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How to Select the Right Carbide Insert (part 1)

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The selection of the most advantageous tooling material for molds and dies has become as sophisticated a factor as the design of the tooling itself. The use of new materials, new harder coatings for materials, pre-hardened materials and combinations of materials in molds and dies continues to complicate the manufacturing engineer's ability to select the optimum cutting tool as it relates to material machineability, hardness and desired surface finish. Couple this with newer high-speed machines and new oil mist and dry machining techniques as well as better surface finish requirements, and you have created a real cutting tool insert selection "dilemma" for the moldmaker (see Figure 1).

Most moldmakers do not have the luxury of a machining lab or even the time to do machining evaluations for different cutting parameters. Short leadtimes and precision machining defines the work.

New insert style cutter designs, particularly for milling, have improved the ability to quickly and efficiently rough-in mold cavities, and perform mold squaring and contouring as well as profiling and finishing. The development of multifunction milling tools that can use a wide variety of different insert style geometries now allows all types of milling to be performed with one cutter - providing more flexibility and lower cost. Some new modular milling system designs even allow the user to simply change milling heads on variable length adapter shanks to provide versatility. And with the addition of new "shrink-fit" toolholder technology that takes advantage of the performance benefits of induction heating, the toolholder and shrink-fitting a cutting tool into it, concentricity and rigidity for deep cavity tools have been greatly improved. These advancements have given the moldmaker better metal cutting options than have been previously available. But when it's all said and done, it's the carbide cutting tool insert that really makes it work. And the options here can be overwhelming. For example, it's not uncommon for a carbide insert manufacturer to have more than 20 different carbide substrate grades that can be coated with titanium nitride (TiN), titanium carbonitride (TiCN), titanium carbide (TiC), aluminum oxide (Al2O3), zirconium carbonitride (ZrCN), titanium aluminum nitride (TiAlN) or aluminum titanium nitride (AlTiN). These coatings can be applied individually or in combination by a physical deposition process (PVD) or a chemical deposition process (CVD) or even in a multi-layer CVD process that permits up to 70 ultra-thin layers to be deposited.

Coatings do significantly improve wear resistance, increase tool life and enable higher speeds and feeds to be achieved. PVD and CVD coatings are differentiated by the deposition processing temperature. The PVD process is a lower temperature process providing a finer crack-resistant coating grain structure with a low coefficient of friction. As a result, PVD-coated inserts can have sharper cutting edges.

Now add several of the cermet ceramic grades to the list and it is easy to see how proper selection can become a real nightmare. So it is important to first consider where and why insert failure occurs as well as some of the application basics for machining different mold materials. The recommendations for specific insert types will serve as a good starting point.