This article was first published in the September 2016 edition of Manufacturing Engineering magazine.

Iscar Metals Inc. (Arlington, TX) has developed a new range of tools, both indexable and solid-carbide, for machining composite materials. Because intensive abrasion of a cutting tool can lead to dramatic deterioration of cutting tool geometry and, as a result, to performance problems, Iscar R&D has focused on wear that can cause delamination during drilling and milling operations. In order to significantly improve the cutting tools’ performance during drilling, Iscar has developed a solution based on interchangeable heads of its SUMOCHAM product line. The new ICF drilling head geometry, which has been especially designed for drilling composite materials, provides low axial forces for smooth penetration during the cutting process without splintering phenomenon. The new heads are based on a new carbide submicron substrate and diamond coating for prolonged and predictable tool life.

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.”

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.”

“PCD diamond-veined tools can be applied to almost every type of machining operation,” said Win. “With structural products for the frame, I think it’s a little easier. What I found in drilling carbon fiber and titanium stacks is that once you clear the carbon material and encounter the titanium material interface, the titanium chips will sometimes score the composite material, again potentially causing a quality issue. To alleviate the problem, we’ve adopted a micro-peck technique in the drilling cycle that allows the formation of smaller titanium chips to clear. In regards to the amplitude of the micro-peck, we are talking on the scale of millimeters. The peck distance is going to be around 0.1–0.2 mm at a frequency of 1.5 to 2.5 micro-pecks in a 360° rotation. What the micro-peck technique does is break up the titanium chips during a drilling cycle and allows for proper evacuation of the smaller more manageable chips. The result is that we’re cutting both materials—the composite and the titanium—with one diamond-veined tool.” Sandvik Coromant is developing a new 88 series geometry PCD-veined tool for CNC applications and a new 86 series for power-feed 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.

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.

Tool geometry can also enhance productive machining of sandwiched composites. For instance, Seco’s Jabro 860 end mills are engineered specifically to machine these types of composite materials. The tool’s double-helix tool flute configuration eliminates fiber breakout, prevents delamination, and improves part edge finishes. As the Jabro 860 rotates, the flute helix on the lower part of the tool forces material upward while the upper helix forces material down. The opposed and balanced cutting forces make for a clean-cutting action.

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

“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.”

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.”

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.”

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.

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.”

Aerospace manufacturers, in particular, rely heavily on sandwiched composite structures in critical components such as aircraft wing skins, fuselage sections, cabin walls and floors. However, machining a stack of materials of differing strengths and physical properties presents several layers of challenges. The main goal is to avoid bending or fraying the core structure or delaminating the face sheets. Sandwiched composite material that is cut unevenly or deformed loses its strength, much as creasing corrugated cardboard destroys its rigidity.

Further complicating machining are that stacks of materials of differing strengths and physical properties can be combined in layers for highly specialized and targeted uses. According to Kennametal’s Composite Machining Guide fiber reinforcement materials include carbon fiber/graphite fiber, glass fibers, ceramic fibers, polymer fibers, and tungsten fibers. Polymer matrix materials include epoxy, phenolic, polymide, and polyetheretherketone (PEEK).

Due to the wide array of applications, no two CFRP materials are exactly alike. Each composite can take on different characteristics by changing the matrix formulation, fiber type, content, orientation, build-up, and the method of forming, according to Precorp.

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.”

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.”

“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.”

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.”

Composite materials for aerospace applications may include polymer matrix composites for structural components for frames and ceramic matrix composites for engine applications. “Ceramic matrix composites developed by GE, Pratt & Whitney, and Rolls Royce for the newest aircraft engines pose special machining challenges,” said Linn Win, industry specialist composites, Sandvik Coromant (Fair Lawn, NJ). “The ceramic matrix composite material is extremely abrasive and very brittle. Adding to the difficulty in machining is the required lightweighting. When you factor in the practical machining for light-weighting applications, it’s very difficult to get the cost savings benefits from the light-weighting components because of the more expensive tooling that is required for complex components like blades and blisks,” said Win.

Sharp end mill cutting edges and high cutting speeds are key factors in cleanly machining sandwiched composite materials. The situation is much like slicing bread—cut too slowly, and the bread compresses instead of shearing cleanly. While on the other hand, fast-moving, sharp cutting edges generate clean cuts. When machining sandwiched composites, slow cutting speeds can distort the face sheet and the honeycomb structure itself.

