Section
4. Cross Abrasive Processes--WJM/USM/ECM/EDM/MAP
Abrasives are commonly
used in both conventional and non-traditional machining processes. Abrasives
are hard grits which acts as micro cutters during mahcining. Mechincal energy
can be good solutions to some difficulties in non-traditional processes when
it is used properly.
Abrasive WJM
Water jet machining(WJM) is mainly used to cut and slit porous nonmetals such
as wood, paper, leather, and foam. However it is not efficient for hard material
machining. When abrasives are mixed in the water jet, Abrasive Waterjet Machining,
a new and more powerful process is realized. Both WJM and AWJM
use the principle of pressurizing water to extremely high pressures, and allowing
the water to escape through a very small opening (orifice). Water jets machining
use the beam of water exiting the orifice (or jewel) to cut soft stuff like
diapers and candy bars, but are not effective for cutting harder materials.
The inlet water is typically pressurized between 20,000 and 55,000 Pounds
Per Square Inch (PSI). This is forced through a tiny hole in the "Jewel",
which is typically 0.010" to 0.015" in diameter. This creates a very high
velocity beam of water! Abrasive water jet machining uses that same beam of
water to accelerate the abrasive particles to speeds fast enough to cut through
much harder materials(Fig. 13). With the help of abrasives, materials of any
hardness can be cut without delamination, without thermal damag, in the same
time, withvery high cutting rate and the capability of cutting very large
thickness. Fig.14 shows a 2" thick piece of 304 stainless steel cut by AWJM[1].
Fig.
13 A diagram of an abrasive jet. Notice that it is just like a water jet with
more stuff underneath the jewel. The high velocity water exiting the jewel
creates a vacuum which sucks abrasive from the abrasive line, which mixes
with the water in the mixing tube to form a high velocity beam of abrasives(Graphic
courtesy of OMAX Corporation).
Fig.
14 A 2" thick piece of 304 stainless steel cut by AWJM. It took under 3 hours
with a very small pump (including programming, setup, etc.) to machine this
to a tolerance of +/-.005". (picture courtesy of OMAX corp.)
Electrochemical
Assistance of Ultrasonic Machining Processes
Ultrasonic
machining is a mechanical material removal process used to erode holes and
cavities in hard or brittle workpieces by using shaped tools, high frequency
mechanical motion, and an abrasive slurry. Ultrasonic machining is able to
effectively machine all materials harder than HRc 40, whether or not the material
is an electrical conductor or an insulator[2]. Due to hardness limits, ultrasonic
machining of some metallic materials is very slow and tool wear becomes a
serious problem[3]. Electrochemical interactions can be integrated into the
ultrasonic machining process to solve this difficulty (Figure 15).
In electrochemical
machining, anode dissolution is accompanied by the formation of a brittle
oxide layer. Therefore, the abrasive grains in EC assisted USM act mainly
on the brittle oxide layer rather than on the underneath material of the workpiece.
Because the oxide film is removed by abrasion, the electrochemical dissolution
process is accelerated due to depassivation of the material surface. The efficiency
of both processes are improved. Machining rate is greater and the tool wear
is less. An increase in the current density enhances the machining productivity
and reduces the tool electrode wear, but decreases the accuracy. Both ultrasonic
interaction and anodic dissolution can result in good surface finish. At the
final step of operation, the machining can use small current density to improve
surface finish.
 |
|
Fig.
15 Ultrasonic Machining with Electrochemical Assistance
|
Abrasive
ECM/EDM/ECDM
The
drawback and advntage of mechanical machining is that it directly contacts
the workpiece and it is usually not sensitive to electrical or chemical fields.
On the other hand, ECM and EDM process may be influenced by the oxidation
or passivation layer. The addition of abrasives can help to breakdown the
passivation layers and improve the machining rate. Also the abrasive grains
that are in contact with the workpiece surface acts to remove the soft nonreactive
oxide layer in ECM, thus exposing fresh metal to further electrolytic action.
Conventional abrasive machining suffers from too fast wheel wear out, while
in abrasive non-traditional machining, the majority of machining is done by
machining other than mechanical machining. Thus the life of the wheel is greatly
elongated. The benifits of both conventional and no-traditional processes
are both enhanced.
Besides
abrasive waterjet machining, which directly uses the intensified mechanical
machining effects of abrasives, other non-traditional abrasive machining processes
are: Abrasive Electrical Discharge Machining (AEDM), Abrasive Electrochemical
Machining (AECM), and Abrasive Electro-Chemical-Discharge Machining (AECDM).
The tools are conducting
electrodes containing abrasive grains, such as grinding wheels or abrasive
sticks using conductive bonding agent or simply free abrasive grit in electrolyte.
The types of tools
and relative movements that are used in various processes are shown in Figure
16. Abrasive Electrochemical Grinding (AECG), Abrasive Discharge Grinding
(AEDG), and Electrochemical Honing (ECH) use abrasive tool with conductive
bond. Abrasive Electrochemical Finishing (AECF), Abrasive Electrical Discharge
Finishing (AEDF), and Ultrasonic Electrochemical Machining (USECM) use free
abrasive grains.
|
|
Fig.
16 Selected methods of Abrasive Electrical/Chemical Machining Processes
(A
– abrasive, D – dielectric, E – electrolyte)
|
Abrasive
Electrochemical Machining Processes
Electrochemical
grinding using conductive bonded abrasive tool (AECG) is particularly effective
for the machining of difficult-to-cut materials, such as sintered carbides,
creep resisting alloys (eg. Inconel, Nimonic), titanium alloys, metallic composites
(eg. PCD-Co, Al-SiC, Al-Al2O3).
