The enlarged surface structure shows the standard PVD process (top), the HiPIMS PVD process of Walter’s WNN10/WSM01 grades (center), and the size of a human hair compared to the HiPIMS surface (bottom). Walter

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.

The machining process is a clever combination of physics, materials science, and mechatronics. During machining, material removal is the result of the force between the workpiece and the tool. The nature of these interactions determines the color and size of the chips. Chips are valuable test and diagnostic data for CNC process engineers; however, if not handled properly, a chip can even reduce machine performance.

Discontinuous chips

OSG’s A-BRAND® ADO-TRS drill is an advanced performance high-feed 3-flute carbide drill. Patented geometry provides stable chip ejection, even with less flute space inherent in 3-flute drills. The result is up to 3X faster cycle times and 3X longer tool life than 2-flute drills.

Although there are various methods of getting rid of the interfering plastic, such as mechanical; using coolant; or „by force,” the most common method of forced detachment is the use of a chip breaker.

These chips are very similar to the continuous chips discussed above but are not as smooth as them. They are mainly formed when cutting hard materials that are processed at high temperatures and speeds. In this case, the friction created between the tool and the workpiece is too great. Therefore, the chips stick to the edges of the machine tool, and when these chips overlap in successive layers, a build-up is created on the edge of the machine being used. During the cutting process, the dimensions of these build-up edges are constantly changing. High temperature and pressure between the tool and the workpiece, the use of improper coolant, high friction on the surface of the machine, and the machining of hard plastics are some of the most common reasons for the formation of this type of swarf.

AlTiN. This is a harder, smoother variant of TiAlN ideal for dry machining and machining titanium alloys, INCONEL, stainless alloys, and cast iron. It maintains a high hardness at very high temperatures. It can offer up to 14 times longer tool life than uncoated tools.

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

what causes built-upedge

They are used for inserts with a variety of coatings and substrates, and their most basic function is to force chips to curl more than they would under natural conditions. Forced wrapping can cause chips to break when they strike the workpiece or tool. Chip breakers increase machining productivity by better controlling waste and reducing cutting forces. Most modern tools of this type take the form of grooves or obstacles in the form of notches in the cutting tools. The goal of chip breaker design is to find the best geometry for a given process scenario, which then puts pressure on the filing and makes it susceptible to fracture because they have a small groove behind the front cutting edge. The geometry of this groove determines the radius of curvature of the chip. Interlocking type chip breakers have a characteristic geometry similar to a step. Obstacle-type chip breakers, on the other hand, have a characteristic geometry similar to a step. The barrier can be integrated into the cutting tool or attached to it. In the case of the „installed” type, they can be adapted to different machining conditions.

Diamond coatings, whether diamond-like carbon coatings (DLC) or polycrystalline diamond (PCD), are applied using a CVD application. These coatings are ideal for extremely abrasive materials like graphite or carbon-fiber-reinforced polymers (CFRP).

Heterogeneous chips, also known as serrated chips, are semi-continuous. They appear saw-like due to low shear deformation and high notch deformation areas. They are typically made of materials with low thermal conductivity or thermally softened mechanical strength. Titanium alloys, nickel, and austenitic stainless steels are examples of workpiece raw materials that can form uneven chips during machining. One reason for this type of filing is the large deformation of the tool chip surface when cutting hard materials at moderate cutting speeds.

Coatings particularly make sense for ferrous or steel-based materials, whether it’s stainless steel, high-temperature alloys, cast iron, or the like. A coating is necessary almost all of the time to allow for increased speeds, but also because they increase tool life. There are big productivity gains with coated tools.

Built up edgeformation

HiPIMS is an advanced, albeit complex, method of PVD that achieves a nearly complete ionization of the coating metal. This technology is characterized by a coating that is extremely thin— in the 1 to 2 μm thickness range—and dense.

Keep up to date with the latest news, events, and technology for all things metal from our pair of monthly magazines written specifically for Canadian manufacturers!

CVD is a method to produce coatings by means of thermally induced chemical reactions. The cutting tool is placed in a CVD reactor, where thin-film coatings are formed as a result of reactions between various gasses and the heated surface of the tool substrate.

“A lot of the coating companies are now offering such a wide range of coating technologies, and they'll try to get very specific in terms of machining application, whether you're roughing or finishing. And they will also look at the workpiece material,” said Walrath. “Today’s coatings are optimized for particular applications.”

The formation of material waste depends on the choice of material and the parameters of the cutting process. Chip removal is an important factor to consider when improving overall productivity and planning autonomous machine operations. Although segmented and continuous chips are self-destructive under certain cutting conditions, the use of chip breakers in systems is a practical principle. Unwanted waste is broken to the proper length by such a tool, preventing chips from becoming entangled in the machine, reducing vibration, and preventing damage. Chip breakers also reduce cutting resistance by preventing chipping and breaking of the cutting edge. Chip control is an extremely important component when we are doing metal cutting. Chip formation and edge overbuilding can prove to be a problem for our CNC machine. For this reason, it is a good idea to constantly analyze this phenomenon. When using a chip breaker, choose the one that is right for the job. For turning operations such as finishing, medium, and roughing, we need to choose a matching accessory for each, as using the correct chip breaker is critical depending on the desired depth of cut, feed rate, spindle speed, and desired poor surface finish.

