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CBN wheels: how to dress for success.

Vitrified-bond super-abrasive wheels are widely used for their higher grinding efficiency, better surface finish, and longer wheel life than either conventional or other types of cubic boron nitride (CBN) wheels. Yet to get these advantages consistently requires proper wheel-surface conditioning, truing, and dressing.

Two GE Superabrasive engineers, Tadayuki Ishikawa, in Gotemba, Japan, and K V Kumar, Worthington, OH, have done extensive studies of the effects on wheel performance (particularly grinding power and surface finish) of variation in wheel-conditioning factors. They were searching for the right combination of truing and dressing to minimize grinding power requirements and maximize surface finish.

Truing restores wheel geometry and sharpens dull abrasive, while dressing opens the wheel surface, relieving bond material around the CBN crystals for better chip flow. Because of vitrified-bond porosity, dressing is little employed, they note, yet has a subtle, but important influence on grinding performance. Vitrified CBN wheels normally show high power consumption, particularly initially u volume of material has been removed and a stable state is reached.

The key grinding factors, the two researchers found, were the fracture mode of the CBN crystals and the amount of bond material around these crystals. For good surface finish and free cutting, it is important to minimize the dispersion of radial positioned CBN cutting edges and remove bond material from around the CBN grains. It is better to true the wheel lightly to remove bond material than to roughen the CBN crystal and generate sharper cutting edges by their macrofracturing. Though sharper, macrofractured crystals mean fewer cutting edges. Although this can reduce grinding power, it worsens surface finish.

Thus, the best compromise for both low grinding power and good surface finish, they found, is a truing depth of 1 micron using a 0.8 speed ratio between truer and wheel in a down-cut mode, followed by dressing with dry blasting of alumina powder.

The upper microphoto illustrates this: proper truing depth and dressing by blasting to get an optimal cutting condition right from the start. The lower photo shows the effect of skipping dressing: initial bond material eventually erodes (at the expense of higher initial power requirements) until a freer-cutting (more open) condition is reached,

For more information from GE Superabrasives, circle 302.
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Publication:Tooling & Production
Date:Dec 1, 1991
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