The increase in performance results from microcutting, electrochemical dissolution,
breakdown of surface oxidation layer, and enhanced flow of the fresh electolyte
in the gap between the grinding wheel and the electrode.
Material is removed
through a combination of electrochemical action and conventional mechanical
grainging. Approximately 90% of the material is removed through electrochemical
action and only 10% by mechanical griding. Because only a small amount of
material is removed by grinding action, the wheel life is typically 10 times
longer than the life of a conventional grinding wheel. An additional factor
contribution to the lone life of electrochemical grinding wheels is the reduced
contact arc--only a fraction of the grinding wheel directly contacts the workpiece
sompared with conventional grinding wheel due to the gap produced by chemical
machining. Figure 17 shows the scheme of ECG.
|
| Fig.
17. Scheme of abrasive electrochemical grinding (AECG) |
The electrochemical
interaction changes the microcutting conditions. Due to chemical effects,
microhardness of the dissolved surface decreases, which reduces the tangential
cutting forces Ft
and normal cutting forces Fn.
The electro-chemical heterogeneity (electric potentials and electro-chemical
coefficients) at individual grains and grain boundariescan induce nonuniform
phase dissolution rate. Mechanical grinding helps reduce these
nonuniformity. Considerable
reduction in cutting force significantly reduces abrasive material wear (q),
and provides notable savings in machining costs when using expensive diamond
grinding wheels. Figure 18 shows the effect of working voltage on power of
the grinding wheel drive, diamond abrasive wear, q, and relative cost
of material volume unit removing, K [4].
 |
Fig.18
Effect of working voltage on power N, wear of diamond q and
relative cost K
(material: stainless steel, electrolyte: 8% NaNO3+5%
Na3PO3) |
Abrasive electrochemical
grinding can be used in the shaping of complex profiles on NC grinder. For
example, Figure 19 illustrates applications for these special tool-electrodes
with PCD abrasives [5].
 |
|
Fig.
19 Examples of AECG Contouring[5]
|
Figure
20 illustrates the influence of combined processes on machined surface finish.
The results of workpiece surface smoothing, that have been obtained in face
grinding using free abrasives (curve 2), electro-chemical grinding without
free abrasives (curve 1), and abrasive – electrochemical grinding (curve
3 for grain size 2 mm.
and curve 4 for grain size 0.5 mm.)
are shown [16]. It can be seen that in the case of electro-chemical smoothing,
maximum height of surface irregularities is Rmax = 0.05 mm.,
i.e. same as the initial value, and in the case of mechanical grinding this
height decreases to Rmax = 0.008 mm.,
while with the combination of both processes it is possible to achieve Rmax
= 0.006 mm.
After changing grain size from 2 mm
to 0.5 mm.,
in only 160 seconds, mirror surface (Rmax = 0.002 mm.)
has been obtained [6]. This
example reveals the effects of electro-abrasive machining processes and their
potential in microfinishing and nanotechnology.
 |
|
Fig.
20 Maximum roughness, Rmax, versus Machining time [6]
|
Abrasive
Electrical Discharge Grinding(AEDG)
Introducing
mechanical effects into EDG process leads to increase in metal removal rate.
For example, in the case of Al-SiC composite, removal rate of the AEDG process
(te = 10 ms,
and n = 500 rpm) is about 5 times greater than that of EDM process,
and about twice of EDG process. The
increase in productivity of AEDG process is attributed to improvements in
hydrodynamic conditions of dielectric flow. Abrasive
EDM has beed discussed in Section 2 and will not be repeated here.
Some
Ultra High Precision Surface Finishing
Processes
Ultra
high precision polishing technologies are used mainly in semiconductor industry.
In such applications, less than 0.1 mm
accuracy is required, the surface roughness should be less than 0.01mm.
In order to obtain such smooth surfaces, the material surface should be machined
with a controllable removal depth less than 0.001mm.
Chemical interactions play more important roles than mechanical or physical
actions at such dimension scales as shown in Table 1 [7].
 |
| Table
1 |
When
the minimal removal depth at the contact between the work surface and the
tool tip is large (approximately 1mm
or larger), removal of the material is caused by the mechanical deformation
mechanism through either plastic deformation or brittle fracture. On the other
hand, when the material surface is machined with a minimal removal depth less
than 0.1 mm,
the chemical effects is the more important material removal mechanism because
the increase in surface area due to particle size reduction. Machining from
chemical action is greater than that from mechanical action. This means that
even in conventional mechanical machining processes, chemical interactions
may be the dominant mechanism of material removal in ultrapresion machining
processes. One typical instance is the Elastic Emission Machining in which
ultrafine abrasive powder mechanically bombards the material surface and elastically
sputters the atoms, molecules or clusters of atoms/molecules from the surface.
It is known that even in this mechanical mode of machining, removal rates
considerably depend on the chemical affinity of the workpiece
material and the abrasive powder[7].
In
Mechanical-Chemical Polishing (MCP), chemically reactive abrasive powders
softer than the workpiece are used. Neither indentations nor scratches of
the abrasives would occur. MCP process using soft powder abrasive is especially
suitable for very hard materials which are difficult-to-machine by conventional
polishing processes. New
polishing processes have been developed with the assistance of various energy
fields, such as magnetic, electrical and ultrasonic field. Figure 21 shows
the Magnetic Abrasive Polishing (MAP) process. Storng
magnetic field drives the magnetic abrasives towards the specimen surface.
This process is applicable not only to flat surfaces but also to inner and
outer sculptured surfaces. For example, inner surface of a stainless steel
pipe can be polished to a roughness of 0.3mm
in a few minutes.
 |
| Fig.
21 Magnetic Polishing [7] |
