In this article we will discuss about the ultrasonic machining:- 1. Ultrasonic Machining (USM) and Ultrasonic Machining Tool 2. Mechanics of USM 3. Process Parameters of USM and Its Effect 4. Components of Ultrasonic Machining 5. Characteristics of Ultrasonic Machining.
- Ultrasonic Machining (USM) and Ultrasonic Machining Tool
- Mechanics of USM
- Process Parameters of USM and its Effect
- Components of Ultrasonic Machining
- Characteristics of USM
1. Ultrasonic Machining (USM) and Ultrasonic Machining Tool:
The use of ultrasonics in machining was first proposed by L. Balamuth in 1945. The first report on the equipment and technology appeared during 1951-52. By 1954, the machine tools, using the ultrasonic principle, had been designed and constructed. Originally, USM used to be a finishing operation for the components processed by the electro spark machines. However, this use became less important because of the developments in electric discharge machining.
But, then, with the boom in solid state electronics, the machining of electrically non-conducting, semi-conductive, and brittle materials became more and more important and, for this reason, ultrasonic machining again gained importance and prominence. In recent years, various types of ultrasonic machine tools have been developed. Of course, the USM technique is still far from perfect.
The basic USM process involves a tool (made of a ductile and tough material) vibrating with a very high frequency and a continuous flow of an abrasive slurry in the small gap between the tool and the work surface. The tool is gradually fed with a uniform force. The impact of the hard abrasive grains fractures the hard and brittle work surface, resulting in the removal of the work material in the form of small wear particles which are carried away by the abrasive slurry. The tool material, being tough and ductile, wears out at a much slower rate.
2. Mechanics of USM:
The physics of ultrasonic machining is neither complete nor uncontroversial.
The reasons of material removal during USM are believed to be:
(i) The hammering of the abrasive particles on the work surface by the tool,
(ii) The impact of the free abrasive particles on the work surface,
(iii) The erosion due to cavitation, and
(iv) The chemical action associated with the fluid used.
A number of researchers have tried to develop the theories to predict the characteristics of ultrasonic machining. The model proposed by M.C. Shaw is generally well-accepted and, despite its limitations, explains the material removal process reasonably well. In this model, the direct impact of the tool on the grains in contact with the work piece (which is responsible for the major portion of the material removal) is taken into consideration.
Also, the assumptions made are that:
(i) The rate of work material removal is proportional to the volume of work material per impact,
(ii) The rate of work material removal is proportional to the number of particles making impact per cycle,
(iii) The rate of work material removal is proportional to the frequency (number of cycles per unit time),
(iv) All impacts are identical,
(v) All abrasive grains are identical and spherical in shape.
Let us now consider the impact of a rigid, spherical abrasive grain of diameter don the work surface. Figure 6.9 shows the indentation caused by such an impact at an instant of time.
If D is the diameter of the indentation at any instant and h the corresponding depth of penetration, we get, from Fig. 6.9,
The various tool positions during a cycle are as shown in Fig. 6.11. The position A indicates the instant the tool face touches the abrasive grain, and the period of movement from A to B represents the impact. The indentations, caused by the grain on the tool and the work surface at the extreme bottom position of the tool are shown in Fig 6.12. If the distance travelled by the tool from the position A to the position B is h (the total indentation), then-
Since the flow stress σ and the Brinell hardness H are the same, equations (6.6) and (6.7) yield –
This rate of material removal is through the direct hammering action of the grains due to the vibrating tool. Some grains, reflected by the fast moving tool face, also impinge on the work face, and we can estimate the indentation caused by such freely moving grains. Figure 6.13 shows a grain reflected by the tool. During vibration, the maximum velocity of the tool face is 2πvA.
Since the original velocity of an abrasive grain is small, its maximum velocity is, obviously, of the order of 2πvA. So, the corresponding maximum kinetic energy of the abrasive grain is given by –
Where ρ is the density of the abrasive material. If we assume that during the indentation caused by such an impinging grain the contact force increases linearly with the indentation, then –
Comparing the values of hw and h’w under normal conditions, we see that h’w is very small as compared with hw, and so it can be concluded that most of the material is removed by the directly impacting abrasive grains.
Relation (6.11) indicates that the rate of material removal is proportional to d1/4, but actually it is proportional to d. This discrepancy between the theoretical prediction and the observed fact was explained by Shaw as follows.
The actual shape of an abrasive grain is not spherical, as shown in Fig. 6.14. Instead of having a smooth surface, it has projections of average diameter d1.
The average diameter of the projections is observed to be proportional to the square of the nominal diameter of the grain (d). So,
Relation (6.18) shows that the mrr is proportional to d, a fact also experimentally confirmed.
The Shaw theory has a number of limitations. For example, it does not correctly predict the effects of variation of A, F, and v. When F is increased, the mrr increases, as shown in Fig. 6.15. This is also confirmed by relation (6.18). However, in practice, Q starts decreasing after some value of F because the abrasive grains get crushed under heavy load.
3. Process Parameters of USM and Its Effect:
The important parameters which affect the process are the:
As can be seen from relation (6.18), the mrr increases linearly with the frequency. In practice also, the mrr increases with the frequency (see Fig. 6.16a) but the actual characteristic is not exactly linear. The mrr tends to be somewhat lower than the theoretically-predicted value.
When the amplitude of vibration is increased, the mrr is expected to increase, as can be seen from relation (6.18). The actual nature of the variation is as shown in Fig. 6.16b for different values of the frequency. Again, the actual characteristic is somewhat different from the theoretically-predicted one. The main source of discrepancy stems from the fact that we calculated the duration of penetration Δt by considering the average velocity (=A/(T/4)). The characteristic of variation of Δt, given by –
is quite different from that obtained from the approximate expression, i.e., (h / A)(T / 4).
