Mixing, pressing and finishing energy are negligible in comparison to the raw processing and sintering energies. All relevant energies for both grinding wheels are summed up in Fig. 8.4. The corundum wheel has only around 36 % of the embodied energy of a CBN grinding wheel of similar dimensions (Fig. 8.4). Then main energy proportion, however, lies in the steel body of CBN wheels.
Nevertheless, CBN grits have much higher wear resistance and the tool body can be re-used. The maximum useful abrasive volume in Table 8.9 depends on stability
aspects, clamping setup, and number of spindle revolutions. Here it is assumed that a diameter of 250 mm is the limit for the conventional grinding wheel. For the superabrasive wheel, it is assumed that the segments can be used down to 1 mm in thickness before they start to lose their stability and begin to crumble away. This leads to a minimum diameter of 392 mm.
With the G-ratios in Table 8.9, the CBN wheel in this case study can produce 13 times more workpieces than the conventional wheel, so that the energy per workpiece volume is only 1.3 J/mm3 for the CBN tool compared to 5.9 J/mm3 for the corundum wheel (Table 8.9). With a tool body re-use of five times, the embodied energy even decreases to only 0.4 J/mm3 for the CBN tool, which is about 7 % of the embodied energy per workpiece volume of the corundum wheel.
Yet there is more to consider. Abrasive tools are often adapted to a special application, e. g. high porosity for high material removal processes, CBN for
Wheel dimensions
400 x 20 x 200 mm Corundum wheel full body CBN wheel
Abrasive layer composed of 40 segments of 5 mm width, glued on a steel body
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Table 8.9 Embodied tool energy per workpiece volume
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—— ► Material Fig. 8.5 Wheel life cycle with qualitative resource streams |
precision grinding of hardened steel, soft bond for internal grinding, etc. Therefore, the comparison of different tools without regarding the application is difficult and not always reasonable. Moreover, the applications of superabrasive and conventional wheels differ in terms of machine tool, coolant supply, dressing, machine periphery, etc. (Fig. 8.5).
Superabrasives are in particular highly wear resistant in combination with high grinding wheel speeds. However, choosing superabrasives as grinding tool material should follow a thorough evaluation of the higher tool costs and the requirements on machine tool and cooling lubricant supply [LINK12b]. Further discussions touch the following aspects:
• Flexibility—In small or single batch production it is often required to use an abrasive tool with several effective surface roughnesses or even different profiles. For this, conventional tools are superior against superabrasives because of their better dressability and lower costs. New conventional tool systems with sol-gel corundum even allow to be used at high cutting speeds with the according advantages [KLOC03].
• Machine park—Often the high efficiency of superabrasives is only emerging from high cutting speeds. High-speed applications hold the advantage of small chip thicknesses resulting in tight workpiece form tolerances and high surface quality or high productivity. However, the complex machine setup needed (spindle power, encapsulation, more complex cooling lubricant system, etc.) might dissolve the technological advantages.
• Tool costs—Superabrasive tools are commonly more expensive than conventional tools, so that their economic efficiency is focussed on larger scale production [KLOC03].
The embodied energy of a product can vary along its life time depending on the intensity of usage and end of life stages [KARA10]. A manufacturer can manage the embodied energy from cradle-to-factory gate better than the users because the usage behavior and maintenance may vary [KARA10].
Kara and Manmek [KARA10] reviewed the embodied energy of composite materials in a cradle-to-gate analysis and found location of the suppliers was a significant factor for embodied energy. The embodied energy could be reduced considerably by carefully selecting local suppliers and by using rail or water transportation in the case of high quantities of raw materials and long distances.