Discontinuouschipformation

“For example, CERATIZIT’s Dragonskin coating is a nanolayer coating with different layers,” said Walrath. “If you take a human hair size, you're looking at 100,000 nm of thickness. This coating is less than 100 nm, and within that thickness there's a multitude of layers of different coatings. Because it has this variety of different chemical compositions, it gives you a very broad base of application, yet you can almost reach total optimization because within that composition is a component that is going to be ideal for that particular machining application.”

The machining of ductile metals, such as steel, with high cutting speeds and large rake angles, results in long chips. The swarf generated at elevated temperatures and continuous high speeds can compromise the safety of machine tool operators, damage products through entanglement in tools, and make swarf removal difficult. Chips can fall off spontaneously or as a result of a forced fracture. When machining plastics, waste materials tend to curl up due to differences in temperature and flow velocity.

Built up edgein metal cutting

Bestcontinuous chip with built up edge

With the various layers within the coating, cutting tool manufacturers can tailor the product to deal with specific application or material challenges. Each layer offers its own characteristics and specifications, like hardness, toughness, wear resistance, lubricity, and combining them together provides more universal benefits.

“First and foremost, coatings increase the life and performance of any tool,” said Urmil Patel, applications engineer, OSG Canada, Burlington, Ont. “Compared to a bright finish, coatings allow for increased cutting parameters. With a coating, there is less friction towards the chips coming out of a drilling application, which means less resistance for chip flow. They also make it easier to identify tool wear. There are many advantages of using a coated tool, but for any heat-resistant applications or tough-to-cut materials, coatings are a must.”

“Because the temperature of the process is quite low, the coatings aren’t as thick as its CVD counterpart,” said Garud. “This means that it can have a much sharper edge. PVDs are typically used for stainless steel turning or machining of superalloys like INCONEL and some titanium applications. PVD grades also are suited for grooving, as it has a very low cutting speed towards the centre of the workpiece, which requires a tough grade. Milling is inherently interrupted cutting, which also requires a tough grade, so you will see a lot more PVD coatings, although we are starting to see some CVD coatings available for grooving and milling.”

TiN. This is a widely used standard coating with universal applications. It can be used in steel, brass, cast iron, as well as aluminum, but liquid cooling is required. It offers three to four times longer tool life than uncoated tools.

“We tend to see uncoated tools with anything in the ISO N series like aluminum, particularly because these materials tend to be soft,” said Scott Walrath, business development manager, CERATIZIT, Schaumburg, Ill. “Depending on the series of aluminum, it can be very abrasive, which may make a coating useful. Uncoated tools have less wear resistance in high-silicon aluminums. The coating is more for the lubricity factor that it offers, where it can lower the coefficient of drag rather than the hardness. In some cases, a ZrN [zirconium nitride] coating will aid in chip evacuation for aluminum."

Because diamond is one of the hardest materials known to man, it can also be used for high-silicon-content aluminums, which tend to be very abrasive. A lot of grooving inserts use a cutting edge with PCD-brazed tips, especially for medical device applications.

PVD technology is based on a physical response method rather than the chemical response of the CVD process. A material vapour is condensed on the substrate’s surface under vacuum conditions. The process uses a lower temperature than CVD and therefore the substrate properties are less affected. This makes it especially suited for fine-grained carbides and other tough materials.

During machining, as the tool plunges into the workpiece, the metal in front of it is compressed. When it exceeds the strength limit, the metal separates from the workpiece and separates as a chip. The shear plane rises upward from the attachment in front of the tool. The value of the cutting angle depends on the type of material and the cutting conditions (tool inclination, speed of rotation, etc.). The smaller the angle, the longer the cutting path, resulting in a thicker chip and higher cutting force, and vice versa. Secondary shearing occurs due to friction as the chip moves along the face. Resistance increases the temperature of the cutting process, causing overheating of the derived materials of the CNC process. In this article, we will try to give you some general knowledge useful to start our adventure, but it is worth mentioning that this is only the tip of the iceberg, and the field of CNC itself is quite extensive and undoubtedly very interesting.

TiCN. Ideal for drilling and reaming of cast iron, high-silicon aluminum alloys, copper, and abrasive materials, TiCN has a very high degree of hardness with good toughness characteristics. Coolant or cooling at the cutting edge is required for higher cutting speeds. It can offer five times the tool life than uncoated tools.

Of the standard coatings available, TiN, titanium aluminium nitride (TiAlN), aluminium titanium nitride (AlTiN), and TiCN are the most common. Using the CVD process, these are the four most common coatings, whereas with the PVD process, these and virtually any other materials can be used as a coating.

