The first and most important decision in recent years is the choice of whether to use conventional, ceramic, or cubic boron nitride (CBN) abrasive. This will depend on the condition of the machine, dressing capability, and part size. In general, high volume, hardened-steel parts under 15 mm with straight bores, for example, tappet rollers, lifter bodies, fuel injector bores, and small bearings, especially M50 steel aerospace, are usually most cost effective with CBN even on older machines. Between 15 mm and about 50 mm, CBN is becoming more and more common on new equipment. Alox — and SG-based wheels dominate over 50 mm. There are several reasons for this. The first is purely the abrasive cost in the wheel. Small-wheel manufacture carries a high percentage of labor in its cost regardless of abrasive type. For CBN, as the wheel size increases, the cost of abrasive soon dominates and wheel cost becomes proportional to size. With alox grain, it being much less expensive, the overall manufacturing cost does not increase as rapidly with diameter.
The second issue governing abrasive selection concerns normal force and its relationship to wheel/part conformance characterized by equivalent wheel diameter de, where
The larger the equivalent wheel diameter, the greater the contact length and, hence, the higher the normal force. As d increases, the ratio d /d must be reduced to reduce d Also the grinding pressure must be reduced further by increasing the structure and grit size in the wheel specification. This also improves chip clearance and coolant access. Unfortunately, the range of grit sizes and bond structures available in vitrified CBN systems, which will still allow the necessary high G — ratios to be maintained, is more limited than those for alox or ceramic wheels. Consequently, de for a CBN wheel is limited to about 150 mm over which the process is compromised either by abrasive cost or cycle time. This will vary by material type: for very difficult to grind materials such as M50 or tool steels, the maximum de value for CBN would be much higher due to gains in productivity by the elimination of dressing, while for simple AISI 52100 plain bearings the maximum de could be nearer 100 mm. Some typical examples of bond specifications for vitrified
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TABLE 18.1 Recommendations for Vitrified CBN Internal Wheels
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CBN wheels grinding hardened steels are given in Table 18.1. The values for ds are for wheels as new and will reduce by up to 20% during use. Bond specifications are based on the Universal (Saint-Gobain Abrasives) VSS bond system. Grades will vary from one wheel manufacturer to another.
For conventional wheels, the range of bond systems is well established by the wheel manufacturers. Table 18.2 provides typical grit size requirements based on de and Q values. Again, wheel grade and structure number will vary depending on the manufacturer and grit type.