In this article we will discuss about the unconventional machining processes:- 1. Abrasive Jet Machining (AJM) 2. Electron Beam Machining (EBM) 3. Laser Beam Machining (LBM) and 4. Plasma Arc Machining (PAM). And also learn about:- Unconventional Machining Process, Characteristics of Unconventional Machining Process and Classifications of Unconventional Machining Processes.
In AJM, the material removal takes place due to the impingement of the fine abrasive particles. These particles move with a high speed air (or gas) stream. Figure 6.1 shows the process along with some typical parameters of the process. The abrasive particles are typically of 0.025 mm diameter and the air discharges at a pressure of several atmospheres.
When an abrasive particle impinges on the work surface at a high velocity, the impact causes a tiny brittle fracture and the following air (or gas) carries away the dislodged small work piece particle (wear particle). This is shown in Figs. 6.2a and 6.2b. Thus, it is obvious that the process is more suitable when the work material is brittle and fragile. A model for estimating the material removal rate (mrr) is available. The mrr due to the chipping of the work surface by the impacting abrasive particles is expressed as –
where Z is the number of abrasive particles impacting per unit time, d is the mean diameter of the abrasive grains, v is the velocity of the abrasive grains, ρ is the density of the abrasive material, Hw is the hardness of the work material (the flow stress), and X is a constant.
Process Parameters of AJM:
The process characteristics can be evaluated by judging – (i) the mrr, (ii) the geometry of the cut, (iii) the roughness of the surface produced, and (iv) the rate of nozzle wear.
The major parameters which control these quantities are:
(i) The abrasive (composition, strength, size, and mass flow rate),
(ii) The gas (composition, pressure, and velocity),
(iii) The nozzle (geometry, material, distance from and inclination to the work surface).
We shall now discuss each of these parameters as also their effects:
i. The Abrasive:
Mainly two types of abrasives are used, viz., – (i) aluminium oxide and (ii) silicon carbide. However, generally aluminium oxide abrasives are preferred in most applications. The shape of these grains is not very important, but, for a satisfactory wear action on the work surface, these should have sharp edges. Al2O3 and SiC powders with a nominal grain diameter of 10-50 μm are available. The best cutting is achieved when the nominal diameter is between 15 μm and 20 μm.
A reuse of the abrasive powder is not recommended as the – (i) cutting capacity decreases after the first application, and (ii) contamination clogs the small orifices in the nozzle. The mass flow rate of the abrasive particles depends on the pressure and the flow rate of the gas. When the mass fraction of the abrasives in the jet (mixing ratio) increases, the mrr initially increases, but with a further increase in the mixing ratio, it reaches a maximum and then drops (Fig. 6.3a). When the mass flow rate of the abrasive increases, the mrr also increases (Fig. 6.3b).
ii. The Gas:
The AJM units normally operate at a pressure of 0.2 N/mm2 to 1 N/mm2. The composition of gas affects the mrr in an indirect manner as the velocity-pressure relation depends on this composition. A high velocity obviously causes a high mrr even if the mass flow rate of the abrasive is kept constant.
iii. The Nozzle:
The nozzle is one of the most vital elements controlling the process characteristics. Since it is continuously in contact with the abrasive grains flowing at a high speed, the material must be very hard to avoid any significant wear. Normally, WC or sapphire is used. For a normal operation, the cross-sectional area of the orifice is between 0.05 mm2 and 0.2 mm2.
The shape of the orifice can be either circular or rectangular. The average life of a nozzle is very difficult to ascertain. A WC nozzle lasts between 12hr and 30hr, whereas a sapphire nozzle lasts for 300hr approximately.
One of the most important factors in AJM is the distance between the work surface and the tip of the nozzle, normally called the Nozzle Tip Distance (NTD). The NTD affects not only the mrr from the work surface but also the shape and size of the cavity produced. Figure 6.5 shows the effect of NTD. When the NTD increases, the velocity of the abrasive particles impinging on the work surface increases due to their acceleration after they leave the nozzle.
This, in turn, increases the mrr. With a further increase in the NTD, the velocity reduces due to the drag of the atmosphere which initially checks the increase in the mrr and finally decreases it. Figure 6.6 shows how the NTD affects the mrr.
The abrasive jet machines are manufactured and marketed by a single manufacturer (namely, S.S. White Co., New York) under the name “Airbrasive”.
Characteristics of AJM:
Electron Beam Machining (EBM):
Basically, electron beam machining is also a thermal process. Here, a stream of high speed electrons impinges on the work surface whereby the kinetic energy, transferred to the work material, produces intense heating. Depending on the intensity of the heat thus generated, the material can melt or vaporize. The process of heating by an electron beam can, depending on the intensity, be used for annealing, welding, or metal removal.