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.”

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.

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.

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.

A carbide blank is slotted and filled with diamond powder. The carbide blank is inserted into one of Precorp’s high-pressure, high-temperature presses and subjected to 270°F (132.2°C) and 876,000 psi (60,398 bar). In this process the diamond powder is compressed and the diamond crystals are bonded to each other and to the carbide blank. The PCD nib is then brazed to a solid-carbide shank. The braze is located sufficiently far away from the tip of the tool to avoid any potential thermal damage. This allows the use of a high-temperature high-strength braze joint between the nib and the carbide shank. The drill geometry is ground to produce the finished PCD tool. This patented process allows for many tool geometries that are impractical and/or impossible using conventional PCD insert processes.

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

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.”

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.”

For instance, a 0.500″ (12.7-mm) diameter Seco Jabro 860 solid-carbide end mill at a 10% radial engagement would run at a speed of 400 sfm (121 m/min) with a feed rate of 0.005 ipt (0.13 mm/t). Such parameters would apply to a sandwiched composite material with an internal Ti-Al honeycomb structure.

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.”

To reduce cycle time and handle increasing amounts of toolpath data, Takumi’s Gilmore noted a pair of solutions.

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.”

Another milling family, EPX, is intended mostly for machining carbon fiber reinforced polymers (CFRP). This family of compression end mills features opposite cutting edge directions—a combination of right and left helix along one flute. This progressive cutting edge geometry reduces delamination and improves tool performance when milling CFRP and the technology is especially recommended for increased feed rates.

As part of continuing efforts to offer lighter, stronger, more cost-efficient products, manufacturers develop and apply high-performance workpiece materials. Sandwiched composites are good examples of that trend, according to Don Graham, manager of education and technical services at Seco Tools LLC (Troy, MI). Here is Graham’s assessment of the current state of composites machining:

Carbon fiber reinforced polymer (CFRP) composite materials deliver the important performance advantages of high strength-to-weight ratio, durability, and extreme corrosion resistance in lightweight structures, valued especially for demanding aerospace and oil and gas industry applications. Difficulty of machining can vary significantly depending on the combinations of matrix material and fiber reinforcements selected. Due to the wide array of applications, no two CFRP materials are exactly alike. Each composite can take on different characteristics by changing the matrix formulation, fiber type, content, orientation, build-up, and the method of forming, according to Precorp Inc. (Spanish Fork, UT), a company since 2013 in the Sandvik Coromant organization.

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.”

Like corrugated cardboard, a sandwiched composite is comprised of a lightweight core structure, usually resembling the hexagonal cells of a honeycomb, backed by rigid facing sheets. Depending on strength requirements, the honeycomb cells may be formed from high-tech paper, cardboard, carbon-fiber-reinforced plastic or aluminum. The face sheets can be paper, plastic, aluminum or titanium and are bonded to the open ends of the honeycomb cells. A balance of bending, compression and shear forces among the elements of sandwiched composites, results in materials that are lightweight, rigid and remarkably strong.

High cutting speeds, however, generate heat, and that poses problems because many of the constituents of sandwiched composites are heat-sensitive. Accordingly, light radial engagement—on the order of 5% of cutter diameter-minimizes heat generation. For the same reason, feed rates are kept low. Despite the light engagement and low feed rates, high cutting speeds help maintain productivity.

Polycrystalline diamond-veined tooling is a technology that has proven to be very effective for machining composites. “PCD is great for its wear resistance properties in composite machining applications, which may lead to longer tool life, but in order to gain the full potential of PCD, the tool will need to have positive cutting geometries,” said Win. “Conventional brazed tooling, which involves brazing a PCD wafer into a pocket, at best allows for minute positive formations, not the optimum for machining composites. PCD-veined tooling produced by Precorp, a subsidiary of Sandvik Coromant, enables PCD cutting tools to be manufactured with the high rake angles and helix angles required to effectively machine composites.

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.

For milling applications, the versatile Multi-Master tool system, with interchangeable heads, features a carbide head with brazed PCD tips. Due to this innovative design, a machined composite workpiece experiences less loading and swarf evacuation issues and surface finish is improved. The main applications for these tools are orbital milling, edging, and ramping down.

