Article From: 8/20/2009 Production Machining
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Inserts are available in a variety of sizes and grades to provide the best wear resistance and toughness to suit each application.
Coating layers vary in thickness (depending on the application for which the insert is being optimized) for the best wear resistance and thermal and chemical stability.
An average shop goes through thousands of inserts in any given year. Every day, an operator might handle dozens of inserts, never thinking about the complicated science behind them. A basic knowledge of what goes into an insert can do more than just provide trivia with which to impress people around the shop.
The Recipe for Carbide Inserts
As with all man-made items, creating an insert begins with the raw materials, or ingredients. The majority of today’s inserts consist of cemented carbide, which results from a combination of tungsten carbide (WC) and cobalt (Co). The hard particles within the insert are WC, while Co can be thought of as the glue that holds the insert together.
The easiest way to change the properties of cemented carbide is through the size of the grains of WC being used. Large grains, in the range of 3 to 5 microns, will create a softer material that wears more easily. Small grains that are less than 1 micron result in a harder material with more wear resistance, but that is more brittle, as well. For applications in very hard metals, an insert with small grains would most likely be best. At the other end of the spectrum, larger grains are preferable when dealing with interrupted cuts or other situations requiring a tougher insert.
Altering the ratio of WC to Co provides another means of manipulating the properties of an insert. Co is a much softer and tougher material than WC, so decreasing its proportion will result in a harder insert. Of course, this again presents the trade-off where a harder insert will have more wear resistance, but also be more brittle. Choosing the proper grain size of WC and ratio of Co for a specific type of application requires a level of scientific knowledge that could fill volumes.
To a varying degree, the trade-off between strength and toughness can be negated through application of the gradient technique. Commonly applied by all the world’s major cutting tool manufacturers, this technique consists of using a higher ratio of Co on the outer layer of an insert than on the inside. More specifically, the outer 15 to 25 microns of the insert receive extra Co, providing something of a “bumper” that allows it to take a bit of a beating without cracking. This allows the insert body to reap the benefits of using a stronger cemented carbide composition.
Once the specifications are determined for the raw materials, the process of actually creating an insert can begin. Powders of tungsten, carbon and cobalt are placed in a mill approximately the size of a washing machine. This process mills the grains to the necessary size and provides even blending of materials. Alcohol and water are added to the mix during the milling, and a thick, dark slurry is produced. The slurry is then placed in a large cyclone dryer that evaporates the liquids and leaves an agglomerate that is reduced back to powder and stored.
The materials begin to look a bit more like an insert during the next stage, where they are mixed with polyethylene glycol (PEG)—a plastic agent that temporarily holds them together in a paste form. Press dies then form the materials into the shape of inserts. Depending on the specific technique, single-axis pressing can be used or multiple-axis pressing can shape the insert from different angles.
Once pressed into the proper shapes, the pieces go into a giant furnace to be exposed to high levels of heat for sintering. This melts the PEG out of the mixture and leaves behind semi-completed cemented carbide inserts. As the PEG leaves the mixture, inserts shrink to their final size. This step of the process requires considerable mathematical calculations, as inserts will shrink different amounts based upon their composition and the final products have tolerances in the lower single-digit micron range.
Applying the Coating
At this point, the products bear a striking resemblance to finished inserts, but must still have coatings applied to maximize performance. The most common process for applying a coating is chemical vapor deposition (CVD), whereby a metal is ionized through high electrical currents and then applied to the insert via vapor condensation. The process can be visualized as ice forming on roads when the blacktop has become extremely cold and the air contains a high amount of humidity. However, instead, the relatively cool inserts are placed in a furnace that can exceed 900°F.
Physical vapor deposition (PVD) is another process used to apply insert coatings. PVD technology creates much thinner layers than CVD. This results in a sharper cutting edge and achieves a benefit in applications dealing with difficult-to-machine metals, such as hardened steels, titanium and heat resistant super alloys.
In a typical CVD process, the first layer of coating applied to an insert consists of titanium carbon nitride (TiCN). This material offers excellent wear resistance and has the added benefit of easily bonding to cemented carbide. Typically, aluminum oxide (Al2O3) is used for the second coating layer. Al2O3 possesses the benefit of being very thermally and chemically stable, protecting the insert from high heat and exposure to chemicals found in coolant.
The amount of TiCN and Al2O3 applied depends upon the type of application for which the insert is to be optimized. When turning hard materials, for instance, substantial protection is needed, and layers of 10 micron of each material might be used. For finishing applications in softer materials, applying a 5-micron layer of TiCN and 2-micron layer of Al2O3 may be more appropriate.
