noted. “Stainless steel flap tracks are an example of aero components that our tools and expertise helped bring to fruition. The manufacturer wanted to use a specific stainless steel that would deliver strength and weight savings—with the added advantage of being virtually maintenance-free over the life of the aircraft.”

This is due to the price of steel and the relative ease of machining insert pockets into a steel body. “If you had a 10" (254-mm) diameter tool, you could easily put 10, 15 or even 18 carbide cutting edges in the steel body,” Pollock said.

Aydt often sees such waste firsthand. When he and his Seco colleagues walk into shops and look in their used carbide insert bucket, “many times we can tell they have thrown away unused edges,” he said. “Used-edge detection can prevent this from happening.”

What makes PH stainless steel alloys relatively tough to machine, Francis explained, is “the high-strength material matrix and an average UTS (ultimate tensile strength) of 200 ksi/1,379 Mpa. If there is forging scale to cut though, the challenge is greater. The scale is very abrasive and can cause depth-of-cut notching. Depending on part shape and complexity, it is sometimes possible to use a high feed or copy mill (round inserts) to remove the scale prior to heavy machining.”

Because stainless steel can build up on the insert, “we use sharper—more positive—rake angles on the top surface of the insert than we would use for materials like steel or iron, where we choose stronger geometries,” added John Pusatera, training specialist at Sandvik Coromant. “It is like using a sharp knife as opposed to using something that has an edge prep for strength. Having a more positive clearance helps make the tool sharper.”

Take aerospace components. A bracket that might have been a separate component is likely to be incorporated into a larger part, requiring more machine precision and flexibility.

“Machining stainless steel can be tough, so components that help ensure rigidity are key pieces to the puzzle—things like solid box or roller ways instead of simple linear ways on all linear axes and large, robust ballscrews to hold the table in position during cutting,” Cope said.

Noting the differences between automotive and aerospace applications for stainless steel, he said, “automotive, at least in my area, seems to be seen differently when it comes to machining. The automotive market is more driving on the CPU, which is driven from cost per edge of the cutting tool, as well as ease of use for machine operators, and eliminating tool handling by the operators. It comes down to what is the cheapest tool that can complete the required operation.”

“While all CAM systems can create HEM (high-efficiency milling) toolpaths that may reduce overall part cycle time, few are ideally optimized to achieve the quickest cycle time while eliminating destructive high cutting tool load,” he explained. “VERICUT software from CG Tech has proven technology that reduces the time to remove large amounts of material with HEM. We witnessed a 25 percent decrease in cycle time in stainless steel by VERICUT processing the toolpath created in CAM software on the Takumi H10 mill.”

But for aerospace, the cost of components and materials used “require different approaches,” said Atul Sharma, aerospace specialist for Seco Tools Canada. “Quality, security and reliability of the tool is paramount. Security and peace of mind [that there will be] no tool failures, and holding tolerance per part, are more of a concern. Rotating an insert edge, or tool edge, is cheaper than risking damage to a part.”

“There are people in this industry who have developed really bad practices when indexing inserts,” she said. “I was surprised at how little education there’s been in that area, and how many shop owners still ask us to [provide] that type of education for their employees.”

Dillaman added, however, that tool designers must be careful not to add so many insert pockets that there is no longer enough steel in the cutter to maintain adequate strength. In addition, users of tools with large numbers of inserts must make sure that their machines are up to the task of using all the cutters.

Mark Gilmore, technical product specialist for Takumi USA-CNC Machine Tools, Indianapolis, echoed the importance of rigid machine design and vertical mill construction to maintain tight tolerances with hardened stainless grades. “While being designed to absorb or isolate the vibration of cutting forces, they must also have the ability to accelerate and maintain speeds required to utilize the cutting tools and toolpaths of today without increasing costs by using high-priced servo motors and drives. The development of roller-type linear rails is replacing box way designs to achieve rigidity and speed and increase accuracy and surface finish.”

“A commonality among all these materials is that the cutting edges are exposed to a great deal of heat, notch wear and built-up edge,” Tucker explained. “Large positive rake angle and clearance is a must,” as is insert geometry that gives minimum contact and friction between the chip and chip face.

Other developments have come in the area of coatings. One of these is Seco Tools’ latest Duratomic coatings, which feature what the company calls “used-edge detection.”

