Superabrasive grinding 101

Cubic boron nitride (CBN) is a superabrasive traditionally used to grind steels and nickel alloys. Though not as hard as diamond (4,500kg/mm² vs. 9,000 kg/mm²), CBN is not chemically reactive with ferrite-based and nickel alloys, allowing it to outperform diamond-based grinding wheels in life and material removal rates on these materials¹. A 1987 study by Hitchiner and Wilks showed that when grinding nickel, the chemical wear of a diamond single-point turning tool exceeded abrasive mechanical wear by 10⁵x, highlighting the importance of using CBN in these applications². Using CBN requires fewer wheel changes and machine downtime, offers low and controlled wear rates, and provides excellent thermal stability. As CBN superabrasive wheels wear with use, the wear rate and wear mechanisms largely determine wheel performance.

Structure

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, macro-fracture, and lower wheel life.

Wear mechanisms

Hitchiner², Malkin³, Upadhyaya⁴, and Ding⁵ all describe four possible modes of wear in grinding with electroplated products.

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.

A visual summary of the four common wear modes is included in Figure 1 (right).

Performance characteristics

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.

Troubleshooting guide

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, focus may be placed on designing a grind cycle that promotes micro-fracture and self-sharpening wheels.

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.

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About the author: Norton | Saint-Gobain Applications Engineer Andrew Biro can be reached at andrew.biro@saint-gobain.com or 508.471.6769.

1. Norton | Saint-Gobain. Superabrasive Wheels for Industrial Applications. 2016.
2. 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.
3. Upadhyaya, R and Malkin, S. “Thermal Aspects of Grinding with Electroplated CBN Wheels.” Transactions of the ASME. 2004, Vol. 126.
4. Upadhyaya, R and J, Fiecoat. “Factors Affecting Grinding Performance with Electroplated CBN Wheels.” CIRP Annual Manufacturing Technology. 2007, Vol. 56.
5. Ding, Wenfeng, et al., et al. “Review on Monolayer CBN Suberabrasive Wheels for Grinding Metallic Materials.” Chinese Journal of Aeronautics. Article in Press, 2017.
6. Ghosh, A and A, Chattopadhyay. “On Grit Failure of an Indigenously Developed Single Layer Brazed CBN Wheel.” Industrial Diamond Review. 2007, Vol. 67.