WATER JET CONFERENCE - Waterjet Technology Association

A small traversal distance creates a step at the end of that depth, which is immediately
subjected to particle impacts at large angles of attack, bringing about the deformation cutting
mechanism (Bitter, 1963) which then controls the cutting action in depth h 2. This step is removed
while another, wider one is formed. This process continues until the jet becomes ineffective in
removing material. The jet at this location, h 3, deflects in an increasing upward direction, which
results in further penetration because of the increase in the rate of change of water momentum
which imparts higher hydrodynamic loading on the particles that, consequently, remove additional
material.
Further jet traversal will engage the jet in the second region cutting pattern, as shown in
Figure 12, for one cycle of penetration. This approximately equals the jet diameter. During this
process, a rough bottom will result that has also been observed for other materials (Saunders, 1982).
When the jet reaches the exit edge and is approximately half way off the material, it will deflect to
the other side of the material causing a contour (a) in Figure 12. The uncut portion, shown in the
same figure, results from curvature of the cutting surface and jet exit behavior.
The photographs shown in Figure 13 were taken of Plexiglas cut by an abrasive waterjet and
clearly show the step formation and progression. Observations of the jet exit flow during the cutting
of relatively thin materials showed the steadiness of the angle of deflection at the sample bottom
surface, γ 2, in Figure 14a. This steadiness supports the previous description of the cutting process
when the sample thickness is equal to or less than the depth of the cutting wear mode, hi. The
complete penetration of relatively thick samples showed that the jet exit flow oscillates backward
and forward, as shown in Figure 14b. At intervals the jet cannot be seen exiting the bottom surface,
but it gradually reappears with a decreasing angle, γ 3 (Figure 14b). This again supports the previous
description of the cutting process during the penetration from h i to h 2. Material thickness, t, in this
case can be expressed as h 2 > t > h l. If the sample depth is less than h i + h 2 + h 3 but larger than h i +
h 2, as the level A indicated in Figure 12, the bottom of the sample will show holes, rather than a
continuous cut as shown in the lower sketch of Figure 12.
The picture in Figure 15 is of the bottom surface of an aluminum sample showing that
phenomenon. The angle γ 1 at the top of the cutting surface (Figure 14) is more dependent on target
material properties than abrasive waterjet and cutting parameters. Relating this angle to material
properties is a key requirement for the development of a cutting model (Hashish, 1983). Careful
experimentation needs to be conducted to determine the sensitivity of this angle to abrasive waterjet
parameters.
Figure 16 shows that the smoothness of the cut varies with its depth, changing from a
smooth cut at the top of the kerf to a straight one for the rest of the depth. This variation can be
related to the different mechanisms of cutting in h i and h 2 described earlier. However, an additional
factor related to the effective jet diameter may be more relevant to this observation. As the jet
penetrates a material, its effective diameter becomes smaller, due to wall friction and a reduction in
abrasive impacts caused by rebounding and jet deflection. The width of the cut will consequently be
smaller as the depth increases, as shown in Figure 17. Because the jet is round, similar narrowing
occurs in the third dimension perpendicular to the directions of both the traverse and the depth.
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