(iii) Static Loading (Feed Force):
With an increase in static loading (i.e., the feed force), the mrr tends to increase. However, in practice, it tends to decrease beyond a certain critical value of the force as the grains start getting crushed. The nature of variation of the mrr with the feed force (for various amplitudes) is shown in Fig. 6.17a.
(iv) Hardness Ratio of the Tool and the Work Piece:
The ratio of the work piece hardness and the tool hardness affects the mrr quite significantly, and the characteristic is as shown in Fig. 6.17b. Apart from the hardness, the brittleness of the work material plays a very dominant role. Table 6.2 indicates the relative material removal rates for different work materials, keeping the other parameters the same. Clearly, a more brittle material is machined more rapidly.
(v) Grain Size:
Relation (6.18) indicates that the mrr should rise proportionately with the mean grain diameter d. However, when d becomes too large and approaches the magnitude of the amplitude A, the crushing tendency increases, resulting in a fall in the mrr as shown in Fig. 6.18a.
(vi) Concentration of Abrasive in the Slurry:
Since the concentration directly controls the number of grains producing impact per cycle and also the magnitude of each impact, the mrr is expected to depend on C. But relation (6.18) shows that the mrr is expected to be proportional to C1/4. The actual variation is shown in Fig. 6.18b for B4C and SiC abrasives. This is in a fairly good agreement with the theoretical prediction. Since the mrr increases as C1/4, the increase in the mrr is quite low after C has crossed 30%. Thus, a further increase in the concentration does not help.
Some physical properties (e.g., viscosity) of the fluid used for the slurry also affect the mrr. Experiments show that the mrr drops as the viscosity increases (Fig. 6.19a).
Though the mrr is a very important consideration for judging performance of an USM operation, the quality of finish obtained has also to be considered for a proper evaluation. In an USM operation, the surface finish depends mainly on the size of the abrasive grains. Figure 6.19b shows a typical variation of the mean value of the surface unevenness with the mean grain size for both glass and tungsten carbide as the work material.
It is clear that the surface finish is much more sensitive to the grain size in the case of glass. This is because of the fact that, for a high hardness, the size of the fragments dislodged through a brittle fracture does not depend much on the size of the impacting particles.
Effects of USM on Materials:
Since the cutting force involved is very small, the process produces no appreciable stress and heating. So, the material structure remains unaffected. However, during cutting through a hole, chipping may occur at the exit side of the hole. To avoid this, the work piece made of a brittle material is fastened to a base usually made of glass.
4. Components of Ultrasonic Machining:
The important components of the machine are:
The acoustic head (Fig. 6.22) is perhaps the most important part of the machine. Its function is to produce a vibration in the tool. It consists of a generator for supplying a high frequency electric current, a transducer to convert this into a mechanical motion in the form of a high frequency vibration, a holder to hold the head, and a concentrator to mechanically amplify the vibration while transmitting it to the tool.
Most transducers work on the magnetostrictive principle because of the high efficiency, high reliability in the 15-30 kHz range, low supply voltage, and simple cooling arrangement. Stampings are used to reduce loss as in transformers. The dimensions are so chosen that the natural frequency coincides with the electric supply frequency. Almost all the modern machines use the magnetostriction transducers made of nickel (stampings of 0.1-0.2 mm thickness).
The main purpose of the concentrator is to increase the amplitude to the level needed for cutting. Various types of concentrators are used (Fig. 6.23a). Figure 6.23b shows how the amplitude of longitudinal vibration of the transducer-concentrator assembly is amplified. It should be noted that the system has to be held to the main body at a nodal point, as shown.
The objective of the feed mechanism is to apply the working force during the machining operation. An instrument showing the movement of the tool indicates the depth of machining.
The basic types of feed mechanisms are the:
(a) Counterweight type,
(b) Spring type,
(c) Pneumatic and hydraulic type,
(d) Motor type.
The tool is made of a strong, but at the same time ductile, metal. Generally, stainless steels and low carbon steels are used for making the tools. Aluminium and brass tools wear ten and five times faster than steel tools, respectively. The geometrical features are decided by the process. The diameter of the circle circumscribed about the tool should not be more than 1.5 to 2 times the diameter of the end of the concentrator, and the tool should be as short and rigid as possible.
When the tool is made hollow, the internal contour should be parallel to the external one to ensure uniform wear. The thickness of any wall or projection should be at least five times the grain size of the abrasive. In a hollow tool, the walls should not be made thinner than 0.5 mm to 0.8 mm. When designing the tool, consideration should be given to the side clearance which is normally of the order of 0.06 mm to 0.36 mm, depending on the grain size of the abrasive.
(iv) Abrasive Slurry:
The most common abrasives are – (i) boron carbide (B4C), (ii) silicon carbide (SiC), (iii) corundum (Al2O3), (iv) diamond, and (v) boron silicarbide (very efficient) whose abrasive power is about 10% more than that of B4C. B4C is the best and most efficient among the rest but it is expensive. SiC is used on glass, germanium, and some ceramics. The cutting time with SiC is about 20-40% more than that with B4C. Corundum is much less efficient and the cutting time is about 3-4 times of that with B4C. Diamond dust is used only for cutting diamonds and rubies.
Though water is the most commonly used fluid in the slurry, other liquids, such as benzene, glycerol, and oils, are also used. It has been found that the mrr tends to decrease with increasing viscosity.
5. Characteristics of USM: