Resin Bonds Are Recommended
Although metal and vitreous (glass-like) bonds are sometimes used in diamond wheels designed for grinding cemented carbides, resin bonds are preferred.
Resin bonds are resilient, cushioning the diamond abrasive against shock that might cause gross fracture. And, since resin bonds are formulated to wear at the same rate as the friable diamond abrasive, new crystals buried in the bond are exposed when needed. As a consequence, resin-bonded diamond wheels generally remove material up to ten times faster than metal-bonded wheels and twice as fast as vitreous-bonded wheels.
The rate of wheel wear is correspondingly higher. But the reduction in labor and overhead cost per cubic inch of carbide ground more than pays for the increase in diamond wheel cost.
"Hardness" (wear resistance) of resin bonds is an important factor in diamond wheel performance. If a resin bond is too hard, the wheel will not be free-cutting. If a bond is too soft, wheel wear will be excessive.
Grinding efficiency can be improved by matching the hardness of the wheel to the specific grinding job. Factors which determine the wheel grade (hardness) are contact area between the wheel and the work, wheel surface speed, wet or dry grinding, grade of carbide, etc.
Basically, a grinding wheel is a toolholder. The tools are crystals (grains) of abrasive embedded in a bond or matrix, which is supported by a core that fits the machine spindle. In a diamond wheel, the abrasive is confined to the wheel rim.
When grinding cemented carbides, the physical properties of the diamond and the bond that holds the abrasive crystals in the wheel rim are of fundamental importance. Using the wrong diamond abrasive or the wrong bond can increase grinding costs substantially.
Diamond content of a wheel is usually expressed in terms of concentration. A 100-concentration wheel contains 72 carats of diamond abrasive per cubic inch of bond (4.4 carats/cm3) A 50-concentration wheel contains 36 carats per cubic inch (2.2 carats/cm3).
Increasing the diamond concentration of a wheel makes it act "harder" - it wears more slowly. With more diamond crystals to share the work, less stress is placed on each crystal.
The degrees to which increasing the diamond concentration of a wheel improves wheel life depends on the mesh size of the diamond abrasive. Fine mesh-size abrasive crystals have very small surface areas, which makes it difficult for the resin bond to hold them securely. Premature pullout of abrasive crystals makes the wheel act softer and shortens its life.
Another way diamond wheel life can be improved in both wet and dry grinding is to increase the width of the wheel being used. This increases the wheel-workpiece contact area and-for a given material-removal rate-reduces the rate of wear of each diamond crystal, hence the rate of wheel wear.
Similar benefits are obtained by increasing the diameter of the diamond wheel. Large-diameter diamond wheels outlast small-diameter wheels by wide margins when run under the same conditions, on the same grade of cemented carbide.
Besides reducing wheel cost per piece, improved wheel life reduces labor and overhead cost per piece. Wheels need to be changed less frequently so the productivity of grinding machines and operators is increased.
Friable Abrasives Cut Fast
Diamond abrasives have many sharp cutting points that remove cemented carbides cleanly and efficiently during grinding.
As grinding progresses, these cutting points become dull. The dull abrasive tends to rub, rather than cut the workpiece surface. Material-removal efficiency is reduced and much grinding energy is dissipated in the form of heat.
For this reason, friable forms of synthetic diamond abrasive are recommended for wheels designed to grind cemented carbides. A friable diamond crystal undergoes controlled microfracturing under the stresses set up by dull cutting points, creating fresh, sharp points. With these self-sharpening diamond abrasives - in the right bond - diamond wheels can grind cemented carbides efficiently, with good wheel life.
Trueing Diamond Wheels
Careful wheel mounting is essential when grinding cemented carbides with diamond wheels. For best performance, the wheel should be mounted perfectly normal and concentric to the machine spindle.
Out-ofroundness is not a major problem with cup and face wheels such as Types 11V(, 12, 15 and 6. Straight diamond wheels such as Type 1A1 usually require trueing after mounting.
An out-of round straight wheel will pound the workpiece surface, reducing wheel life and impairing surface finish. This can be avoided by trueing the wheel so its entire surface is concentric with the spindle within 0.0005 inch (.0127mm.)
Resin-bonded diamond wheels can be quickly trued, using a silicon carbide brake controlled trueing device. However, every effort should be made to mount the wheel to within the required concentricity limits before using the trueing device. This will reduce the loss of valuable diamond wheel rim in the trueing process.
Dressing Improves Wheel Performance
In trueing, the surface of the wheel is uniformly abraded to bring it into truth with the center of rotation. This operation also blunts the diamond abrasive, which is level with the bond after trueing. In this condition, the wheel cannot grind.
To restore the wheel to full cutting efficiency, resin is abraded away with a dressing stick containing fine aluminum oxide abrasive. This exposes the cutting points of the diamond abrasive. When the wheel has been dressed sufficiently, it abrades the dressing stick very rapidly, indicating that the wheel is in good condition for grinding.
Maintaining the Wheel
If a diamond wheel frequently tends to lose its free-cutting effectiveness in service, operating conditions probably aren't just right for that particular job. The wheel can be reconditioned by dressing. But the way the wheel is being used should be critically reexamined.
Never exceed the maximum safe operating speed specified in the Safety Code ANSI-B 7.1 or marked on the wheel. The specified limits are:
9,500 SFPM for resin bond diamond and CBN wheels
6,000 to 7,500 SFPM is recommended for wet grinding with short arc of contact (type 1A1 wheels).