Using CBN abrasives.

Making grinding more accurate, cost-effective

The use of CBN (cubic boron nitride) grinding in the manufacturing process has grown dramatically since General Electric introduced the material in 1969. This superabrasive material is second only to diamond with respect to hardness and offers great thermal conductivity and abrasion resistance. Grinding has become an efficient, accurate, cost-saving process with the use of superabrasives like CBN.

The use of CBN first became popular for the sharpening and repair of high speed steel cutting tools such as endmills, drills, broaches, and reamers. The very slow wear characteristics of the CBN wheel ensure more accurate tools, while CBN's sharp, cool grinding action doesn't anneal the sharp cutting edges, making a better, longer lasting tool. Not surprisingly, it has encouraged the use of grinding for the manufacture of high speed steel cutting tools.

Without question, the greatest early development of advanced CBN application technology took place in Japan. For years, Japan consumed nearly 50% of all the CBN produced in the free world (excluding China and Russia). Today, metalworking manufacturers around the world are realizing the benefits of using CBN in their grinding processes. Machine up time, geometric accuracy and consistency of ground surfaces, improved residual stress characteristics, shorter cycle times, and lower overall costs are among these benefits.

Tool development

The first machine tools to utilize grinding wheels containing CBN abrasives were conventional machines designed to operate under conditions optimum for aluminum oxide wheels. However, coolant and wheel speed limitations, low spindle horsepower as well as the lack of superabrasive wheel truing and dressing capabilities and overall machine accuracy and rigidity, were major limiting factors to the use of CBN wheels.

More recently, a number of new generations of grinding machine tools have been developed with improvements that capitalize on CBN's benefits. Some machines manufactured by Elb, Excello, Hoglund Campbell and others may be "single-purpose" machines, designed and built for a special application. For instance, they are quite common in the aerospace industry. Other machines built by Toyoda, Landis, Junker, Makino, Huffman, Walter, and others are built for large-scale, more common applications such as automotive camshaft grinding and cutting tool manufacturing. Today, there are literally hundreds of different grinding machines designed to utilize CBN technology-machines characterized by high spindle speed and horsepower, enclosed grind chambers, high pressure, high volume coolant delivery systems, as well as highly accurate feed and wheel conditioning mechanisms.

The development of CBN grinding processes has taken great strides since its first use on simple dry and wet grinding applications. Wheel-conditioning equipment and controls developed during the past 10 to 15 years make it possible to form and/or sharpen the grinding wheel while removing as little as a few microns from the wheel. This adds significantly to the life of the wheel and provides increased performance consistency. It was also found that increasing wheel speed, in CBN grinding, is advantageous to increasing wheel life and removal rate capabilities as well as improving surface finish. Increasing the speed, while taking into account the additional factors of wheel speed limitations designated by the wheel manufacture, wheel balance, coolant and safetyguarding requirements, will dramatically improve performance in most applications. Recommended wheel speeds are now considered to be in the range of 6500 to 9000 sfpm (32 to 45 m/sec), though many new machine tools and processes are designed to operate at wheel speeds far above this range. Universities and others are experimenting with CBN wheel speeds as high as Mach 1, the speed of sound.

Coolant has also been found to have a profound effect on CBN wheel performance. Not only is the type of coolant critical to performance but, so is the method of delivery: Depending upon the process and material being ground, wheel life may be improved by a factor of 40 to 50X when comparing the best with the worst type of coolant. High pressure delivery systems with adequate fluid filtration and chilling capabilities can dramatically improve performance.

Wheel development

` Superabrasive grinding wheels were first developed around the use of diamond for the grinding of tungsten carbide and other difficult-to-grind materials. The bond matrix system for these wheels were typically comprised of phenolic resins with secondary abrasives and/or metal powders used as fillers. When CBN was first introduced, it was used with the same bond formulations as previously developed for diamond. Although performance was outstanding, when compared with conventional aluminum oxide wheels with respect to grinding ratio and form holding capabilities, it needed its own bond formulations to be optimized for the "chip" producing process of steel grinding. With the improvements in resin bond systems for CBN, it grew and gained acceptance in tool and cutter, surface, and cylindrical grinding applications.

The greatest growth in the use of CBN wheels came with the development of automated production grinders. More "user-friendly" grinding wheels, utilizing bond systems such as vitreous (glass) and single-layer, electroplated matrices were developed for use. Automated grinding systems, comprised of CNC grinders, vitrified and/or electroplated CBN wheels, and improved grinding fluid delivery, rapidly made their way into production grinding operations in the automotive, bearing, and aerospace industries Today, CBN grinding wheels, in all bond systems, are being utilized in nearly all metalworking industries. Vitreous and single layer bond systems continue to be the fastest growing.