Very high velocities can be obtained by using enough voltage; for example, an accelerating voltage of 150,000 V can produce an electron velocity of 228,478 km/sec. Since an electron beam can be focused to a point with 10-200μm diameter, the power density can go up to 6500 billion W / mm2. Such a power density can vaporize any substance immediately. Thus, EBM is nothing but a very precisely controlled vaporization process. EBM is a suitable process for drilling fine holes and cutting narrow slots.
Holes with 25-125μm diameter can be drilled almost instantaneously in sheets with thicknesses up to 1.25 mm. The narrowest slot which can be cut by EBM has a width of 25μm. Moreover, an electron beam can be maneuvered by the magnetic deflection coils, making the machining of complex contours easy. However, to avoid a collision of the accelerating electrons with the air molecules, the process has to be conducted in vacuum (about 10-5 mm Hg); this makes the process unsuitable for very large work pieces.
To indicate the wide range of applications of the electron beam, a plot of the power density versus the hot spot diameter is given in Fig. 6.69. It is obvious that the range of the electron beam is the largest. This is why the electron beam is used not only for machining but also for the other thermal processes.
The electrons are emitted from the cathode (a hot tungsten filament), the beam is shaped by the grid cup, and the electrons are accelerated due to a large potential difference between the cathode and the anode. The beam is focused with the help of the electromagnetic lenses. The deflecting coils are used to control the beam movement in any required manner.
In case of drilling holes the hole diameter depends on the beam diameter and the energy density. When the diameter of the required hole is larger than the beam diameter, the beam is deflected in a circular path with proper radius. Most holes drilled with EBM are characterized by a small crater on the beam incident side of the work. The drilled holes also possess a little taper (2°—4°) when the sheet thickness is more than 0.1 mm. Some idea about the performance characteristics of drilling holes with EBM can be obtained from Table 6.5.
While cutting a slot, the machining speed normally depends on the rate of material removal, i.e., the cross-section of the slot to be cut. The sides of a slot in a sheet with thickness up to 0.1 mm are almost parallel. A taper of 1° to 2° is observed in a slot cut in a thicker plate. A small amount of material splatter occurs on the beam incident side. Table 6.6 gives some idea about the slot cutting capabilities of the electron beam.
The power requirement is found to be approximately proportional to the rate of metal removal. So, P ≈ CQ, C being the constant of proportionality. Table 6.7 gives the approximate values of C for different work materials.
A very rough estimation of the machining speed for the given conditions is possible, using Table 6.7.
Mechanics of EBM:
Electrons are the smallest stable elementary particles with a mass of 9.109 x 10-31 kg and a negative charge of 1.602 x 10-19 coulomb. When an electron is accelerated through a potential difference of V volts, the change in the kinetic energy can be expressed as 1/2me (u2 –u02) eV, where me is the electron mass, u is the final velocity, and u0 is die initial velocity. If we assume the initial velocity of the emitting electrons to be negligible, the final expression for the electron velocity u in km/sec is –
u ≈ 600√V (6.67)
When a fast moving electron impinges on a material surface, it penetrates through a layer undisturbed. Then, it starts colliding with the molecules, and, ultimately, is brought to rest (Fig. 6.71). The layer through which the electron penetrates undisturbed is called the transparent layer.
Only when the electron begins colliding with the lattice atoms does it start giving up its kinetic energy, and heat is generated. So, it is clear that the generation of heat takes place inside the material, i.e., below the transparent skin. The total range to which the electron can penetrate (δ) depends on the kinetic energy, i.e., on the accelerating voltage V. It has been found that –
Where δ is the range in mm, V is the accelerating voltage in volts, and p is the density of the material in kg / mm3.
Effects of EBM on Materials:
Since machining by an electron beam is achieved without raising the temperature of the surrounding material (except an extremely thin layer), there is no effect on the work material. Because of the extremely high energy density, the work material 25-50μm away from the machining spot remains at the room temperature. Apart from this, the chance of contamination of the work is also less as the process is accomplished in vacuum.
Summary of EBM Characteristics:
Laser Beam Machining (LBM):
Like a beam of high velocity electrons, a laser beam is also capable of producing very high power density. Laser is a highly coherent (in space and time) beam of electromagnetic radiation with wavelength varying from 0.1μm to 70μm. However, the power requirement for a machining operation restricts the effectively usable wavelength range to 0.4-0.6μm.
Because of the fact that the rays of a laser beam are perfectly parallel and monochromatic, it can be focused to a very small diameter and can produce a power density as high as 107 W/mm2. For developing a high power, normally a pulsed ruby laser is used. The continuous CO2-N2 laser has also been successfully used in machining operations.
A coiled xenon flash tube is placed around the ruby rod and the internal surface of the container walls is made highly reflecting so that maximum light falls on the ruby rod for the pumping operation. The capacitor is charged and a very high voltage is applied to the triggering electrode for initiation of the flash. The emitted laser beam is focused by a lens system and the focused beam meets the work surface, removing a small portion of the material by vaporization and high speed ablation.
