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Superabrasives’ Virtues

Dave Goetz
By Dave Goetz Corporate Application Engineer, Norton | Saint-Gobain

Increased demand for economically manufacturing challenging aerospace parts faster with higher quality surface finishes is spurring on the development of new bonds and grains for grinding wheels. The latest superabrasives can be an excellent choice for aerospace manufacturers doing production grinding.

Different from conventional abrasives, which are typically natural grains used or blended with other abrasive grains, superabrasives are natural or synthetic.

Synthetic may include diamond as well as cubic boron nitride (cBN). Diamond is the hardest known substance known with a Knoop hardness of around 7,000, and cBN is the second hardest at a Knoop value of 4,700, compared with conventional grains that are in the 2,100 to 2,500 range.

The hardness, as well as durability, of superabrasives make them ideal for grinding hard, challenging materials.

Superabrasives have significantly higher G-ratios (ratio of volume of material ground / volume of wheel wear) in the 500 – 10,000+ range, depending on the application.

Conventional abrasives are usually in the 1 – 20 range and ceramics in the 10 – 200 range. Higher G-ratios usually equate to shorter grind cycles and more jobs per hour.

Superabrasives also offer greater thermal stability and durability, resulting in less dressing and longer grinding wheel life. Wheels can last a year or more, depending on the application.

Superabrasives wheels require fewer wheel changes, making them ideal for large volume production runs where up-time is critical.

High stock removal capabilities allow superabrasives to replace machining steps, enabling operators to move from machining-to-grinding where grinding and polishing the final form is done from the blank or solid stock. One manufacturer of a compressor crankshaft switched to a superabrasives grinding wheel, saving over 30% on “per part” costs.

There are several factors for choosing the right superabrasives wheel. Grit size is dependent on the required finish. Part geometry / form / profile determine grade, concentration and bond type. When selecting a bond, consider that metal bonds offer good thermal conductivity and form holding capabilities, and resin bonds are good for applications where a higher finish and a lower cost wheel is desired. Vitrified bonds are well suited for most other applications. Wheel composition is best determined by the material being ground.

Though there will be exceptions, diamond is typically suitable for non-metallic and non-ferrous materials, while cBN is generally used for grinding metals. Single layer products (plated or MSL) are good for high stock removal and complex forms. Machine tool rigidity is also a key consideration when using superabrasives, due to their higher speed capabilities.

Superabrasives can run in the 8,500 SFPM to 16,000 SFPM range, depending on the type of wheel and application, while conventional abrasives safely max out much lower, usually at around 6,500 SFPM (8,500 SFPM with special speed / safety ratings).

Aerospace manufacturers are increasingly using more wear-resistant coatings which make materials harder to grind. At the same time, manufacturers need to reduce cost.

New superabrasives bonds, such as Norton Paradigm, which has a metal-like bond, use considerably lower power, resulting in higher feed rates that significantly reduce cycle times. The new Norton Paradigm cBN grinding wheels easily cut difficult-to-grind steel parts Rc 50 or harder, as well as difficult-to-grind wear coatings. Norton Paradigm removes 30% more material and performs at 2.5 times the G-ratios of previous superabrasives.

Despite the higher cost of a superabrasives grinding wheel when compared with a conventional wheel, manufacturers can increase the number of parts per hour and reduce overall production costs.

In addition, increasing demands for lower stress or stress- free grinding are propelling developments for new, sharper and freer cutting solutions that stay sharp longer and grind with lower energy and stress. These new grains also require new bond technologies which provide better form holding and grain retention, allowing for further improvements in wheel life, and performance.

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