Costs per part were evaluated for each grinding wheel using redress life from the trials. Cost comparisons were made based on conditions yielding best results from previous trials.
Figure 19.28 shows typical redress life results. The end of the redress life is indicated when the process no longer yields roughness, roundness, or any other quality parameter within tolerance.
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TABLE 19.3 Wheel, Labor, and Machine Cost Factors
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19.10.8 Machine Conditions and Cost Factors
The cost factors used in the evaluation are given in Table 19.3.
A high-speed vitrified CBN wheel was tested on a special-purpose high-speed grinding machine for speeds up to 140 m/s. The results were compared with conventional alumina wheels and CBN wheels tested on a conventional machine at speeds up to 45 m/s.
The special-purpose machine was designed to employ smaller diameter wheels so that wheel costs were lower for the highest wheel speeds. The cost of the special-purpose machine was two and a half times the cost of the conventional machine.
For high-speed CBN grinding, it was necessary to employ a rotary-disk diamond dressing tool with a hydraulic drive. “Touch dressing” was employed to allow very small dressing cuts to be taken. The application of such techniques is essential for the successful application of vitrified CBN. The higher costs have to be justified by increased productivity and lower total cost per part. A 6-month payback period was assumed consisting of 20 working days per month at 16 hr per day.