The complex tool design with multiple cutting edges and the complex chip formation mechanisms complicate the analysis of the grinding process [KLOM86]. Figure 7.7 shows all input and output streams that can be considered in grinding and provides a basis for a life cycle inventory. The items have different relevance for different applications.
The tooling is part of non-product material. Grinding wheel design, tool conditioning, and dressing tools are described in Sects. 6.3.3 “G-Ratio” and 6.5 “Tool Conditioning”. Cooling lubricant is important to cool and lubricate the grinding process, clean, transport chips, cool the machine tool, and protect against corrosion [BRIN99, KLOC09, MARI07].
Grinding oil, water-based emulsions or watery solutions are common cooling lubricants. These coolants have a different amount of non-renewable material content and affect the environmental attributes of water differently, such as contents of oil, acids, alkalis, toxic compounds, etc. [JAIN12, p. 497 ff]. Furthermore, the grinding fluid can attract bacteria and fungi or be irritating to the worker’s skin.
Water scarcity is a local measure, which predicts the long-term sustainability of a manufacturing location [REIC09]. The importance of water use is perceived differently in different geographical regions. The German industry, for example, does not perceive water scarcity in the same way as the Californian industry does. Research on new coolant media is ongoing to address the growing concerns on recyclability, toxicity and water consumption [KALI11, ZEIN11b].
The total energy consumed to generate part shape and surface by grinding consists of the processing energy and the energy consumed by machine tool and periphery (Fig. 7.7) [CRAT10, LINK12]. The processing energy or specific grinding energy, ec, is defined as energy to remove one volumetric unit of material and is used for forming grinding chips, plowing material, and mastering friction between grinding grits, tool bond and the workpiece [MALK08, OLIV09].
Commonly, the specific grinding energy, ec, is calculated from the grinding power, Pc, and the material removal rate, Qw, after Eq. 7.1 [KLOC09]. Grinding power consists of the forces and speeds in tangential, normal and axial direction (Eq. 7.2) [ROWE09, p. 25]. However, normal and axial feed rates are much smaller than cutting speed in tangential direction and workpiece speed is smaller than the wheel speed, so that the simplified Eq. 7.3 is commonly used.
Qw material removal rate Ft tangential grinding force vs grinding wheel speed vw workpiece speed Fn normal grinding force vfr radial feed rate Fa axial grinding force vfa axial feed rate
Researchers have developed several grinding force models in close correla — tionship to the undeformed chip thickness, but these grinding force models are empirical and hardly applicable for generic applications [TONS92]. Grinding energy cannot be predicted accurately and variations in wheel sharpness lead to large variations in grinding energy [ROWE09]. Table 7.2 gives example processing energies.
Machine tool energy and peripheral energy can add much to the total energy [DAHM04]. This includes energy to run machine control, hydraulics, lighting, coolant system, compressed air, etc. Some machine power profiles have been published and databases provide basic information on machine power demands [ZEIN11, DENK05, KLOC10, BANI05]. Coolant pumps can account for a big portion of grinding energy as well as heating, ventilation and air conditioning (HVAC) and lighting [LINK12, DIAZ10].
The total grinding energy per part, Etotah can be calculated from processing, handling, setup and dressing time per single part (Eq. 7.4, Fig. 7.8) [LINK12]. Here, the power consumed by the dressing spindle and axes is neglected.
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Table 7.2 Examples for specific grinding energies [LINK12]
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Higher material removal rate decreases process energy for the same volume of material removed [MARI07, LINK11, ZEIN11, KLOC10]. This results from the decreasing processing time, which dominates over the increasing processing power demand. Nevertheless, higher material removal rates lead to higher process forces, larger tool wear, and higher surface roughness.
Waste streams from the grinding process include heat waste. The common assumption is that nearly the total energy in the contact zone is converted to the total heat flux, qt (Eq. 7.6) [MARI07]. Abrasive grits with higher thermal conductivity can reduce temperatures drastically, e. g. CBN instead of Al2O3, [ROWE09].
Grinding debris and filter material are another waste stream from the grinding process [ECKE00]. Grinding debris can be composed of 10-80 % of chips, 2-75 % of grinding tool swarf and up to 50 % of filter aid [SCH003]. The oil or emulsion content defines the recyclability of the grinding debris. Recycling options for abrasive tools are addressed in Sect. 4.8 “Tool End of Life”.
Machine tools have a life cycle of their own [DIAZ10]. Enparantza et al. [ENPA06] calculated life cycle costs for a centerless grinding machine tool. In this case study, 80 % of the life cycle costs happen during the use phase due to the grinding operation itself and maintenance. Direct labor accounted for 51 % of the total costs, the grinding wheel for 13 %, the machine tool purchase for 8 % and energy consumption for 6 % [ENPA06].
qt total heat flux
F0 specific tangential force
vc cutting speed
lc contact length (Eq. 6.7)
qch heat flux to the chip
qcool heat flux to the cooling lubricant
qs heat flux to the wheel
qw heat flux to the workpiece