Discontinuous chips have a non-uniform shape and are usually deformed by multiple cracks. Workpieces made of hard and brittle metals such as cast iron, brass, and bronze have been known to produce discrete, small chips. They can also be generated from very tough workpieces where the friction between the workpiece and the tool is high. The production of discontinuous chips is caused by low feed rate, high cutting speed, low rake angle, deep cutting of the material, and so on. Their discontinuous formation on brittle materials improves surface finish and reduces energy consumption. However, at the same time, when machining ductile workpieces, it causes poor machining of the field we are developing and increases the duration of the entire process.

It is very common for most cutting tools today to have some sort of coating added to the substrate. It’s important to strike a balance between a quality substrate and a quality coating—the company can provide the greatest coatings available, but without a high-quality substrate, the coating can be rendered ineffective. Also, having a quality substrate is essential in applications where uncoated tools are used.

A typical CVD coating is generally between 17 to 20 μm in thickness but can be as thin as 5 μm. And for the most part, CVD coatings include titanium carbide (TiC), titanium carbon nitride (TiCN), titanium nitride (TiN), and aluminum oxide (Al2O3).

Another benefit of multilayer coatings is they allow for a multitude of different chemical compositions to layer through the coating structure. This means that the coating can offer a wider application range.

Titanium carbonitride (TiCN) coating is ideal for drilling and reaming of cast iron, high-silicon aluminum alloys, copper, and abrasive materials. Matveev_Aleksandr/iStock/Getty Images Plus

There is an exception to the rule that uncoated tools make sense for cutting aluminum and aluminum alloys with high silicon content.

Built up edgechips

TiAlN. This is a general purpose coating that also is suited for interrupted cuts. It maintains high-temperature hardness and oxidation resistance, high cutting speeds, with cooling not being essential to its success. In some applications, it can have a tool life that is 10 times longer than uncoated options.

“With diamond-like carbon coated tools, this coating enables good flute polishing, meaning that it has a lower coefficient of friction with non-ferrous materials,” said Patel. “DLC coatings especially make sense in high-quantity production environments where it is important to consider the longevity of the tool. This is where DLCs really shine.”

“We are starting to see more multilayer coatings in solid carbide drilling, but also turning and milling,” said Garud. “Multilayer coatings are really the biggest trend right now. Multilayer coatings tend to be quite a bit more elastic, as it has extremely well-bonded layers compared to single layer coatings. It doesn’t tend to flake, making the life expectancy of the tool to be that much higher. Tiger-tec Gold for turning steels is one such example of latest generation of multilayer CVD coatings.”

Types of chips in metal cutting

“Uncoated tools are primarily used in aluminums and non-ferrous materials,” said Sarang Garud, product manager, Walter Tools, Greer, S.C. “The benefit of uncoated is that it can still be polished, so, for example, with a polished uncoated carbide, the chips won’t stick. This is helpful when cutting aluminum as it keeps the grade tough. There is no built-up edge. Uncoated tools also have very sharp cutting edges as there is no cutting thickness to add an artificial hone.”

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“Some of the technologies like medium-temperature titanium carbon nitride have been around since the late 1980s, early 1990s,” said Garud. “These are great options for reducing flank wear.”

Diamond-like carbon coated tools allow for good flute polishing, meaning that it has a lower coefficient of friction with non-ferrous materials. OSG Tool

Multilayer coatings are becoming increasingly popular. They offer extreme toughness in very thin layers, sometimes only 1 or 2 nm thick. One of the advantages of using a multilayer coating is that it prevents any cracks from spreading beyond the originating layers during the machining process.

CVD coatings are ideal for general turning operations, especially in steel and stainless applications. The thickness of the coating provides wear resistance, particularly in applications where crater wear is common.

CERATIZIT’S Dragonskin is applied using nanocoating technology that produces tough layers and reduces cutting edge breakage. CERATIZIT

“CVDs are generally a thicker coating and ideal for cast iron, steels, or any application that requires a more robust tool or high load bearing capacity,” said Garud. “With turning, especially if the cuts are continuous rather than interrupted, you can use the thick CVD coating for a long tool life at the highest of cutting speeds and feeds.”

“While it is very thin, it’s the density that makes it special,” said Garud. “This makes it exceptionally smooth, almost as if it is machining with a mirror. This coating limits built-up edge and prevents the chips from adhering to the substrate or tool. It is ideal for some aluminum and superalloy applications. They tend to be more expensive because we tend to put them on ground inserts for machining things like INCONEL or a finishing grade. And the price reflects the combination of the grade and the coating.”

Also called ribbon chips, they are homogeneous and have no cracks or segments, which makes them by all means unique. Moreover, they are interconnected to form a long coil and are obtained when cutting plastic materials such as aluminum, mild steel, mild steel, etc. They are characterized by having small notches on the top side, while the underside is smooth and shiny. Continuous chips are formed when machining parts made of hard materials. High cutting speed, high rake angle, low depth of cut, low material share, and low friction are other factors that contribute to the formation of continuous chips. Generally, this type of waste material also occurs with the use of lubricants or coolants and the use of a sharp cutting edge. The ribbon filings discussed in this paragraph provide a clean surface finish, longer tool life, and less power consumption. On the other hand, removing these types of chips presents a challenge to the operator. To improve the disposal conditions, chip breakers are necessary.