Functional Requirement of Controlling Workpiece Surface Grooves

The workpiece surface profile is defined by the generated surface grooves. A small roughness band needs shallow grooves and small chip thicknesses (Figs. 7.22 and 7.19). Shallow grooves can only be generated when both groove bottom shape and wheel deflection are controlled. The groove bottom is shallow for grits with large cutting edge radius and small depth of cut. Bond elasticity defines wheel deflection and groove generation [BORK92].

7.3.2.2 Functional Requirement of Reducing Heat Generation

Process heat is a dominant challenge in grinding technology and affects the part’s surface integrity. It is favorable that little heat is generated, existing heat is removed, and chemical reactions are suppressed to reduce the impact on surface integrity (Fig. 7.23).

Control of heat generation includes low heat per single grit interaction, few grit interactions per time, and short interaction time between workpiece and grinding tool. Heat generation per grit is very complex and includes heat generated by rubbing, plowing and cutting during all three phases of grit engagement (Fig. 7.20).

Although Fig. 7.20 applies for ductile material, brittle material experiences similar chip formation phases, but cracks are induced and expanded in phases II and III and particles will break out rather than chips formed.

Sliding heat can be reduced by lubricants with high lubrication ability, a small contact area in normal direction, and short kinematic contact length, lk (Fig. 7.24). The kinematic contact length, lk, evolves from the contact arc and the grit engagement angle (Eq. 7.10). Malkin and Guo propose to obtain the sliding energy by measurements of the grit wear flat area [MALK08].

lk kinematical contact length lg geometrical contact length (Eq. 6.7)

q speed ratio between vs and vw, positive for down-grinding, negative for up — grinding

There are only few examinations and models for the heat from plowing [ROWE09]. Contact conditions and shape of grit contact area seem to be most important.

Heat from cutting is produced at different shear zones within the single grit engagement (Fig. 7.20) [TONS92]. Shear zones are beneath the grit (c, d), at the grit rake face (b) as well as in the chip formation zone (a). The friction work between chip and tool bond (e) can be reduced by a higher grit protrusion (Fig. 7.25).

Rowe argues that shear energy at the shear plane (zone a) and at the rake face (zone b) add up to the total energy depending on the shear plane angle [ROWE09, p. 343 f., ROWE79]. A favorable shear plane angle near 45° exists with minimum shear energy. The mechanisms are not modeled. Qualitatively, the favorable shear plane angle has to regard grit shape and friction conditions. Furthermore, shear energy is reduced by a small shear strain rate, i. e. small grinding wheel speed, vs, and by a small chip cross-sectional area, which can be achieved by a small unde­formed chip thickness (Fig. 7.19).

The heat sources at zones b and d can be minimized by changing rake angle respectively clearance angle (Fig. 7.25). These strategies are derived from machining with defined cutting edges and can therefore only be applied if grit shape and orientation on the grinding tool are taken into account [KLOC11]. The heat at zones b, c, and d seem to have minor influence and a sensitivity analysis can indicate their relevance for process heat.

Few grit interactions per time express the second design parameter to serve the requirement of reduced heat generation during grinding (Figs. 7.23 and 7.26). On the one hand, the contact area between workpiece and tool has to be decreased, for example by a small wheel width, bs (Fig. 7.26) [METZ86, 78]. One example for a small wheel width is the traverse grinding variant “Quick Point Grinding”.

On the other hand, the active cutting edge density should be minimal, for example by a low number of kinematic cutting edges. The kinematic cutting edges, Nkin, are the only ones from the overall static number of cutting edges, Nstat, that are exposed to the workpiece (see Sect. 6.2). Therefore, Nkin is influenced by Nstat, by process parameters, tool wear and grinding tool deflection (Fig. 7.26).

A short interaction time of workpiece and grinding wheel comes from increased heat source speed, which is the workpiece speed, vw (Fig. 7.26).