1.Introduction
Ultrasonic machining was a non-traditional machining process. USM was grouped under the mechanical group NTM processes. briefly depicts the USM process.
In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 μm over the workpiece. Generally the tool was pressed downward with a feed force, F. Between the tool and workpiece, the machining zone was flooded with hard abrasive particles generally in the form of a water based slurry.
As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material was brittle, due to crack initiation, propagation and brittle fracture of the material. Hence, USM was mainly used for machining brittle materials {which are poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-discharge machining (ECM and ED)}.
2. Mechanisms of Material Removal in USM and its modelling
As had been mentioned earlier, USM was generally used for machining brittle work material. Material removal primarily occurs due to the indentation of the hard abrasive grits on the brittle work material. As the tool vibrates, it leads to indentation of the abrasive grits. During indentation, due to Hertzian contact stresses, cracks would develop just below the contact site, then as indentation progresses the cracks would propagate due to increase in stress and ultimately lead to brittle fracture of the work material under each individual interaction site between the abrasive grits and the workpiece. The tool material should be such that indentation by the abrasive grits does not lead to brittle failure. Thus the tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys.
Other than this brittle failure of the work material due to indentation some material removal may occur due to free flowing impact of the abrasives against the work material and related solid-solid impact erosion, but it was estimated to be rather insignificant. Thus, in the current model, material removal would be assumed to take place only due to impact of abrasives between tool and workpiece, followed by indentation and brittle fracture of the workpiece. The model does consider the deformation of the tool.
3. Machine
• Slurry delivery and return system
• Feed mechanism to provide a downward feed force on the tool during machining
• The transducer, which generates the ultrasonic vibration
• The horn or concentrator, which mechanically amplifies the vibration to the required amplitude of 15 – 50 μm and accommodates the tool at its tip.
The ultrasonic vibrations are produced by the transducer. The transducer was driven by suitable signal generator followed by power amplifier. The transducer for USM works on the following principle
• Piezoelectric effect
• Magnetostrictive effect
• Electrostrictive effect
Magnetostrictive transducers are most popular and robust amongst all.
Fig. 9.2.8 shows a typical magnetostrictive transducer along with horn. The horn or concentrator was a wave-guide, which amplifies and concentrates the vibration to the tool from the transducer.
4.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.
5.Limitations
• Low MRR
• Rather high tool wear
• Low depth of hole