A very small fraction of the molten metal is vaporized so quickly that a substantial mechanical impulse is generated, throwing out a large portion of the liquid metal. Since the energy released by the flash tube is much more than the energy emitted by the laser head in the form of a laser beam, the system must be properly cooled.
The efficiency of the LBM process is very low—about 0.3-0.5%. The typical output energy of a laser is 20 J with a pulse duration of 1 millisecond. The peak power reaches a value 20,000 W. The divergence of the beam is around 2 x 10-3 rad, and, using a lens with a focal length of 25 mm, the spot diameter becomes about 50μm.
Like the electron beam, the laser beam is also used for drilling micro holes and cutting very narrow slots. Holes up to 250μm diameter can be easily drilled by a laser. The dimensional accuracy is around ±0.025 mm. When the work piece thickness is more than 0.25 mm, a taper of 0.05 mm per mm is noticed.
Mechanics of LBM:
Machining by a laser beam is achieved through the following phases:
(i) Interaction of laser beam with work material,
(ii) Heat conduction and temperature rise, and
An accurate analysis of the whole process is difficult and beyond the scope of this text. We shall, however, discuss certain simple aspects of fundamental importance, considering only the increase in temperature of the work material up to the melting point; vaporization and ablation will not be taken into account in our analysis.
(i) Interaction of Laser Beam with Work:
The application of a laser beam in machining depends on the thermo-optic interaction between the beam and the solid work material. So, it is obvious that the work surface should not reflect back too much of the incident beam energy. Figure 6.74 shows a laser beam falling on a solid surface. The absorbed light propagates into the medium and its energy is gradually transferred to the lattice atoms in the form of heat. The absorption is described by Lambert’s law as –
I(Z) = I(0)e–μz
Where I(z) denotes the light intensity at a depth z (Fig. 6.74) and μ is the absorption coefficient. Most of the energy is absorbed in a very thin layer at the surface (typical thickness 0.01μm). So, it is quite reasonable to assume that the absorbed light energy is converted into heat at the surface itself, and the laser beam may be considered to be equivalent to a heat flux.
(ii) Heat Conduction and Temperature Rise:
Re-radiation from the surface at a temperature of 3000 K is of the order of only 600 W / cm2 and it is negligible as compared with the input flux 105-107 W / cm2. To make our analysis one-dimensional, the diameter of the beam spot is assumed to be larger than the depth of penetration. Also, the thermal properties, e.g., conductivity and specific heat, are considered to remain unaffected by the temperature change.
So, the equivalent heat conduction problem is represented by a uniform heat flux H(t) at the surface (Fig. 6.75) of a semi-infinite body. The equation for heat conduction for the region z > 0 is –
The determination of the dimensions of the molten portion of the material is quite complicated. However, if the molten pit (or hole) is deep and narrow, the major portion of heat conduction from the molten hole takes place through the side walls. When the heat input rate is equal to the rate of heat loss by the molten portion, it maintains its shape and size. In such a steady state condition, the rate of heat loss by the molten portion (Fig. 6.77) is given by –
From experience. it has been found that D ≈ 55d. So, In (D/d) may be approximately taken to be 4, and equating the heat input rate to the heat loss rate, the relation we obtain is –
When the beam intensity is very high (>107 W / cm2), the heating is very rapid, and the mechanism we have just given is not valid. The incident beam heats up the surface quickly and vaporizes it. Thus, the surface of the work where the beam falls recedes as the material vaporizes. So, if v is the velocity with which the surface recedes, the rate of heat input required to vaporize the material (equal to the rate of heat input from the incident beam) is-
H ≈ vL, (6.82)
Where L is the amount of energy to vaporize a unit volume of the material.
Summary of LBM Characteristics:
Plasma Arc Machining (PAM):
A plasma is a high temperature ionized gas. The plasma arc machining is done with a high speed jet of a high temperature plasma. The plasma jet heats up the work piece (where the jet impinges on it), causing a quick melting. PAM can be used on all materials which conduct electricity, including those which are resistant to oxy-fuel gas cutting. This process is extensively used for profile cutting of stainless steel, monel, and super alloy plates.
A plasma is generated by subjecting a flowing gas to the electron bombardment of an arc. For this, the arc is set up between the electrode and the anodic nozzle; the gas is forced to flow through this arc.
The high velocity electrons of the arc collide with the gas molecules, causing a dissociation of the diatomic molecules or atoms into ions and electrons resulting in a substantial increase in the conductivity of the gas which is now in plasma state. The free electrons, subsequently, accelerate and cause more ionization and heating. Afterwards, a further increase in temperature takes place when the ions and free electrons recombine into atoms or when the atoms recombine into molecules as these are exothermic processes.
So, a high temperature plasma is generated which is forced through the nozzle in the form of a jet. The mechanics of material removal is based on – (i) heating and melting, and (ii) removal of the molten metal by the blasting action of the plasma jet.
For more details, see the standard handbooks and reference books. Here, we shall list the basic characteristics to familiarize the reader with the process.