Tuesday, November 13, 2018

Making the Grade: How Insert Grades Work

When it comes to optimizing your metalcutting operations, the first place to look is the point of contact: the cutting edge of the tool. And in modern manufacturing, that often means an insert. Most manufacturers understand the exterior features of an insert, like wiper flats, chipbreakers and other elements of an insert’s geometry. The grade, however, describes an insert’s surface treatment and interior composition, and it may be the most important aspect of how an insert performs in various materials.

Since the 1920s, cemented carbides – primarily made from an alloy of carbide, usually tungsten, and cobalt – have been the material of choice for inserts. And the ratio between the carbide and cobalt can be best understood as a compromise between toughness, or resistance to chipping and fracturing, and hardness, or the ability to resist wear and maintain an edge. The more cobalt, the greater the toughness; the more carbide, the greater the hardness.

Furthermore, many grades feature additional materials. For example, steels require much greater hardness, especially at high temperatures, and cubic carbides such as titanium carbide, tantalum carbide and/or niobium carbide can improve chemical stability or grain-size control to achieve that goal at the cost of reduced toughness. But for greater customization, inserts require coatings, most of which arose in the 1970s in order to further increase chemical stability and surface hardness.

In contemporary machining of ferrous materials, uncoated inserts have become rare. Instead, manufacturers opt to use a variety of coatings to achieve greater material removal rates. For thick coatings that include aluminum oxide (Al2O3), chemical vapor deposition (CVD) creates highly wear-resistant inserts at the expense of toughness; physical vapor deposition (PVD) creates thinner coatings such as titanium aluminum nitride (TiAlN) that maintain inserts’ edge strength and smoothness at the cost of providing somewhat less protection from wear and cratering.

Some grades, such as those produced with Seco’s DURATOMIC™ coating process, have additional features that positively affect machining. DURATOMIC involves an a-based Al2O3 coating with uniquely arranged aluminum and oxygen atoms. This so-called “textured coating” produces a grade with improved mechanical properties and greater thermal and chemical inertness without a significant loss of hardness.

Other grades dispense with cemented carbide altogether. Cermet, a material with excellent build-up resistance produced by combining metals and ceramics, is ideal for creating high-quality surface finishes in stainless steels despite being somewhat less tough overall than coated carbides. Likewise, pure ceramics, polycrystalline cubic boron nitrides, polycrystalline diamond and diamond coatings have numerous advantages over typical cutting tool materials, but have alternate requirements that may interfere with upstream or downstream processes within the entire manufacturing cycle.

How, then, should operators approach insert grade selection? The easiest solution is to rely on your tooling partners at Seco, which provides comprehensive, proven recommendations based on machining strategies and material properties. With the Suggest tool at secotools.com, it’s easy to find the grade that’s perfect for your application, while our Machining Navigator catalogs include all the information you need to find the right solution for your process. And should you have any questions, your local Seco distributor can help you find the answer.