Product development

In time, the need for new performance-oriented CBN products became apparent. Stronger crystals, able to provide long life in demanding single-layer applications, along with special coatings and surface treatments designed to provide added abrasive retention in impregnated grinding wheel bond matrices, were next to be developed. Microcrystalline CBN, with its characteristics of extreme fracture strength and thermal stability, were next to evolve for use in honing and grinding of hardened-, as well as unhardened-steels, nickel, and cobalt-bases superalloys, cast irons, and stainless steels. Crystals with high fracture strength and thermal stability, angular particle shapes, and modified fracture characteristics have become the latest innovations.

The development of new, and the improvement of older materials means an ever-changing challenge for the manufacturing engineer. The grmndability of various materials can vary greatly from one material to the next and can be measured by means of the grinding ratio, "G" (volumetric ratio of material removed divided by the volume of grinding wheel consumed during the process). Tougher, more difficult-to-grind materials will consume more energy and will wear the grinding wheel at a higher rate than "easier-to-grind" materials. (See chart on page 45 for relative grindability.) The relative grindability (M-2 = 100) of a number of common materials. All materials, where applicable, were ground in their hardened state. With the exception of cast iron and 52100 (bearing steel), M-2 (a common tool steel) was the easiest to grind.

One improvement in the quality of tool steel has emerged from the technology of powder metallurgy. While powder metal can be defined by the sintering of powders of various alloying elements, one manufacturer has found benefits in the quality of their powder metal steels by thoroughly alloying the elements before the particles are formed. Once HIP'ed and sintered, this process, called particle metallurgy (PM), provides a dense, homogeneous material with tougher, more wear-resistant properties.

In a study to investigate the relative grindability of materials produced using PM technology using two different grades of CBN abrasives in resin bond grinding wheels, it was found that dramatic improvements in process efficiencies could be realized by choosing the right abrasive.

Two CBN abrasives, designated "A" and "B" were selected for this program, along with three tool steels: 1) M-2, produced under conventional methods, 2) PM 10V, produced using PM technology, and 3) PM 15V, also produced using PM technology. M-2 was chosen because of the vast amount of data on its grindability and because it is often used as a benchmark in similar testing. All tests were conducted under identical parameters.

Grinding wheel specifications, with the exception of the abrasive, were also produced under identical conditions. Abrasive A is a medium strength CBN product with blocky shaped particles, designed with a microfracture mode of crystal breakdown. This product is coated with Ni to a coating level of 60 weight percent. Abrasive B is a specially engineered abrasive with strong, angular shaped, fracture resistant crystals. These crystals are then treated with titanium and coated with nickel to 60 weight percent. The combination of these characteristics, along with its macrofracture mode of breakdown, gives the grinding wheel long life, free cutting ability, and good surface finish. According to a simulated impact fracture test, CBN B tends to fracture along natural weakness planes, keeping it sharp and free cutting. The special coating on this product is chemically bonded to the crystal and provides texture and exceptional heat capacitance to aid in crystal retention in the bond system.

In the wet surface grinding of AISI M2 steel, the wheel containing CBN B produced a grinding ratio (G 200% of that produced with the wheel containing CBN A. This was achieved with a minimal increase in grinding energy and a 6% improvement in surface finish (Ra).

The performance advantage seen when grinding the PM tool steels used in this test with different CBN abrasives is even more dramatic. The results indicate that CBN A produced a 63% higher G when grinding PM 10V than it did when grinding AISI M2. This was in spite of the fact that PM 10V has a 3X higher wear resistance than M2 with similar hardness. (Wear resistance was measured by means of "crossed-cylinder" and "pin-abrasion" wear tests.) The G obtained when using CBN A to grind PM 15V was still 9% greater than obtained on AISI M2, even though this material has a five-times-greater wear-resistance.

The most impressive result of this test program, however, is the performance achieved when comparing the performance of abrasives in the grinding of PM materials. CBN B produced a G 450% that of CBN A when grinding PM 10V, with a minimal 13% increase in grinding energy and a 6% improvement in surface finish. A G 290% that of CBN A results were obtained with CBN B on PM 15V, despite its nearly 15% vanadium content. A 9% increase in grinding energy was experienced along with a 24% improvement in surface finish.

Mode: Surface Grinding

Wheel Speed: 5500 sfpm (28 m/sec)

Coolant: 10% HD Soluble Oil in Water

Specific MRR: Q' = 0.6in3/in/min (6.4mm3/mm/sec)

Wheel Type: 5D x 3/16W in. (125D x 5W mm)

Bond: Resin

Concentration: 100

Abrasives: 120/140 mesh Borazon CBN Type II 120/140 mesh Borazon CBN 420