Once TiCN and Al2O3 have been applied, an insert is very close to being functionally complete. Unfortunately, Al2O3 is completely black in color, making it extremely difficult for users to tell which sides of an insert have been used and how the cutting edge has held up. To work around this problem, most manufacturers apply a final coating of titanium nitride (TiN). Bright gold in color, TiN serves no purpose other than providing a highly visible means of assessing a used insert’s condition.
Until recently, the application of TiN marked the completion of an insert. In recent years, a final process has become somewhat widespread. When an insert begins to cool from the CVD or PVD process, the various materials within it contract to differing degrees. Because of this, stress is introduced and small micro cracks appear within the layers. An advanced technique of blasting the insert with a mixture of alcohol, aluminum oxide and fine sand has been found to relieve these stresses and minimize micro cracking. Once this blasting has been completed, a finished insert exists.
The Role of Geometries
When geometry is mentioned in regards to inserts, most manufacturers immediately picture macro-geometry, or the physical shape of the component. Micro-geometry, dealing with the microscopic shape of an insert’s cutting edge, is a rapidly developing field that deserves just as much attention.
On the macro level, insert geometry deals with determining the best possible shape for chip control. Depending on the material and application, different shapes and angles will provide optimal results in breaking chips and efficiently transporting them away from the cutting zone. Macro-geometry is a well-established field that most major cutting tool manufacturers have mastered.
Only recently have developments in technology reached the point of enabling control of an insert’s micro-geometry. Using very advanced processes, the cutting surface of an insert can be given a round, oval or angled edge. Microscopic chamfers, or grooves, can also be introduced into an insert’s edge. As innovations in honing and measurement have enabled this level of detail, significant benefits in insert life and stability have emerged. It is safe to say that further technological advances will drive further development in the field and even more substantial achievements will occur.
Ceramic Insert Technology
While the vast majority of inserts are made of cemented carbide, a growing number are produced from other materials. Ceramics may be the most prominent among the alternatives. As exotic materials such as Inconel have become more prominent in parts for aerospace and other industries, ceramics have received acclaim for their high performance in these applications.
Ceramic inserts are created in a process very similar to that used for cemented carbide. Because ceramics do not bond as easily as other materials, much higher temperatures must be used during sintering. High pressures are also used.
Silicon carbide (SiC) whiskers are often used to provide additional strength in ceramic inserts. These small fibers provide the same effect as using rebar to reinforce concrete. In the past, the benefits of including SiC have been relatively small, but recent breakthroughs are changing that. New processes allow SiC whiskers to be oriented in a specific direction, greatly improving their effectiveness. Ceramics tend to be more brittle than other materials, and defects occur somewhat regularly. The inclusion of properly oriented SiC whiskers significantly slows down the deterioration of the insert, as it takes much more energy for a micro crack to traverse the aligned whiskers. As this and similar technologies continue to develop, ceramic inserts will become a more viable solution for a range of applications.
Getting More from Inserts
From a decision-making standpoint, one of the most important things to remember about inserts is the significance of aspects that cannot be seen. Even under careful scrutiny, the difference between a high quality and low quality insert might be unidentifiable without testing. Substituting cheap inserts because they look the same will inevitably lead to increased costs down the road.
When choosing an insert grade, the ideal solution is to consult with an expert from a cutting tool manufacturer. Outside of that, some basic concepts can be used to narrow down the available selections. Most cutting tool manufacturers number inserts in a way that reflects their properties. At Sandvik Coromant, for instance, the first number of an insert grade reflects the general category it falls into. The number 4 is used for steel grades, 3 is used for cast iron and 2 is used for stainless steels. Within each category, the final two digits indicate the insert’s hardness, with low numbers signifying harder, but more brittle and high numbers representing softer, but tougher. To find the category of insert needed, it is best for a shop to start in the middle of the list and work up or down the list, depending on performance.
Lastly, if an insert is not performing optimally, evidence already exists that can help to determine a solution. Looking closely at the cutting edge with an eyeglass can reveal the nature of the problem. When examination shows that an insert edge is experiencing significant abrasive wear or small deformations, a harder grade is required. If chipping is occurring and small pieces are missing, a softer, tougher grade will likely remedy the situation. By understanding how inserts are created and how different grades are tailored to specific applications, much can be done to boost productivity and reduce costs.