Wheel manufacturers such as Universal Grinding Wheel (Saint-Gobain Abrasives) have taken this concept further, and produced wheels with multiple pore former size distributions to create both macroporosity for high permeability and microporosity for controlled fracture of the bond. This type of wheel, with trade names such as Poros 2, has proved very effective for creep-feed grinding where coolant delivery into the grinding contact zone is critical for avoidance of burn (Figure 6.9).
6.5.5 Ultrahigh Porosity Vitrified Wheels
The introduction of extruded Seeded Gel needle-shaped grains has provided another opportunity for creating extremely porous and permeable structures. The natural packing density of grains with an aspect ratio of 8:1 is about 30% by volume. Norton (Saint-Gobain Abrasives) has recently developed a product called Altos with a totally interlinked porosity as high as 65 to 70%. The structure contains only a few percent of bond, but is, nevertheless, very strong because the bond migrates and sinters at the contact points between grains acting analogous to “spot welds.” The high-structural permeability allows prodigious amounts of coolant to be carried into the grind zone. This type of wheel gives probably the highest stock removal rates of any vitrified wheel, higher even than those possible with vitrified CBN, together with excellent G-ratios for a conventional abrasive. It is, therefore, finding major opportunities for grinding difficult burn-sensitive materials such as nickel-based alloys for the aerospace and land-based power generation industries.
6.5.6 Combining Grade and Structure
In very broad terms, wheel grades E thru I are considered soft and are usually used with high — structure numbers (11 to 20 with induced porosity) for creep-feed and burn-sensitive applications. Grades J through M are considered medium grade, usually used with lower-structure numbers for steels and regular cylindrical and internal grinding. Very hard wheels are produced for applications such as ball-bearing grinding. These wheels are X or Z grade and can contain as little as 2% porosity. Specifications of this hardness are produced by either hot pressing or by oversintering such that the bond fills all the pores. In this type of application, structural numbers can vary from 8 to >24. Their use is limited to relatively few specialist applications such as the grinding of ball bearings.
6.5.7 Lubricated Vitrified Wheels
The pores may also be filled with lubricants such as sulfur, wax, or resin by impregnating regular wheel structures after firing. Sulfur, a good high-temperature extreme pressure (EP) lubricant, is common in the bearing industry for internal wheels although it is becoming less popular due to environmental issues.