Tech Talk
What is "Designer CBN"?
In many grinding operations, there is a continuous need to balance part dimensions and surface quality against production rates, all within a well-defined envelope of costs. Most manufacturers would face extinction if they made finely-toleranced parts with near-perfect finish, but took too much time making each piece. Conversely, most manufacturers would face a similar fate if they made part quotas, but the parts were out of tolerance or poorly finished. Fortunately, advances in grinding are making it easier to balance the equation.
Grinding improvements are ultimately being driven by designs that call for stronger, better parts. Design engineers have always asked for the impossible; but now market realities are also requiring near perfection. In the automotive industry, hotter-running engines, producing the most power out of the least volume and weight, require finer tolerances, often out of increasingly hard and difficult-to-grind materials. Also, increasingly compact and sophisticated transmissions need stronger, more finely finished components. The need for tighter tolerances and finishes has spread beyond automotive into plastics dies, tooling, and machine designs, all demanding precise and rapid grinding.
In response to this drive for faster production of better parts, manufacturers of grinding machines have developed new designs with increased capabilities. Higher spindle speeds combine with stiffer frames and better bearings to increase performance, often to the point where grinding machine designs are considered "old" if they were made as little as five years ago. Likewise, large improvements in motion and machine controls help the newer machines provide more control over grinding rates and geometries.
As important as machine parameters are - and they are critical - they mean very little unless the grinding wheel supports these improvements. With recent advances in the manufacture of grinding wheels, it has become possible to specify an exact type, grade, wheel bond, and production parameter for a given part. To illustrate this, consider the evolution of cubic boron nitride (CBN).
CBN was first developed by General Electric in 1957 as part of a program to make a substance harder than diamond. Scientists proved CBN was not as hard as diamond (CBN is second only to diamond in hardness: diamond at Knoop 7,000 kg/sq.-mm and CBN at 4,700 kg/sq.-mm), and it was essentially shelved for 10 years. During that period, GE experimented with CBN and found that it was well suited for grinding hardened ferrous materials. In fact, it was even better than diamond on these alloys because diamond, being pure carbon, tends to react chemically with ferrous materials at the high grinding temperatures.
Since its commercial introductions in 1969, CBN has evolved steadily with widespread applications in ferrous and superalloy materials. Today, this is nearly $400 million in wheel sales, worldwide. Advances in machine tool design, machine controls, wheel bond technology, wheel conditioning devices, adapative grinding process methods, coolant, and coolant application have all contributed to the success of CBN in these applications areas. Equally important have been breakthroughs in microstructure and coatings of CBN products.
CBN microstructure can be braodly classified as monocrystalline and microcrystalline. Each classification provides a specific grinding result, and advanced coatings and treatments within each allow the grinding wheel manufacturer to choose precisely the type of cutting or grinding effect required.
Monocrystalline CBN is manufactured by transforming hexagonal boron nitride (hBN) into a cubic, diamond-like structure (CBN) in the presence of a catalyst at very high pressures and temperatures. The growth cycle and catalyst can be varied to obtain single crystal CBN particles with specific physical characteristics such as color, shape, size, and fracture characteristics. Type I, 500, and 400 are examples of monocrystalline products.
Microcrystalline CBN is produced by direct conversion of hBN in the absence of a catalyst at high pressures and temperatures. The 550 family of products is representative of microcrystalline CBN. The microstructure of each mesh size particle consists of submicron to micron size crystallites of CBN bonded together and controlled to achieve desirable cutting characteristics during grinding.
The latest product in the evolution of CBN was engineered to achieve lower grinding power, lower normal grinding forces, and longer wheel life when compared to other CBN products. To do this, it was necessary to develop a strong, wear resistant crystal with smooth faces and sharp cutting edges. It also was important for the cutting edge to regenerate by cleaving and to maintain a sharp edge longer in the grinding process.
Type I is a black crystal with considerable surface texture. CBN 500 is a gold colored crystal with semi-smooth faces and rough edges. CBN 400 is a tan colored crystal with smooth faces and sharp edges. It has the most angular (higher aspect ratio or more irregular) shape and yet maintains a relatively high toughness.
Extensive testing of Borazon CBN 400 and 420 has proved this material's ability for high stock removal rates without workpiece burn. For example, in one test of plunge creep feed grinding of M2 tool steel (Rockwell C hardness of 62), test pieces showed no burn at matieral removal rates to 6.3 sq.-mm per second. The CBN 400 product shows stepwise cleaving and maintains a sharp edge, while the Type I product shows a microfracture wear pattern.
Similar tests have shown the same kind of gains with vitrified bond wheels. For example, in OD plunge grinding of a case-hardened steel alloy (Rc 60 +/-3), power consumption on the CBN 400 wheel averaged 25% lower than that of Type I. Reduction in grinding power should have significant application benefits by increasing the safety margin for the heat affected zone, and enable reduction in cycle times. Applications include steel cam lobes, IDs of bearing rings, and other precision grinding operations.
In another test, nickel-coated CBN 420 in a resin bond wheel produced nearly twice the grinding (a measurement of wheel life characteristics). A reactive coating is used with CBN 420, producing a chemo-mechanical bond between the crystal and the coating, helping to prevent premature loss from the bond matrix. All crystals are held securely and continue to contribute to wheel performance. This improved crystal retention, combined with CBN 400's cleaving behavior, result is a wheel that lasts longer and grinds with lower grinding forces.
Improving the performance of a grinding wheel system by matching the type of CBN with the optimal bond system can provide significant cost, quality, and performance benefits over conventional grinding. Long wheel life, infrequent wheel conditioning (if required), higher quality and more consistent parts, shorter cycle times, and overall lower grinding costs should be the goal in any grinding operation.