The components of sandwiched composites can be abrasive, so the tools used to machine them usually are manufactured from micrograin carbide to maximize edge integrity and wear resistance. To further resist abrasive wear, some cutting edges receive diamond coatings. Seco’s DURA thin diamond coating, for example, is applied via CVD and combines low surface roughness for lubricity with high adhesion characteristics that reinforce its wear resistance. The coating requires a balance in tool engineering in that it is thin enough to minimally affect sharpness but thick enough to provide resistance to abrasion.

“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.

“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.

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.”

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.”

“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.”

Machining characteristics of CFRP/CFRP and CFRP/metal materials are impacted by the abrasiveness of fibers, by fiber size, fiber diameter, fiber length, volume of fibers (percentage), and fiber layout, unidirectional or fabric weave. For example, abrasiveness of fiber increases with strength and diameter. Short fibers tend to delaminate, as do unidirectional layered composites. To counter the tendency to delaminate, Kennametal has developed compression-style routers that generate cutting forces at top and bottom of the materials’ surfaces, as well as other tools including burr-style routers, down cut-style routers, and ball end routers.

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.

Sandwiched composite parts typically are flat or mildly curved panels that range in thickness from 0.250 to 0.500″ (6.35–12.7 mm). The panels are fabricated to near-net-shape and finish-machined to trim outer edges, mill out widows and other various shaped openings and holes. For finish machining, shops must use high-speed end mills specifically designed for such sandwiched composites.

SUMOCHAM for composites is suitable for use on any type of machine-tools such as CNC machines, robots, and even powered feed machines (ADU) for which special thread connectors are available. The fast head replacement and high positioning repeatability provide minimum machine downtime. Relatively small indexable drilling heads with diamond coating provide an economic advantage, compared to long full solid-carbide drills, as well as easy stock management. The SUMOCHAM range for composite materials covers today a diameter range from 0.250 to 0.500″ (6.35–12.7 mm).

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.

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.

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.”

Iscar also offers a range of solid-carbide drills, starting from 0.118″ (3 mm). The tool geometry of the CFD family has been designed with a stepped point and with two working sections, considerably improving surface finish and allowing a smooth cut on very difficult-to-machine composites, like RTM or thermoplastic materials.

Motion control systems are also critical, particularly for optimum surface finish and, to some degree, part tolerance.

“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.”

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.”

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.”

Iscar’s EPN-F family of solid-carbide end mills feature cutting edges that are divided into sections. This design results in better distribution of load on the end mill and machined workpiece, and thus provides increased tool life and improved surface finish, especially when machining carbon fiber and honeycomb composites.

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.”

“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.”

Machining the open construction and lightweight materials characteristic of sandwiched composites involves very low cutting forces, so high-torque machine tool spindles are often unnecessary. However, most sandwiched composite parts are big, such as expansive aerospace wing skins, and the machines that cut them are very large in size as well as powerful.

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.”

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.”

“We have found that in drilling applications in ceramic matrix composites, we do not encounter too many issues when entering the material; the problems occur when we exit the material. On the exit surface, we find that the breakout can be quite poor, due to the high axial pressures that are being applied to the workpiece upon the tools exit. The back side of the material in an unsupported environment tends to blow out, which may lead to quality issues. In an unsupported application, we believe there is not an effective way to machine ceramic matrix composites, due to the materials inherent brittleness,” said Win. “But what we have found is that when we apply positive tool geometry with the PCD-veined diamond tooling, you can actually reduce the amount of stress forces that being applied because now you have freer cutting edge and are able to produce the features that are needed with lower thrust force.”

“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.”

Industry is examining different methods of machining sandwiched composites, including waterjet and other abrasive cutting methods. All the alternatives have advantages and disadvantages. The bottom line is that flawless machining of sandwiched composites, especially those intended for aerospace applications, is crucial. Any imperfection on the skin of the wing can be a crack initiation site, and vibration and other forces in the aircraft will cause a crack to grow. In the interest of reliability and safety, manufacturers will continue to employ the cutting tools and techniques that have been developed and proven over time when machining sandwiched composite materials, concludes Seco’s Graham.

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.”