Cubic Boron Nitride (CBN)

Cubic boron nitride — CBN — is a synthetic superhard material, second in hardness only to diamond, and the one superhard material that can cut steel. Where diamond fails on iron and carbide fails on hardened steel, CBN keeps a keen edge at high speed and high heat, which is what makes hard turning — machining workpieces in the mid-fifties HRC and above that once had to be ground — an ordinary CNC operation.

A material made to order

Cubic boron nitride does not exist in useful quantity in nature; it is made. Boron and nitrogen are heated and pressed in the same high-pressure, high-temperature process that makes synthetic diamond, forcing the atoms into a cubic crystal nearly as hard as diamond’s. First produced in the 1950s, it became an industrial abrasive for grinding, and then — in sintered form — a cutting-tool material. The tool market uses the two names loosely: “CBN” and “PCBN” (polycrystalline cubic boron nitride) both appear for the same class of insert. Strictly, PCBN is the sintered composite — countless fine CBN crystals bonded together with a ceramic or metallic binder — and it exists because a mass of bonded crystals is tougher and more predictable than any single crystal could be.

CBN cutting edges reach the toolholder in two main ways. Solid inserts are made wholly of sintered CBN, so they carry several usable cutting edges and take the heavier depths and more demanding roughing. Brazed inserts are a small sintered CBN tip joined onto a carbide body: fewer edges, but cheaper to make and the usual form for finishing, where the CBN only has to be where the cut is. Grinding wheels also use CBN as an abrasive grain, but the cutting-tool world is the concern here.

The problem only CBN solves

Hardness alone is not enough to cut steel, as diamond proves. Diamond is the hardest material known, yet it cannot machine iron: at cutting temperature the carbon of the diamond dissolves into the steel or turns back to graphite, and the edge is destroyed in seconds. Superhard cutting of ferrous work therefore needs a material that is nearly as hard as diamond but chemically indifferent to iron. CBN is that material. It stays hard to temperatures far above where carbide softens, and it does not react with iron-based work even at the high edge temperatures of a fast cut. That combination — superhardness plus chemical stability to steel — is the whole basis of its existence in the shop.

What CBN does in the cut

CBN’s useful behaviour follows from that chemistry. Its hardness is second only to diamond’s and far above carbide’s, so an edge holds size through long runs on abrasive, hardened work. It keeps its hardness at high temperature, which lets it cut at speeds that would soften or wear out carbide and ceramic edges. It conducts heat well, drawing warmth out of the edge zone and into the chip instead of letting it cook the tool. And it can run without coolant: hard turning is often done dry, partly because the heat is not the problem it is for lesser tools, and partly because alternate heating and quenching from coolant can crack a brittle superhard edge.

CBN’s limits are those of a superhard material. It is not tough. Interrupted cuts, scale, a hard spot, chatter or a flexible setup will chip or shatter the edge, so CBN demands a rigid machine, a rigid workpiece setup and a stable cut. Cutting edges are usually prepared with a strengthening chamfer or negative land rather than a razor point, and the typical job is light finishing and semi-finishing with small depths of cut at high speed, not heavy roughing in a shaky fixture. Grade matters as it does in carbide: the CBN content and grain size are tuned, coarser and tougher for interrupted work, finer for the best surface finish.

CBN at work: hard turning instead of grinding

The signature application is hard turning. A part is hardened first — through-hardened, case-hardened or nitrided steel, hardened cast iron, bearing races, gears, shafts, dies and rolls — and then turned, faced or bored on a lathe or turning centre at hardnesses from about 45–50 HRC well into the sixties. Before CBN this work went to the grinding machine: wheels, dressing, coolant, several setups and long cycle times. Hard turning replaces much of that with a single-point cut. One setup finishes the part, no wheel is dressed, the swarf is dry and recycled, and the surface finish and size holding reach grinding quality at a higher metal-removal rate. CBN tooling also machines hard cast iron in milling — brake discs and similar high-volume work — where its wear life far outruns carbide.

Hard turning is not a casual swap. The machine and the part must be rigid enough for a light, high-speed finishing cut to hold tenths; workholding must support the part against the cut; and runout and vibration that a grinding wheel would shrug off are fatal to a brittle CBN edge. Where those conditions are met, the process removes a whole grinding operation from the route.

Choosing carbide, ceramic or CBN

For hardened ferrous work the tool ladder is clear. Carbide, especially coated carbide, is the tough, cheap choice for steels up to roughly 45–50 HRC and for interrupted work. Ceramic tools cut the harder range at high speed and lower cost than CBN, but they are brittle and poor at interrupted cuts and thermal shock. CBN takes over above about 55 HRC as the highest-performing ferrous tool material, at a premium price that only pays when its speed and life are used. The deciding factors are the workpiece hardness, whether the cut is continuous or interrupted, how rigid the machine is, and — always — the cost per part rather than the price of the insert. And the choice cuts both ways: CBN is the wrong tool on aluminium or other non-ferrous work, where diamond-based tooling is the superhard answer, and it is wasted on soft steel, which carbide removes cheaply and easily.

CBN is one rung on the ladder that starts at high-speed steel and rises through carbide and ceramic to the superhard materials. It only performs where the machine is stiff enough to protect a brittle edge — the subject of machine structure and rigidity — and its speed and feed decisions are made in the same feeds and speeds framework as every other tool within CNC machining.