Superalloys (nickel-, cobalt-, and iron-based) are hard, very tough, and generate even higher cutting temperatures at the same cutting speeds as steels. Because they also work-harden very easily, they tend to create depth-of-cut notching, said Ludeking.

"These properties make them much more challenging for the coating because there are additional wear modes to protect against," said Ludeking.

"About 80 percent of inserts used today are coated carbide grades. They give machinists the ability to increase metal removal rates and cutting speeds, while increasing tool life well beyond what is possible with an uncoated insert," said Graham.

"A turning operation, because it's typically a continuous cut, creates a lot of heat," explained Don Graham, manager of education and technical services for Seco Tools. "This makes it essential that a coated tool be used."

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Textured coating typically has an inner TiCN base layer, which adheres to the substrate and creates strength at the cutting edge, and a top layer of Al2O3 that forms a thermal barrier, permitting higher cutting speeds.

As with CVD, the PVD coating type is chosen based on the intended application and the wear modes that will be created during the machining process. Each coating has unique hardness, chemical, and physical characteristics that make it appropriate for different material classes.

Like any metal cutting operation, the best place to start is by examining the workpiece material. The type of material being turned plays a key role in determining whether a coated or uncoated carbide insert is needed.

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The PVD coating process provides a smooth coating that adheres well to sharp edges. Because PVDs are thinner than CVD coatings, and because they are applied at lower temperatures, they don't affect the toughness of the tool the way CVD coatings do.

For example, carbon and alloy steels generate high temperatures during the turning process. They will also react chemically with the carbide insert. The tool's coating must be able to resist the heat and allow the chips to flow smoothly to reduce crater wear and flank wear. The different properties of other materials result in different wear modes, which the coating needs to resist.

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"The CVD coating process depends on chemical reactions, and the coatings are bonded to the substrate both chemically and physically," explained Ludeking. "This makes it possible to create relatively thick coatings that provide excellent crater and flank wear resistance, as well as an excellent thermal barrier protecting the substrate."

Texturing enables engineers to control the growth of the Al2O3 crystal structure, aligning it in such a manner that it creates a much smoother surface. This engineering of the coating at the atomic level achieves three goals:

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"Manufacturers can design coatings that combine the best properties of the different coatings and provide excellent resistance to several wear modes," said Ludeking.

The NeoMill-Alu-QBig indexable insert milling cutter from Mapal stands for top performance in high-volume milling of aluminium. The tool manufacturer thus offers a very economical solution for use on high-performance machines, such as those found primarily in the aerospace industry.

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While the coating's makeup itself is important, it also needs to be applied to the correct substrate to get the maximum benefits.

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"The workpiece material properties tell us quite a bit about what the machining conditions will be like at the cutting edge," said Kurt Ludeking, product manager for Walter Tools. "The coating must be chosen to resist the wear modes that will operate during machining and the high temperatures created during cutting."

Using steel as an example, a multilayer coating provides crater wear, flank wear, and plastic deformation resistance far better than TiN, TiCN, or Al2O3 could provide on their own.

In the future look for continued development of harder, more wear-resistant PVD coatings that can run at higher speeds, getting closer to the performance of CVD coatings in that respect.

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"Fortunately for end users, cutting tool manufacturers match the many technical details of substrates and coatings into highly effective products, so all that is required is to choose the correct grade for the application," said Ludeking.

Chemical vapor deposition (CVD) and physical vapor deposition (PVD) are the two main coating processes applied to carbide inserts. CVD coatings are the thicker (often 5 to 20 microns) of the two and are very wear-resistant. This makes them suitable for steels and cast irons.

The 3D Finish tools are available in a barrel, oval, taper or lens shape and are ideal for the five-axis finishing of free-form surfaces, such as in tool and die production or, in the aviation and aeronautical industries. The cutters are available in a wide variety of size and style to ensure the ideal cutter geometry for any component shape. Also, in terms of application they are universal in that the 3D Finish milling cutters are suitable for machining steel, stainless steels, cast iron, non-ferrous metals, heat-resistant steels and hardened steel.

