Finally, for the roughest dressing of large cylindrical or centerless wheels there are grit tools and cluster impregnated tools (Figure 7.12).
Grit tools represent the most cost-effective dressing tool for the commonest applications using straight wheels. Clusters consist of a single layer of five to seven large natural diamonds semiexposed on a round flat surface held in a sintered metal matrix. As with other tools, they are inclined up to 15° with the tool center line intersecting the wheel center line. Their large head diameter results in fast traverse rates for reduced dress cycle times relative to other tools.
Grit tools consist of a consumable layer of diamond grains held in a highly wear-resistant sintered metal matrix. The tools wear progressively over time exposing new grains. Diamond size selection is governed by the abrasive grain size in the wheel, while the tool width, bd (and length, L), are dictated by the diameter and width of the wheel (Table 7.4). Note that the tool now consists of a random collection of diamond-cutting points whose action will depend on their exposure during any point in the life of the tool. Also the tool will wear a radius to the shape of the wheel. Overall, the process is not as consistent as a single point but in most cases acceptable and offset by the fact the tools are cheap, easy to make, and long lasting. Dressing forces with grit tools, however, must be respected; forces are typically five to eight times greater than those for single-point diamonds. The tool must, therefore, be clamped extremely rigidly with little or no overhang. Minimum dress depth is 10 pm because of the relatively dull dress action. They can handle dress depths up to 125 pm dressing conventional alumina wheels and 50 to 25 pm with ceramic-type abrasives.