Q&A Polycrystalline diamond and cubic born nitride tools make machining hard material easy. BIG DAISHOWA manufactures some of the hardest cutters on the planet one that involves some intense processes and chemistry. Polycrystalline diamond (PCD) tools and cubic boron nitride (CBN) are one and two, respectively, on the list of hardest insert materials, but their makeups are quite different. Without getting into the real nitty-gritty of the chemistry, extreme heat, sintering, and pressure all are used to form the different grades of these inserts based on their material-to-binding ratio. We specializes in holders but also offer CBN- and PCD-tipped inserts with custom chipbreaker forms for our boring tools, we often get questions about this style of tooling. Here, we share our answers. CHOOSING PCD AND CBN INSERTS
Q: As a toolholding expert, how do you describe the importance of the tool/holder interface? A: Only the most stable conditions are suited for PCD and CBN. With high hardness comes highly brittle characteristics. If the rigidity of the tooling or fixturing is not ideal, or if a workpiece has severe harmonics caused by excessive voids, vibrations can manifest in the cut. This leads to poor tool life of the diamond cutting edge. In these cases, it is best to stick with carbide inserts. Q: What’s the biggest difference between PCD and CBN inserts? A: A PCD insert should be the first choice in aluminum and magnesium applications, as well as for carbon fiber parts, because its extreme hardness provides very long tool life in these non-ferrous materials. Because it is a synthetic diamond (almost pure carbon), it cannot be used to machine ferrous materials, such as steels and cast irons, because at temperatures over 700 degrees F, carbon atoms react with the part material to form iron carbide and cause the diamond structures to collapse. PCD is especially required when aluminum has more than 5 percent silicon content to resist the abrasiveness that leads to wear. CBN uses boron as the primary element. While boron has a similar crystal structure to carbon, it lacks an affinity to iron. It is the second-hardest known material and can tolerate cutting temperatures over 1,800 degrees F. CBN is suitable for steel or iron machining and is one of the first choices for working with hardened steels, usually more than at least 50 HRC, and should be used
for finishing only. Any interrupted cutting requires the highest-stability tool possible, otherwise it won’t work well. CBN comes in different grades, in which binders, such as ceramics and metals are added to change the characteristics. Grain sizes of the crystal structure also can be tweaked for certain characteristics. High CBN content with ultrafine grain sizes is the hardest combination, suitable for continuous machining of hardened cast iron and powdered metals. CBN inserts also can be coated. For example, a medium-grain-size CBN material with a TiCN coating is recommended for machining hardened steels. Q: What is the best minimum/maximum depth of cuts for PCD and CBN inserts? A: PCD and CBN inserts are best reserved for light semi-finishing and finishing operations. I’d generally recommend a maximum of 0.040 in. stock on diameter and minimums down to 0.004 in., depending on the insert’s nose radius. Q: What is the relationship between surface finish and a tool’s nose radius? A: A larger nose radius produces a better surface finish for a given feed rate. Therefore, the larger the radius, the faster you can feed the tool and still get a good finish. The trade-off comes in tool pressure. A
Cubic boron nitride tools come in different grades, in which binders, such as ceramics and metals, are added to change the characteristics.
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