Today, the desire for process improvements and the need for higher productivity and efficiency are driving the grinding process. Taking a new approach to grinding system optimization for the manufacture of automotive components has many benefits.
For highest productivity, the goal is to increase the number of parts between dress cycles, or to reduce the dress depth (compensation), resulting in more parts per wheel and fewer wheel changes. A common approach to do this has been to try the latest and greatest conventional grinding wheel specifications in an effort to reduce wheel usage.
However, in order to fully optimize a process, it is best to explore all possible solutions including the use of superabrasives. Whether upgrading existing superabrasive wheels or converting to superabrasives from older conventional technology, new grains and bond technologies will open up new grinding system possibilities. Using these new technologies, nearly 30% improvement in wheel life, a 34% higher G-Ratio, and a higher Q’ at the same or lower power can be achieved.
The Shift to Vitrified cBN Technology
The shift from conventional abrasives to vitrified cBN technology in grinding automotive components has been driven mainly by the machine tool builders serving this industry. Machine tool companies recognized the advantages of using cBN to develop robust processes, providing lower grinding costs to the end user.
Increased demand from machine tool builders, as well as from automotive component manufacturers, for higher production is driving abrasives manufacturers to develop new vitrified cBN products. The new developments lower the cost per unit by making longer lasting, faster grinding abrasives with higher material removal rates.
Advancements in vitrified bond technology coupled with the correct choice of superabrasive grain provide better form holding while grinding at the same or lower power. This is a key benefit because new advanced materials such as High-Velocity Oxygen Fuel (HVOF) coatings are being introduced for today’s gas engine components. It is also important that the risk of introducing heat into the part be eliminated to meet the changing needs in grinding.
The need for reduced stress or even stress-free grinding is pushing abrasives manufacturers to develop new, sharper, freer cutting superabrasives. New cBN grains that resist dulling and stay sharper longer, provide higher material removal rates (Q’) and also grind more efficiently. These grains require a much lower energy-to-grind ratio and self-sharpen, making them freer cutting, resulting in lower residual part stress after grinding.
Coupling these new grains with newly developed bond technologies help the grains achieve maximum performance. The new bond system has far better grain retention, allowing the wheel to hold form longer, allowing the cBN grain to work as intended by maximizing its potential and performance.
Application-Driven Superabrasives Solutions
The ever-expanding usage of wear-resistant materials and the need to finish them requires abrasives that are more durable.
For example, the finishing of ceramic coatings that are applied to provide wear resistance, make superabrasives a good choice to grind these materials economically.
Although vitrified cBN is still the most common technology, we are seeing more plated and MSL (Metal Single Layer) options and opportunities for operations with complex forms and geometries, where higher stock removal and reduced cycle times are required.
- Vitrified bond is still the best for high-volume production for improved form holding and on-machine dress capabilities.
- Single-layer plated is recommended for high stock removal in applications with complex forms and when form generation is not possible.
- Metal bonds are an excellent choice when good thermal conductivity and form-holding capabilities are required.
- Resin bonds are most suitable for applications where a high finish at a low cost is desired.
Superabrasive product selection is process driven. Part geometry, stock removal, and finish requirements, as well as the material being ground, all help determine the grain size and type, the grade, concentration, and bond.
Some of the main factors used to help manufacturers specify the proper grinding wheel are:
- Material to be ground and its hardness.
- Part geometry and finish requirements.
- Machine tool stiffness and accuracy.
- On-machine dressing capabilities.
- Area of grinding contact.
- Wheel speed required
- Horsepower required
Proper coolant type should also be considered. Coolant removes the chips and heat from the grinding zone, while protecting against chemical reactions caused by the grinding process. Oil-based fluids are best when working with vitrified cBN to maximize wheel life. Low vaporization temperatures limit the lubricity of water-based fluids and the high-temperature steam promotes oxidation of the cBN crystal.
To help understand why there is a shift to superabrasives, it is useful to compare conventional and superabrasives to highlight the differences that make superabrasives more productive.
- Superabrasives are more thermally stable and durable than conventional abrasives. These features usually provide a longer wheel life, particularly in demanding applications.
- Superabrasives require higher machine stiffness compared to conventional abrasives, as they are less forgiving.
- Superabrasives typically offer a much higher G-Ratio (Ratio of Volume of Material Ground / Volume of Wheel Wear). Conventional abrasives are usually in the 1–20 range, ceramics in the 10–200 range, while superabrasives are in the 500–10,000+ range depending on the application. Since higher G-Ratios typically equate to shorter grind cycles and more jobs per hour, grinding with superabrasives can increase productivity.
- Superabrasive wheels require less dressing and fewer wheel changes, providing longer life (some lasting 12 months or more) so they work very well in large production run applications where up-time is critical.
Companies such as Norton | Saint-Gobain offer process solutions that capitalize on the benefits provided by superabrasives.
These benefits include:
- Grinds at low specific Grinding Energy (S.G.E.).
- Achieve G-Ratios at or above 3400 (OD grinding).
- Retains cBN grain crystals allowing for slower degradation of surface finish and more parts between dress cycles.
- Cycle time reductions for higher productivity.
- Possibility to grind at higher MRR.
- Easy to dress with rotary dressing disks.
All of which deliver the performance and cost-saving advantages that justify the move from conventional abrasives.
The hardness and durability of the superabrasive also make it an excellent choice to grind hard and difficult-to-grind materials. In some applications, they also allow replacing machining operations. With the right product, finished parts can be produced from blank or solid stock by eliminating the machining operations in a process Norton refers to as “machining-to-grinding.”
It can be demonstrated time and again that selecting and properly utilizing the correct superabrasive can optimize the grinding process in most automotive related grinding operations. Converting to superabrasives can make the process more productive and offer greater cost-saving options than most conventional abrasives, including some of today’s premium conventional abrasive products.
Dave Goetz is a corporate application engineer of Norton | Saint-Gobain.