The vast majority of standard grinders still use conventional abrasives at wheel speeds of 43 m/s or less. The wheel life is relatively long so any benefits from upgrading to CBN have to come from improved cycle time or quality. However, an analysis of about 30 different cylindrical grinders currently on the market reveals the following power capability per unit width of wheel as a function of wheel size and speed (Table 17.1).
Some of the power is required to overcome the inertia of the wheel and flange assembly. So assuming an average power available given by median value for each speed and assuming typical wheel widths between one third and the maximum allowed, the maximum stock removal rates Q (mm3/mm/s) are also estimated based on hardened steel. These numbers are limited by the system stiffness and cannot be easily increased to justify CBN. The only exceptions, therefore, are grinding very difficult materials such as tool steels or nickel and cobalt-based superalloys where there is a loss of cycle time from dressing, or for very narrow profiles where higher removal rates still remain within the machine stiffness parameters. One area where vitrified CBN is used successfully, for example, at speeds of 60 m/s or less, is groove grinding in transmission shafts.
Since the system can be relatively weak, work/wheel speed ratios are comparable to those for internal grinding; 1:50 for the weakest grinding processes up to 1:150 for stiffer grinding processes. However, the equivalent wheel diameter de always remains under 250 mm (with the possible exception of grinding rolls for the steel industry) and more typically is under 75 mm. Coolant access is, therefore, relatively easy and wheel grades are more a function of material and finish requirement and readily available in wheel makers’ specification manuals.
Nevertheless de does have an impact once standard speeds are exceeded. A standard speed for Alox wheels is 43 m/s and this is not just for safety reasons. At speeds up to this value, standard wheel grades can grind parts with the full range of de values using the spindle power provided. However, once 43 m/s is exceeded, the burn level is rapidly exceeded for all but the smallest de value such that by 60 m/s the wheels are limited to de of <25 mm and burn-insensitive workpieces unless the wheel grade and, hence, wheel life are reduced drastically, thus negating much of the reason for higher speeds in the first place.
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TABLE 17.1 Wheel Speed, Wheel Diameter, and Power Required per Unit Width
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Similar effects are seen with vitrified CBN as will be discussed below. The high thermal diffusivity of CBN shifts the wheel speed limit. For example, grinding hardened steel in water — based coolants, the value for CBN is shifted from 43 m/s to 80 m/s. Essentially, machines designed specifically for alox or for CBN abrasives require two quite different wheel-speed ranges although there is a recent approach to use hard-grade vitrified CBN wheels at speeds traditional to alox wheels.