Abrasive Water Jet Machining (AWJM) is being used in different industries for
a long time. This technique gives a clean cut without any heat affected zone.
AWJM is especially suitable for machining of brittle materials like glass, ceramics
and stones as well as for composite materials and ferrous and non-ferrous materials.
The characteristics of surface produced by this technique depend on many factors
like jet pressure, type of abrasive used, stand-off distance of the nozzle from
the target, feed rate, abrasive flow rate, work materials, etc. Many researches
have been carried out on different parameters of AWJM. Fekaier et al.
and Ohlsson and Magnusson investigated the force parameters involved
during AWJ machining. Tikhomirov worked on the possible feed rate
depending on the stand-off distance of the nozzle. Andreas and Kavaeevic
investigated the properties and structures of high speed water jets. Momer et
al. investigated the influence of abrasive grain size distribution
on abrasive water jet machining process. Though abrasive water jet machining
is a cool cutting process and stress development on the machined surface is
insignificant, some interesting works have been done by Arola and Ramulu
on residual stress of metals machined with abrasive water jet. In the present
study detailed investigations have been carried out on the effect of a few parameters
like stand-off distance, feed rate and jet pressure on width of cut, taper of
cut and surface finish produced during machining of aluminum.
MATERIALS AND METHODS
The experiments were carried out in an abrasive water jet machine of model WJ 4080. The machine was equipped with the controller type Acramatic 2100 CNC control. The work material used was aluminum with the following main properties: tensile strength-90MPa, modulus of elasticity-69 GPa and density-2.71 g cm-3. The abrasive used was garnet with mesh size of 80 and hardness of 7.5 Mohs. The cutting parameters investigated were stand-off distance of the nozzle from the work (from 1 to 5 mm), feed rate of the work table (from 10 to 50 mm min-1) and jet pressure (from 12 to 50 ksi). The jet was perpendicular to the work surface and the nozzle diameter was 0.75 mm. Small rectangular aluminum blocks were cut by abrasive water jet half way along the length. This was done to measure the width of cut at the top and bottom of the block. After measuring the width of cut, the blocks were cut to the full length and the surface roughness was measured on the vertical cut surface. Surface roughness was measured by a surface roughness measuring equipment model SURFPAK SV-514. Surface roughness was measured at distances of 10, 20, 30 and 40 mm from the top of the surface.
RESULTS AND DISCUSSION
Effect on width of cut
Effect of stand off distance on width of cut: Figure 1
shows the effect of stand-off distance on widths of cut (Btop-width
at the top and Bbottom-width at the bottom) of the vertical cut surface
of the aluminum blocks. During the cutting process the jet pressure was 30 ksi
and the work feed rate was 15 mm min-1. Btop has increased
by 0.542 mm due to the increase in stand-off distance from 1 to 2 mm. Then,
it has changed its trend and width of cut decreases until the stand-off of 5
mm. It can be concluded that increasing the standoff distance will increase
the width of cut up to the stand-off distance of 2 mm, because of the taper
shape of the jet. Some researchers[7,9] also concludes that width
of cut increases with nozzle distance. But as the stand-off distance increases
beyond 2 mm, the stream becomes weaker and as a result, the width of cut decreases.
Effect of pressure on width of cut: Figure 2 shows
the relationship between width of cut and abrasive water jet pressure ranging
from 10 to 50 ksi. Both Btop and Bbottom show the same
|| Effect of stand-off distance on width of cut
|| Effect of pressure on width of cut
Their increasing trend is almost parallel, starting from 10 to 50 ksi. It is
obvious that with an increase in jet pressure the energy of the stream increases
which causes the widening of the kerfs. Similar results were obtained by Momber.
Effect on taper of cut: The taper of cut (TR) is defined
as a non-dimensional ratio between the top cut width and the bottom cut width.
It can be calculated as follows:
TR = (Btop-Bbottom)/2. Machining parameters
should be selected so that the taper of cut is as small as possible.
Effect of stand-off distance on taper of cut: Figure 3
shows the trend of change of taper of cut with the change in stand-off distance
of the nozzle from the work. The taper of cut gradually reduces to the minimum
value as the stand-off distance is increased from 1 to 5 mm. At low stand-off
distance the jet is strong and it has a diverging shape. Thats why the
top width of the cut is smaller than that of the lower and the taper angle is
|| Effect of stand-off distance on taper of cut
|| Effect of feed rate on taper of cut
|| Effect of pressure on taper of cut
But as the stand-off distance is increased, the jet becomes weaker and near
the exit the jet looses its kinetic energy and the width of cut at the bottom
becomes smaller than that of the top one. The negative values indicate that
the upper width of cut is lower than bottom width of cut.