Among Redzepagic’s top suggestions in the area of insert indexing and replacement: When there is a shim in the insert pocket, take a good look at it. And if the insert has left an impression in that shim, make sure to change the shim because the insert needs to lie on a flat surface. If it doesn’t, the insert will be able to move around in the pocket—and that motion can ruin both the insert and the cutter body.

Zertivo features improved adhesion between substrate and coating and optimized cutting edge integrity, the company said; GC2334 grades are optimized for indexable drilling in stainless steel.

The company’s small-diameter indexable milling cutters feature very small inserts. Stusak claims his company is the industry leader when it comes to five-axis pressing of inserts. “We have the ability to press and form inserts at very small diameters and hold tight tolerances,” he said.

The steel body of an indexable tool is used over and over while only the cutting edge is replaced. Normally, carbide inserts will have more than one cutting edge. When worn, the cutting edge is changed by turning or flipping the insert—i.e., indexing—to an unused cutting edge. According to Pollock, some tool designers focus on creating as many cutting edges as possible on one insert to improve the cost benefit of using their tools.

Besides the correct tooling and the right speeds and feeds, shops need heavy and well-built machines with quality components and a solid casting foundation, said Mike Cope, product technical specialist for Hurco Companies, Indianapolis.

For roughing, Tucker advised, “cutting edges should have the smallest possible reinforcement land on the edge.” Machine shops should “employ large cutting depths and feed rates in combination with lower cutting speed, rather than low depths and higher speeds.”

While indexable tools are the most popular choice for roughing, the situation is different for cutting applications that require smaller tool diameters or greater precision. In holemaking, for example, indexable tools have a much lower share of the U.S. cutting tool market, accounting for just 30 percent in 2017, according to McKinsey & Co., while solid-carbide tools accounted for 40 percent and solid non-carbide accounted for the remaining 30 percent.

Tooling for cutting stainless steel must resist high heat, excessive cutting edge buildup and wear. Additives like sulfur can improve machinability “but cannot eliminate the challenges completely,” cautioned Hurco’s Cope. “These additives aren’t allowed in some of the tougher grades of stainless to machine, such as 304 and 316.”

Positive chip breakers, Duratomic coating, chip splitters and wiper inserts help achieve better finish and productivity, he added.

Nevertheless, some companies make tools with replaceable cutting edges that are aimed at what has traditionally been solid-carbide’s turf. For example, Iscar Metals Inc., Arlington, Texas, recently introduced indexable end mills with diameters that go to 5/16" (7.9 mm). The idea is to “complement solid-carbide tools, not totally replace them,” said Bryan Stusak, Iscar’s national milling product manager.

The choice of machine tools for aerospace parts is changing, according to Gifford. Mitsui Seiki’s horizontal machine centers run from 630 mm to 2.5 m and produce everything from actuator housings and latch-type components to larger parts like flap tracks for wings. “Over 1 m is where we really have seen a large increase as parts have become more complex,” Gifford said. Parts previously suited for a 630-mm machine have been “meshed with another part and another part, and now it’s a much larger structural piece that has to be machined.”

According to Dan Tucker, product manager, Western U.S. for Sandvik Coromant, Fair Lawn, N.J., the company’s more recent technologies, such as Inveio and Zertivo, have improved durability and prolonged insert edge integrity for greater tool life when machining stainless steels.

When cutting stainless steels, coolant may not be the right choice, he added. “I’ve found the coatings on the cutting tool like heat; they have a better lubrication effect as they heat up.” That’s why he tends to recommend machining most 15-5 grades dry, especially when the insert tool is present. Under those conditions, tools have a tendency to crack when subjected to high heat and quick cooling. “Using an air blast to blow chips out of way so you’re not recutting the chips maintains temperature within the tool and is more stable for the process.”

At Greenleaf Corp., Saegertown, Pa., tool designers found that adding more pockets to their face-milling tools actually adds to the life of the inserts in the tools. What makes the difference is the extra inserts in those additional pockets. More inserts stabilize the cut and reduce the forces on each insert, which in turn reduces the number of times the inserts must be indexed, explained Martin Dillaman, Greenleaf’s applications engineering manager. “You may be adding some cost by adding an insert or two to the tool, but the productivity increases outweigh that,” he said.