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Also expected is the further development of CVD processes that take advantage of preferred orientation of the coating grains, known as texturing, for higher wear and heat resistance, combined with postcoating processes that make the coatings tougher and more resistant to chipping.

It is this thermal barrier that is important in high-speed operations in which plastic deformation is the key concern.

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Manufacturing companies are constantly looking to reduce machining times, their search may be over with the arrival of WNT’s 3D Finish range, which provides quick and reliable results, especially on time-intensive finishing processes, where it offers huge potential for shortening cycle times and improving productivity.

This structural change creates a coating that has better mechanical and thermal properties, as well as better wear resistance and toughness, said Graham. Seco's textured coating, for example, has an inner TiCN base layer, which adheres to the substrate and creates strength at the cutting edge, and a top layer of Al2O3 that forms a thermal barrier, permitting higher cutting speeds.

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"Selecting the correct coating type means taking both coating processes and all of the coating materials into consideration," said Graham.

The new 3D Finish from WNT (part of the Ceratizit Group) is said to enable users to significantly reduce the time spent on finishing processes carried out on 5-axis machines, while achieving improved surface qualities at the same time. The WNT 3D Finish tool range is a development of the ball-nose end milling cutter and has been designed with a large radius (up to 1,500mm) in the cutting area. This feature allows the cutters to achieve considerably increased stepovers when compared to conventional ball-nose cutters with the same tool diameter. The result is processing times in pre-finish and finish machining reduced by up to 90%, without the workpiece, tool and machine being subjected to a higher load. Much greater widths of cut (ap) are also possible so, in addition to reduced cycle times, surface quality is maintained, but is the user chooses to reduce the step-over then surface quality can be improved significantly. A typical Ø 20mm ball nosed cutter would achieve a surface quality of Ra1.6 µ with a stepover of 0.5mm, but with the 3D Finish and a radius of 1,500mm, the stepover could be increased to 6.93mm with the same surface quality result.

Coating type is chosen based on a number of variables, but the most important factors are the workpiece’s material properties and the application. Image courtesy of Seco Tools.

Manufacturing companies are constantly looking to reduce machining times, their search may be over with the arrival of WNT’s 3D Finish range, which provides quick and reliable results, especially on time-intensive finishing processes, where it offers huge potential for shortening cycle times and improving productivity. The new 3D Finish from WNT (part of the Ceratizit Group) is said to enable users to significantly reduce the time spent on finishing processes carried out on 5-axis machines, while achieving improved surface qualities at the same time. The WNT 3D Finish tool range is a development of the ball-nose end milling cutter and has been designed with a large radius (up to 1,500mm) in the cutting area. This feature allows the cutters to achieve considerably increased stepovers when compared to conventional ball-nose cutters with the same tool diameter. The result is processing times in pre-finish and finish machining reduced by up to 90%, without the workpiece, tool and machine being subjected to a higher load. Much greater widths of cut (ap) are also possible so, in addition to reduced cycle times, surface quality is maintained, but is the user chooses to reduce the step-over then surface quality can be improved significantly. A typical Ø 20mm ball nosed cutter would achieve a surface quality of Ra1.6 µ with a stepover of 0.5mm, but with the 3D Finish and a radius of 1,500mm, the stepover could be increased to 6.93mm with the same surface quality result. The 3D Finish tools are available in a barrel, oval, taper or lens shape and are ideal for the five-axis finishing of free-form surfaces, such as in tool and die production or, in the aviation and aeronautical industries. The cutters are available in a wide variety of size and style to ensure the ideal cutter geometry for any component shape. Also, in terms of application they are universal in that the 3D Finish milling cutters are suitable for machining steel, stainless steels, cast iron, non-ferrous metals, heat-resistant steels and hardened steel. In assessing the cost-effectiveness of the WNT 3D Finish tools, the machining experts at the Ceratizit Group carried out extensive trials. These highlighted the major time savings that can be achieved when compared to a conventional 10 mm diameter ball nose cutter. In the trial a 100 x 200mm component was milled at a feed of Vc 200m/min. Owing to the higher stepover of 1.5mm for the 3D Finish (compared to 0.1mm for the ball nose cutter), the total milling travel was reduced from 200m to just 13m, with a corresponding reduction in cycle time from 79 minutes to 7 minutes and a resulting reduction in machining cost of 88%. To fully exploit the advantages of the 3D Finish tools fully, a modern 5-axis machining centre and compatible CAD/CAM software are required, such as Open Mind’s hyperMILL MAXX, which supports the tool geometry, other software suppliers also offer CAD/CAM solutions. www.wnt.com/uk/cutting-tools/milling-tools/3d-finish.html

