6.5.3.1 Overlap Ratio
Overlap ratio, Ud, is an important metric for dressing processes with axial feed rate [SALJ84]. It indicates how frequently the dressing tool overtravels a point on the grinding wheel. Overlap ratio is calculated from engagement width of the dressing tool, apd, and axial feed per grinding wheel revolution, fad (Eq. 6.24).
(6.24)
bd + fad
with apd = 2
apd engagement width fad axial feed
bd active dressing tool width
For inclined contour elements of the grinding wheel profile, the overlap ratio can be calculated for the direction of the contour, axial, or radial direction [HARB97]. For cylindrical dressing tools and in most industrial applications the simplified Eq. 6.26 is used [KLOC05a, VDI04]. Active dressing tool width, bd, depends on the depth of dressing cut, aed.
bd active dressing tool width fad axial feed
Small dressing feed or high overlap ratio results in grinding wheel topography with small active surface roughness, low grit protrusion, and a high number of kinematic cutting edges [KLOC05a, MARI04, SCHU96, STUF96]. The favorable small part surface roughness is accompanied by the danger of higher grinding forces and stronger grinding wheel loading. Therefore, grinding wheels for roughing operations are dressed with small overlap ratios.
Workpiece surface roughness decreases with increasing overlap ratio, so that a maximum overlap ratio exists above which the part surface roughness cannot be improved significantly. Messer [MESS83] found a maximum value, Udmax, in empirical tests, which depends on grinding grit size (Eq. 6.27).
grit size in US Mesh
Udmax — (6-27)