The Evolution of Vitrified Grinding Wheels (Part 2)
Any review of vitrified grinding wheel development should cover both the abrasive materials and the bond medium. Part 1 of this series addressed materials; here we turn to the bond.
THE BOND MEDIUM
Grinding wheels probably originated in ancient Egypt where they were likely cut from sandstone. They also appear in sketches by Leonardo da Vinci and it's thought Belgian gem-polishers were using a cast iron wheel impregnated with diamond powder during the late 1700's, but little else is known until the early nineteenth century.
Around this time the first solid-bonded abrasive wheels appeared. These were made in India for hand-grinding gems and used emery or corundum abrasive in a gum resin shellac binder.
Rubber bond grinding wheels were introduced around 1860, again using corundum.
The first Vitrified (glass) bond wheels were commercialized about ten years later. What are today known as Resin (plastic) bonded wheels didn't appear until 1923. Metal bonds for diamond grinding wheels weren't introduced until the early 1940's.
CREATING A VITRIFIED BOND GRINDING WHEEL
The term “Vitrified" in its simplest form means “glassy” or “glass bond." Abrasive Materials such as Aluminum Oxide, Silicon Carbide, Diamond, and CBN are mixed with glass frit (ground glass) and other ceramic materials (clays, feldspars, fluxes, etc…). More recent advances in “Vitrified” bonds has led to the development of “Ceramic” bonds. In general terms, Ceramic bonds are those in which some or all of the glass phase has been converted to a crystalline phase to enhance certain material properties (i.e. higher strengths). Blended grinding wheel compositions are then formed either by “Hot Pressing” or “Cold Pressing” processes.
In Hot Pressing, the blended wheel materials are placed into a suitable mold and simultaneously pressed and sintered (baked). In sintering, the glass & ceramic components are fused and melted together forming the hard Vitrified Bond that holds the abrasive materials in place. Because the grinding wheel is simultaneously pressed and sintered at high temperatures, mold material selection has to withstand the processing temperature. Hot Pressed Vitrified wheels usually have low porosity levels and are very different in their use applications when compared with Cold Pressed Vitrified grinding wheels.
In Cold Pressed Vitrified Products, an extra material called a binder is blended into the wheel composition. The purpose of the binder is to provide handling strength to grinding wheels that are pressed in molds at room or low temperature and then removed for subsequent and separate sintering in kilns (high temperature ovens). In the sintering process, the temporary binder is removed at a low temperature and the glass is fused together at a high temperature forming the rigid hard Vitrified bond. Cold Pressed Vitrified grinding wheels generally have high levels of porosity in their structures.
Grinding wheel properties are adjusted by varying the percentage of abrasives, size of abrasives, blends of abrasives, different bonds and bond types, manufacturing process, etc… Varying the amount of the abrasive in Superabrasive Grinding wheels is referred to as the “Concentration”. Higher Concentrations contain more material and in general provide longer grinding wheel life as there are more Diamond or CBN cutting points to remove material.
CREATING POROSITY IN VITRIFIED GRINDING WHEELS
Porosity carries coolant through the grinding zone and provides space for chip clearance. By providing space to remove grinding swarf (used grinding wheel and removed work piece material), the grinding wheels are able to grind faster, provide better surface finishes (especially if the grinding fluid is filtered to remove the debris). Because open spacing in a wheel structure brings liquid coolants to the grinding zone, they are also known to grind cooler. In so doing, Vitrified Grinding wheels can remove the heat generated during grinding much more efficiently thus allowing faster grinding cycle times and less thermal damage to sensitive materials like Steels and Aerospace Alloys.
Porosity in wheels structures can be created by a variety of methods. In cold press and sinter manufacturing, the wheels are pressed to limited densities and sintering is controlled to limit shrinkage and densification. Another method for creating porosity in grinding wheel structure is through the use of additives in the wheel compositions that are then removed in the sintering stage, thus leaving voids (porosity) where the material once resided. Additional porosity can be generated by adding constituents to wheel compositions that remain within the wheel through manufacturing but are initiated when used. For example, some wheel compositions include material that doesn’t melt in processing but is dissolved in coolant upon use (i.e. Salt). Another example of induced porosity would be the use of hollow microspheres which upon use are broken open, leaving the open void as a pore.
Stay tuned for part 3 of this series where we will explore the advantages and limitations of vitrified diamond and cbn wheels in various grinding applications.