06-10-2016, 11:11 AM
1457975307-paper1.pdf (Size: 196.38 KB / Downloads: 5)
INTRODUCTION
38.1.1 Brief Description of Process
Abrasive jet machining is a grinding process where the abrasive particles are moved through the
workpiece by a stream of high speed water rather than by a solid wheel. By mass flow, the stream is
approximately 90 percent water and 10 percent abrasive. This provides excellent cooling and there
is no heat-affected zone at the cut surface. The jet is typically about 0.030" in diameter and virtually
any two- dimensional shape that can be drawn can be cut. A typical setup is shown in Fig. 38.1.
The jet is formed by pumping clean water at 40,000–60,000 psi through a fine orifice, then
entraining dry abrasive into the stream and passing the mixture through a mixing tube where the water
accelerates the abrasive up to speed. In general the process is most competitive for materials between
1/8'' and 2'' in thickness. A collection of parts made by abrasive jet machining is shown in Fig. 38.2.
38.1.2 History
High pressure water without abrasives began to be used as a cutting tool for soft materials in about
1970. The increase in cutting power by the addition of abrasives came in about 1980. The first abrasive
jets were crude devices that were often sold as the tool of last resort for severing difficult material.
Dimensional control was comparable with oxy-acetylene burning. Then, advances in material technology
allowed the manufacture of long life nozzles that could keep their shape during a long cut. At about the
same time computational power became very low cost so that advanced control systems could be built.
These two advances increased the precision of the process to permit cutting within 0.001 in of the true
line in some cases and within 0.005 in. in almost all cases. Moreover, the advanced controls allowed
inexperienced operators to make rapid setups and produce good parts without trial and error.
38.1.3 Factors Influencing Selection of Abrasive Jet Machining
Abrasive jet machining is chosen for many reasons, but all boil down to consideration of costs. Cost
savings come from many areas including
1. Low capital cost
2. High cutting speed in difficult materials
3. Minimal clamping and fixturing required for the workpiece
4. Rapid set up and programming
5. No heat affected zones on the cut surface
6. Waste in the form of valuable large pieces, not oily chips
7. Very thin sections can be made without deformation or melting.
Abrasive jet machining can also be compared with other processes. It is faster than wire EDM
machining. It can make the same general shapes as EDM but is less accurate. The abrasive jet is not
sensitive to slag and scale on or within the material that can cause problems by breaking wires in
EDM machining. It can also be used on insulating materials.
The abrasive jet is also often considered as an alternate to various thermal cutting processes.
Lasers are fast and accurate in thin materials where they outperform abrasive jets in terms of cutting
speed. However, many materials (copper, brass, glass, ceramics, and others) can not be cut with
lasers. The abrasive jet provides better precision and speed than lasers in thicker materials (1/2 in and up).
Plasma cutting and oxy-acetylene burning are faster than both lasers and abrasive jets in thicker materials,
but the range of materials cut by these processes is even more restricted than for lasers and
these other thermal processes are, in general, less accurate than either lasers or abrasive jets. Finally,
all the thermal processes leave a heat affected edge that often interferes with subsequent welding or
machining while the abrasive jet does not.
Production of flat parts on machining centers requires fixturing and tooling and short runs are
often impractical under CNC control. The abrasive jet can make parts from plate without tooling and
fixtures directly from a CAD file. For this reason, abrasive jets are often found in prototype shops
where they very effectively compete with manual machinery.
38.2 THE CUTTING PROCESS
38.2.1 Geometry of Jet and Character of the Cut Surface
In general, the jet cannot be regarded as a rigid tool. Abrasive jets bend as they pass through the cut
material so that the exit point lags the entry point. At sufficiently high speeds there is a small sideto-side
motion as well that produces striations on the cut surface. In Fig. 38.3 we see a part with five
fingers cut at differing traverse speeds but equal times to produce each finger. In the long finger we
can see the lag as recorded by the striations. The lag is usually not important for straight line cutting
where maximum cutting speeds are determined by the striation that can be tolerated. In shape cutting,
the lag can produce serious geometry errors on tight radii and it is the lag that sets the maximum
speed for radii and corners.
Utility Requirements
Clean mineral free water is required for long nozzle operation. The same elements in hard water that
cause boiler scale form deposits on waterjet nozzles. Water softeners and sometimes reverse osmosis
systems are often required to achieve long nozzle life. In extreme cases, hard water can also form
harmful deposits within a pump. A cutting nozzle will use from 0.1 to 2 gallons per minute depending
on size. In addition often 1–3 gallons per minute is required for intensifier cooling. Usual tap
water pressures are sufficient for supply, but some manufacturers include a boost pump to insure that
there is no loss of pumping due to unexpected supply pressure dips.
Most systems require three-phase electric power and use normal factory voltage levels. Power
requirements would range from 15 kW to run a 20-hp pump up to 150 kW for a 200-hp pump. Small
shops without three phase power can install a phase converter if they have sufficient single phase
power. Another option, but a rarely used one for manufacturing applications, is to drive the pump
with diesel power.
Often the control systems associated with jet cutting require a small supply of compressed air at
normal shop pressures of 80–100 psi. This would almost certainly be a requirement if automatic on-off
valves are used as they are most commonly air actuated.
The waste stream includes water, bits of the material being cut and abrasives. Usually the water
is routed to a drain and the solids are removed separately. However, if the material being cut is
hazardous, for example lead or beryllium, the waste water cannot be put in the drain. Closed loop
water recycle systems must be used in these cases. Otherwise, the solid waste is directly put with
waste routed to a land fill. The solid waste stream from abrasive jet cutting can be recycled to
reclaim the spent abrasive in cases where the usage is large enough to justify the cost of the recycle
system.
SAFETY
The largest danger is the jet itself. It can easily cut through human flesh and bone and an exposed jet
generates noise levels that can easily harm human hearing. These dangers are most difficult to deal
with in fully three-dimensional cutting in applications like removing risers from castings with abrasive
jets. The solution for these applications is usually completely sealing the entire apparatus in a cutting
box or room. In cutting shapes from or slitting flat materials, the problem is much less severe. The
nozzle is kept very close to the material for both safety and best cutting performance and the jet is
received in a catcher immediately below the material. When cutting materials where wetting is not
an issue, it is a good idea to cut 1/2 in or more under water. This almost totally eliminates any jet
noise and makes a cleaner operation by suppressing splash back and mist that carries fine abrasive dust.