WATER JET CONFERENCE - Waterjet Technology Association

Effect of Abrasive Hardness, Shape and Fragmentation
Figure 9 shows the trend for the relationship between the depth of cut and the relative
hardness number, Hr. which is defined as the ratio of abrasive particle to material hardness, H a and
H m, respectively. The trend indicates that no further improvement in depth of cut can be obtained
when the hardness ratio exceeds a critical value. We have observed that, at the same parameters,
aluminum can be cut with softer abrasives, such as silica sand, as well as with harder abrasives,
such as garnet. Because the cost of abrasives increases with their hardness, it is important to
determine the critical hardness number, H r. at which δh/δH r = 0. However, changing abrasive
materials also changes other characteristics, such as the specific weight and shape of particles. The
effect of particle hardness alone on cutting results requires additional research.
Round particles have proved to be less effective than sharp-edged particles for both brittle
and ductile materials. Cutting with glass beads (Figure l0a) produces up to 70% shallower depths in
both ductile and brittle materials than does cutting with sharp silica sand particles (Figure l0b). Both
have similar hardness values. For ductile materials, sharp particles help to remove material by
plowing and raising lips (Finnie, 1958), while, for brittle materials, sharp particles induce more
stress concentrations than round particles.
We have observed that particles fragment during the cutting process, depending on the
relative hardness between the abrasive and target materials. however, abrasives harder than the
target material also fragment. When cutting steel or aluminum, silica sand with a mesh number of
70 breaks down to a mesh number of 100. No quantitative data are available yet to characterize the
percentage of abrasive particles that break or to what degree these particles fragment. Silicon
carbide abrasives showed minimum fragmentation when cutting steel and were used again after
drying with similar results. The design of an abrasive recycling system will depend on particle
fragmentation data, as recycling may not be feasible if excessive fragmentation occurs. Also, to
understand the cutting mechanism, it is important to identify the role of particle fragmentation in
producing erosion in excess of that obtained by the initial impact.
OBSERVATIONS OF THE CUT KERF
Visualization of the cutting process was accomplished by taking high-speed movies of the
cutting of transparent samples of Plexiglas, as shown in Figure 11. Figure 12 shows cutting surface
contours at different intervals with the cutting process divided into two regions. In the first, the
entry region, the jet travels a certain distance before reaching the maximum depth, which is equal to
h i + h 2 + h 3, where h i, h 2 and h 3 represent depths in which the amount of material removed varies as
described below. A steady depth, h i, occurs primarily by the jet's leading "edge." Particles strike that
surface at shallow angles of impact causing cutting wear to control the mechanism of material
removal (Ruff and Wiederhorn, 1979, and Finnie, 1958). Over the cutting surface corresponding to
h i, the material removal rate equals the material displacement rate. Because the material removal
rates at deeper distances from the jet are less than the material removal rates at upper locations, the
curvature of the cutting surface increases with the depth. At the end of the depth h i, particle
deflection causes the cutting wear mechanism to terminate.
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