A&L Machine, a shop that serves primarily the energy sector and its often on-demand needs, is always on the lookout for ways to expedite critical processes. For Allen Dvoracek, A&L Machine president, this occasionally means travelling to suppliers for a firsthand look at how their products work.

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.”

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.

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.”

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.

“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.

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

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.”

Single- or multiple-point precision tool used to bring an existing hole within dimensional tolerance. The head attaches to a standard toolholder and a mechanism permits fine adjustments to be made to the head within a diameter range.

“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.”

Just in time (JIT) manufacturing, by nature, increases setups and shrinks batch sizes. This puts shops in a bind, because more setups and smaller batches are typically less profitable; but saying "no" to work can be even more painful. The burden to search out creative ways to reduce cycle and setup time in order to realize a profit falls to the shops that commonly perform JIT work and on the shops that can’t afford to turn it down when the opportunity arises.

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.”

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“This tooling has proven much more durable and efficient than the original end mill setup,” Dvoracek added. “We’ve run more than 10,000 parts and have never had to change an insert. And we’ve only changed the offset once, and that was only because we originally set it on the high side of the limit and decided to put it on the low side.”

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.”

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.”

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.”

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.

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.

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.”

Machine designed to use a serrated-tooth blade to cut metal or other material. Comes in a wide variety of styles but takes one of four basic forms: hacksaw (a simple, rugged machine that uses a reciprocating motion to part metal or other material); cold or circular saw (powers a circular blade that cuts structural materials); bandsaw (runs an endless band; the two basic types are cutoff and contour band machines, which cut intricate contours and shapes); and abrasive cutoff saw (similar in appearance to the cold saw, but uses an abrasive disc that rotates at high speeds rather than a blade with serrated teeth).

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.”

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.”

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

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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.”

“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.

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.”

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.

Philosophy based on identifying, then removing, impediments to productivity. Applies to machining processes, inventory control, rejects, changeover time and other elements affecting production.

“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.

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.

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.”

Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

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.”

One such trip took Dvoracek to Europe to visit a machine tool builder. But, it was a side trip to Switzerland that proved most fruitful. Dvoracek swung through Rǖmlang, Switzerland, and KAISER Precision Tooling’s headquarters in search of insight into how Swiss toolmakers are addressing high-cost JIT production.

Matt Tegelman is the Kaiser product manager for BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill. For more information about the company’s boring tools and other products, call (888) TOOL-PRO or visit www.bigkaiser.com.

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.”

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.

Vacio agreed with Dvoracek’s initial instinct, that the SW Twin Cutter could improve the process. He also explained that the head’s versatility, whether running as a dedicated tool or only as needed for one of its varying capabilities, fit right in with the JIT work typical at A&L.

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.”

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.”

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.”

“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.”

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.”

“The results were immediate and undeniable, settling in at application metrics of 590sf and .002ipr,” Dvoracek said. “We’re getting better parts off the machine sooner, allowing us to get to the next job ahead of schedule. Reducing the cycle time by 22 seconds per part will save approximately 40 machine hours a year on this job alone. That’s 40 extra hours of machine utilization that we didn’t [previously] have because of a fairly simple tooling solution.”

While on the tour, Dvoracek saw a demonstration of SW Twin Cutter heads, but the KAISER representative made a point to show that this body could facilitate much more than just common boring jobs. He demonstrated face grooving, balanced- and stepped-boring, and even compatibility with insert holders for back boring and chamfering. That got Dvoracek thinking about a challenging feature that A&L is often called on to produce that requires two different types of tools, and makes for long, complex cycles.

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.”

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.”

“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.”

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.

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.”

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.”

As Vacio had forecasted to Dvoracek, when subsequent batches of the part arrived for finishing, the print didn’t call for face grooving. It wasn’t a problem. The outside grooving insert was simply re-positioned to face the OD boss and reset to the boss diameter for rotationally symmetric rouging (RSS). Two inserts performing the same cut doubled the feed rate, reducing production costs in additional ways.

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.

“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.”

When he returned home, Dvoracek reached out to BIG KAISER, the exclusive dealer of KAISER tooling, and explained what he saw on his trip. When Gerard Vacio, BIG KAISER’s regional sales and support engineer, visited A&L, Dvoracek showed him how they were using the combination of a small-diameter, long-projection end mill and a grooving tool for the job. The end mill machined an OD boss and the inside wall of a face groove at the bottom of the boss. The grooving tool then cut the remaining face groove geometry. Dvoracek explained to Vacio that in addition to the time involved with resetting depth offsets to blend the end mill and grooving tool, the end mill was prone to breakage.

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.

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.”

“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.”

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.

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.

So confident it was a match, Vacio offered a no-risk trial of the boring head and Dvoracek agreed. He took the print specifications to the BIG KAISER engineering team and was able to quickly turnaround a ready-to-run solution. The head was configured with two insert holders. The inside grooving tool would machine the boss. Once the face of the part was reached, the outside grooving tool finished the outside of the groove.