"These properties make PVD coatings very useful for workpiece materials that are tough, sticky, or work-harden easily like stainless, high-temperature alloys, and nonferrous alloys," said Ludeking.

PVD coatings are thinner (often 1.5 to 4 microns) than CVD but are tougher and smoother. This makes them the correct choice for difficult-to-cut materials.

These high temperatures within the cutting zone can enter the cutting edge, leading to plastic deformation of the tool, which reduces tool life. Applying a coating to the carbide insert improves wear resistance, allowing a single insert to remove more material. In most cases, higher machining speeds also can be achieved.

"Since aluminum is a 'soft' material, BUE is a major concern," said Ludeking. "Preventing this requires an extremely smooth surface, as well as a very sharp cutting edge. Because nonferrous materials don't generate the high temperatures at the cutting edge, uncoated inserts are still effective on these materials. Coated inserts will still have a significant speed and tool life advantage, although it is smaller in nonferrous alloys. "

The increasing use of difficult-to-machine materials continues to place pressure on tooling manufacturers to develop new products that can withstand the heat generated from the cutting process, while providing longer tool life.

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"These trends will continue to move the industry toward the ideal cutting tool—hard, wear-resistant, and with excellent edge toughness—enabling higher-productivity machining across the whole spectrum of materials," said Ludeking.

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"Because each layer has its own function, such as adding toughness, lubricity, or wear resistance, combining them together gives us the best of each."

Inserts with a combination of coatings are very common in turning operations, so these tools can be used in a wider variety of materials and conditions.

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"By creating a coating with a smoother surface, less friction occurs and the chips flow across the surface of the insert better," said Graham. "We do this by orienting the crystal structure of the aluminum oxide layer parallel with the insert's surface."

In assessing the cost-effectiveness of the WNT 3D Finish tools, the machining experts at the Ceratizit Group carried out extensive trials. These highlighted the major time savings that can be achieved when compared to a conventional 10 mm diameter ball nose cutter. In the trial a 100 x 200mm component was milled at a feed of Vc 200m/min. Owing to the higher stepover of 1.5mm for the 3D Finish (compared to 0.1mm for the ball nose cutter), the total milling travel was reduced from 200m to just 13m, with a corresponding reduction in cycle time from 79 minutes to 7 minutes and a resulting reduction in machining cost of 88%. To fully exploit the advantages of the 3D Finish tools fully, a modern 5-axis machining centre and compatible CAD/CAM software are required, such as Open Mind’s hyperMILL MAXX, which supports the tool geometry, other software suppliers also offer CAD/CAM solutions.

The substrate is the foundation for the coating. Having the correct foundation enables the coating to be supported and bonded in the best possible way and enhances the overall performance of the tool.

Titanium, another high-temperature material, has most of the same properties as the superalloys, except that crater wear tends to be the most common wear mode, and BUE tends to be more common as well.

The coating type is chosen based on a number of variables, but the most important factors are the workpiece's material properties and the application range that the insert is designed for. For steels, good crater wear and abrasive wear resistance are critical, so aluminum oxide (Al2O3) is almost always a component of coatings intended for steel machining.

"By layering the coatings we can tailor a product to tackle a specific problem or material," said Graham.