“Heat-treated 15-5 is not quite as gummy, so we were able to get chips to come off the cutting tools better,” Gifford noted, whereas 17-4 “tends to be more abrasive on the cutting tool, so it wears your edge a bit faster.”

Another advance in the indexables field is Iscar’s development of proprietary insert grades designed for machining heat-resistant superalloys used in aerospace applications. Stusak claims these insert grades normally last about 20 percent longer than a typical grade in Iscar’s portfolio. Though he wouldn’t discuss the specifics, he did reveal that there are alloying elements in the carbide substrate of the inserts that give it a higher hardness in elevated machining temperatures.

YG-1 Tool (USA) Co., Vernon Hills, Illinois, has introduced a similar product. “Traditionally, indexable milling had its lane that it stayed in and solid tools had their lane. You used indexable tooling for roughing and went with solid for more intricate details, tight tolerances or fine finishes,” said Jan Andersson, the company’s product management director for indexables.

Due to the relatively high cost of carbide compared to steel, solid-carbide tools tend to be smaller than indexable tools. While the diameter of a common solid-carbide end mill might be ½" (12.7 mm) or ¾" (19 mm), Pollock said, indexable tools normally start at ¾" (19 mm), with 2" (50.8 mm) and 3" (76.2 mm) diameter sizes more common.

Redzepagic also advises shop personnel who remove inserts to make sure to clean the pockets, completely blowing out any chips that could scratch a surface and adversely affect insert fastening. If this isn’t done, the result, again, can be insert movement that wrecks the tool body.

Today, Andersson stated, “you have a little bit of overlap” between indexable and solid tools with diameters roughly in the range of ½" (12.7 mm) to ¾" (19 mm). A key development in this area is the appearance of what Andersson calls “semi-indexable” tools with a solid-carbide front end that threads into a steel or carbide carrier. With semi-indexable tools, he noted, “you still are getting the advantage of the indexable in that you are only replacing part of the tool, and you get more of the precision you get with solid tools.”

For popular grades like precipitated hardened 15-5 and 17-4, machine builders and toolmakers continue to innovate machining options. Learning how to machine stainless steel continues to evolve.

With a more uniform coating surface, cutting edges produce parts with better surface finishes, Dillaman noted. In addition, he said, coatings that lack thinner (and therefore more vulnerable) areas will do a better job of protecting insert substrates from heat.

In instances such as this, high-feed milling and dynamic milling provided the best removal rate without putting stress on the machine or part, Lawrence added. “With good software, it’s easy to achieve part tolerances.”

Once on the job, indexables can last for longer periods of time than solid-carbide tooling. “You can keep using [an indexable tool] for three, five or even 10 years, depending upon what you’re using it for,” Aydt said. He added, however, that the carrier will always wear out eventually, with the pockets deforming over time because of the immense amount of heat and pressure applied to them.

Like the EH system, YG-1’s product in this category, called iSmart, significantly reduces carbide-related tool costs and changeout time. On the downside, however, the presence of the threaded joint means semi-indexable tools are incapable of matching the precision of solid-carbide tools. As a result, Andersson places them in the semi-finishing category, between tools designed for roughing and those capable of superfinishing.

At Mitsui Seiki, cutting tool development trials with heat-treated 15-5 achieved material removal rates of about 42 in3 (688 cm3) per minute while maintaining superior edge life—without coolant.

The company builds its box way machines to remain rigid and robust to handle the types of cuts needed, he said. “We can turn the rpm up and take lighter cuts fast, say for finishing,” he said. “But for roughing, we have a big, robust spindle and castings that provide a rigid machine that can take the larger depth of cut and remove material at higher rates.”

To understand how this works, consider an indexable tool with black inserts. An operator looking at the edges of one of these inserts may not be able to tell whether they have been used. As a result, the insert may be unnecessarily indexed or even replaced.

And for semi-finishing, “there should be sufficient material left for finishing to allow the tool to go beyond the deformation-hardening zone. Avoid excessive flank wear; this leads to a dull cutting edge, creating a work hardening zone.”

“We are currently working with a major automotive OEM on a small-engine turbo manifold flange produced from a 17-4 cast stainless steel,” Lawrence explained. “Machining characteristics and makeup are close to a 310 stainless. Previous applications for these were cast iron, but due to [the need to] withstand heating cycles, stainless steel seems to handle expansion and contraction better.”