Cubic boron nitride (CBN) is a
To understand the wear mechanisms in single-layer CBN wheels, it is first important to understand their construction. Single-layer wheels typically consist of a steel core that has been machined or ground to a specific size and tolerance. Next, wheels are masked to keep only the abrasive regions exposed and the CBN grains are then tacked in place. The wheels are submerged in an electrolytic solution, and a current is passed through the wheel and solution, which draws the Ni-based bond onto the wheel as a cathodic reaction occurs2. Manufacturers can control the thickness of the bond layer by varying the amount of time the wheel is plated.
Plating thickness plays a significant role in CBN grain wear mechanisms during grinding. Thick coatings tend to hold grains at the expense of material removal rate (due to lower grit protrusion from the top surface of the bond) and higher grinding temperatures (due to less room for coolant to enter and grinding swarf to escape). Thinner coatings can increase grit protrusion, at the risk of grain pullout,
Hitchiner2, Malkin3, Upadhyaya4, and Ding5 all describe four possible modes of wear in grinding with electroplated products.
Grain micro-fracture (microcrystalline splintering) – Grinding forces are sufficient to cause
Grain macro-fracture or large-scale cleaving (partial grain break-off) – When forces are substantially high, grains may cleave or break off in large fragments. This typically results in low wheel life and is an indication that the grinding parameters are too aggressive.Grain/bond interface wear or bond wear (causing total grain breakoff) – Though not common, it can happen if the grain exposure level (percentage of the grain that is above the plating bond material) is high (>50%) and the forces are high. In grain pullout, the grains are ripped out of the bond, resulting in low wheel life. This was highlighted by Ghosh and Chattopadhyay6 who note the relationship between plating thickness and
A visual summary of the four common wear modes is included in Figure 1 (right).
Assuming a grinding process has been developed that results predominantly in micro-fracture of CBN grains, common and predictable wheel performance trends are often observed with single-layer wheels. As described by Hitchiner2 and Upadhyaya3, 4, wheel wear and grinding power of new plated wheels tend to increase quickly during break-in, where only the tips of the highest grains are cutting the material, leading to a lower active grit density and rougher surface.
However, this break-in period is often short, replaced by more stable grinding where rates of change for wheel wear, power, and surface finish can decrease by up to 10x after the initial break-in rates. Wheel failure tends to occur once the grains have worn, causing grinding power to increase and surface roughness to drop, often resulting in workpiece burn. Failure can also be caused by grain and bond stripping off the steel core during more aggressive applications. In both failure modes, predicting the end of life is difficult, and is best estimated using empirical life data from previous wheels.
The final performance is only partially a function of the wheel. Other influential factors include coolant system filtration, setup, and nozzles, machine stiffness and forces, and the workpiece fixture. The final stress-state of the abrasive grain is a result of thermal and mechanical wear, which are driven by machine parameters, coolant, and lubricity. Once the entire system has been evaluated,
If the wear modes are observed when using single layer products, it is recommended to check troubleshooting topics to improve the wheel performance and part quality in the application.Norton | Saint-Gobain
- Norton | Saint-Gobain. Superabrasive Wheels for Industrial Applications. 2016.
- Hitchiner, Mike, et al., Handbook of Machining with Grinding Wheels. Boca Raton: Taylor & Francis Group - CRC Press, 2007. pp. 93-98, 103-107,185-193.
- Upadhyaya, R and Malkin, S. “Thermal Aspects of Grinding with Electroplated CBN Wheels.” Transactions of the ASME. 2004, Vol. 126.
- Upadhyaya, R
andJ, Fiecoat. “Factors Affecting Grinding Performance with Electroplated CBN Wheels.” CIRP Annual Manufacturing Technology. 2007, Vol. 56.
- Ding, Wenfeng, et al., et al. “Review on Monolayer CBN
SuberabrasiveWheels for Grinding Metallic Materials.” Chinese Journal of Aeronautics. Articlein Press, 2017.
- Ghosh, A and A, Chattopadhyay. “On Grit Failure of an Indigenously Developed Single Layer Brazed CBN Wheel.” Industrial Diamond Review. 2007, Vol. 67.