Ultra sonic machining

USM

1. ULTRA SONIC MACHINING

2. Introduction USM is a used to erode holes and cavities in hard and brittle w/p. It is happened by means of high frequency mechanical motion and abrasive slurry. It can machine materials which can be electrically conductive or not. Also it is a silent process.

3. Principle : Process and cutting tool The process is performed by a cutting tool, which oscillates at high frequency , typically 20-40 kHz, in abrasive slurry. The shape of the tool corresponds to the shape to be produced in the work piece. The high-speed reciprocations of the tool drive the abrasive grains across a small gap against the work piece . The tool is gradually fed with a uniform force. The impact of the abrasive is the energy principally responsible for material removal in the form of small wear particles that are carried away by the abrasive slurry. The tool material, being tough and ductile, wears out at a much slower rate.

4. Type of transducer Transducers in Usm is used to convert the applied energy to mechanical motion. Magnetostrictive Transducer A magnetostriction transducer is a device that is used to convert mechanical energy into magnetic energy and vice versa . Change in length as a result of magnetisation . These transducer are made of nickel , nickel alloy sheets. Such transducers are available up to 2000 Watts. The maximum change in length can be achieved is about 25 microns

5. Piezoelectric Transducer A piezoelectric crystal transducer/sensor is an active sensor and it does not need the help of an external power as it is self-generating. A quartz crystal is a piezoelectric material that can generate a voltage proportional to the stress applied upon it.  If a force is applied on the pressure port, the same force will fall on the force summing member. Thus a potential difference will be generated on the crystal due to its property These transducer generate a small electric current when they are compressed. Also when the electric current is passed though crystal it expands. When the current is removed , crystal attains its original size and shape. Such transducers are available up to 900 Watts. Piezo electric crystals have high conversion efficiency of 95%.

6. Horn The oscillation amplitude obtained from the magnetostrictive transducer is usually around 5 microns, which is too small for removal of material from the workpiece . The function of the concentrator (also called mechanical amplifiers, acoustic horn and tool cone) is to amplify the amplitude of vibration of the magnetostrictive transducer from 5 to 40–50 microns.

7. The horn or concentrator can be of different shape like • Tapered or conical • Exponential • Stepped

8. Nodal point clamping Rigid system should be made without any loss in the mounting . So damped at the nodal points To reduce the loss and fatigue failure clamping is made in the nodal point

9. Feed Mechanism There are four types of feed mechanism which are commonly used in USM: Gravity feed mechanism Spring loaded feed mechanism Pneumatic or hydraulic feed mechanism Motor controlled feed mechanism.

10. Gravity feed mechanism In this mechanism counter, balance weights are used to apply the required load to the head through pulley and rope arrangement. In order to reduce friction ball, bearings are used. Gravity feed mechanisms are simple in construction, but this mechanism is insensitive and inconvenient to adjust.

11. 2. Spring load feed mechanism In this mechanism spring pressure is used to feed the tool during the machining operation. This type of mechanism is quite sensitive and easy to adjust.

12. 3. Pneumatic or hydraulic feed In this mechanism, hydraulic cylinder is used to give a linear motion of the tool. High feed rate and accurate positioning are possible with hydraulic feed mechanism.

13. 4. Motor controlled feed mechanism This mechanism is used for precise control of the tool feed movement.

14. Material Removal Rate Using the theory of Shaw (1956), material removal by USM due to cavitations under the tool and chemical corrosion due to slurry media are considered insignificant. Therefore, the material removal due to these two factors has been ignored. The material removal by abrasive particles due to throwing and hammering only has been considered.

15. Abrasive particles are considered spherical in shape having diameter d. Abrasive particles, suspended in a carrier, move under the high-frequency vibrating tool. There are two possibilities when the tool hits an abrasive particle. If the size of the particle is small and the gap between the bottom of the tool and the work surface is large enough, then the particle will be thrown by the tool to hit the work surface (throwing model). Under the reverse condition, the particle will be hammered over the workpiece surface. In both cases, the particle creates a crater of depth hp and radius rp .

16. Process Parameter Amplitude of vibration ( ao ) – 15 – 50 μm Frequency of vibration (f) – 19 – 25 kHz Feed force (F) – related to tool dimensions Feed pressure (p) Abrasive size – 15 μm – 150 μm Abrasive material – Al2O3 - SiC - B4C - Boronsilicarbide - Diamond Flow strength of work material Flow strength of the tool material Contact area of the tool – A Volume concentration of abrasive in water slurry – C

17. Advantages It can be used machine hard, brittle, fragile and non conductive material No heat is generated in work, therefore no significant changes in physical structure of work material Non-metal (because of the poor electrical conductivity) that cannot be machined by EDM and ECM can very well be machined by USM. It is burr less and distortion less processes. It can be adopted in conjunction with other new technologies like EDM,ECG,ECM.

18. Disadvantages Low Metal removal rate. It is difficult to drill deep holes, as slurry movement is restricted. Tool wear rate is high due to abrasive particles. USM can be used only when the hardness of work is more than 45 HRC.

19. Applications Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc. Used for machining round, square, irregular shaped holes and surface impressions. Machining, wire drawing, punching or small blanking dies