Effect of feed on taper of cut: Figure 4 shows the relationship between taper of cut and feed rate. The general trend of the curve shows that increase in work feed rate results in increase in taper of cut. Rankin and Kavocevic also shows some interesting relationship between feed rate and depth of cut. Momber showed that with increase in traverse rate, the top width of cut decreases. But the decrease in width of cut with increase in work feed rate is more prominent near the exit of the slot since at the exit the jet becomes weaker. As a result taper of cut gradually increases with increase in work feed rate.
Effect of pressure on taper of cut: Figure 5 shows the trend in change in taper of cut with increase in jet pressure during machining of aluminum. As the jet pressure is increased from 10 ksi, the taper of cut gradually increases to a maximum value at a jet pressure of 30 ksi. This trend is similar to the results obtained by Momber and Kovacevic. But the taper angle reduces again with further increase in jet pressure. This is due to the more straightening of the jet at a higher pressure.
Effect on surface finish
Effect of stand off distance on surface finish: Figure 6
shows the effect of stand-off distance of the nozzle from the work surface and
the surface roughness of the vertical cut surface at different depths from the
top surface. Jet pressure and work feed rate in this illustration are 30 ksi
and 15 mm min-1, respectively. The general trend of increase in surface
roughness with increase in stand-off distance is obvious. It is also to be observed
that work surface roughness increases rapidly beyond a stand-off distance of
3 mm. At a stand-off distance of 4 mm it is clear that further the surface is
from the top, rougher it is.
Effect of feed rate on surface finish: Figure 7 shows the effect of feed rate on roughness of the vertical machined surface at various depths from the top surface. In the Fig. 7 the jet pressure and the stand-off distance of the nozzle were 30 ksi and 3 mm, respectively. Increasing the feed rate causes the increasing of striation formation of the jet and hence the surface roughness increases with increase in feed rate. Thus at a feed rate of 50 mm min-1 the value of surface roughness at a depth of 40 mm from the top of the surface is 11.84 μm where as the value is 5.58 μm at a feed rate of 10 mm min-1 at the same depth. It can also be observed from the Fig. 7 that in general the machined surface is smoother near the top of the surface and becomes rougher at greater depths from the top surface.
|| Effect of stand-off distance on surface finish
Effect of pressure on surface finish: Figure 8 shows the effect of jet pressure on surface roughness at different depths from the top surface (at a work feed rate of 15 mm min-1 and a stand-off distance of the nozzle of 3 mm). It can be observed from the Fig. 8 that as the pressure increases, the surface roughness gradually decreases. This result is similar to those obtained by Kovacevic. With increase in jet pressure, brittle abrasives break down into small ones. As a result of reduction of size of the abrasives the surface becomes smoother. Again, due to increase in jet pressure, the kinetic energy of the particles increases which results in smoother machined surface. It can also be observed that the highest value of surface roughness is at the greatest depth of the machined surface.
In the present study experimental investigations have been carried out for specific cutting parameters of jet pressure, stand-off distance of the nozzle and feed rate. It was observed that for a moderate pressure of 30 ksi and feed rate of 15 mm min-1, the width of cut increases with increase in stand-off distance, but further increase in stand-off distance causes narrowing the slot due to a weak jet. Again, both the top and the bottom widths increase with the increase in pressure due to higher kinetic energy of the abrasives and water. Therefore, pressure should be kept as low as possible in order to reduce the wastage of material, though it would reduce the productivity of machining.
Taper of the kerf is another important factor in machining accuracy and it should be as small as possible. Experimental investigations reveal that a stand-off distance of around 4 mm (at a feed rate of 15 mm min-1 and a jet pressure of 30 ksi) produces the smallest width of cut. Again, in order to minimize the width of cut (at a jet pressure of 30 ksi and a stand-Off distance of 3 mm) feed rate should be kept within 40 mm.
Surface finish is of great concern for AWJM. It was found that the cut surface is smoother near the top surface and gradually becomes rougher at higher depths. Surface roughness increases with increase in stand-off distance and the results show that the stand-off distance should be kept within 3 mm (at a pressure of 30 ksi and a work feed rate of 15 mm min-1) in order to obtain a reasonably smooth surface. Similarly, increase in work feed rate results a poorer surface. It is advisable to limit the feed rate within 30 mm min-1 (at a jet pressure of 30 ksi and a stand-off distance of 3 mm) which gives a good surface finish.
Jet pressure plays an important role in surface finish. As the jet pressure increases, surface becomes smoother. This is due to fragmentation of the abrasive particles into smaller ones as a result of high pressure and small particles give a smoother surface. Again, surface is smoother near the top of the work surface and gradually it becomes rougher at higher depths.