Working Of Ultrasonic Machining (USM) | Process Parameters
Ultrasonic machining is a non-traditional machining process. USM is grouped under the mechanical group NTM processes. Fig. briefly depicts the USM process.
Principle Of Ultrasonic Machining :
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 is pressed downward with a feed force, F. Between the tool and workpiece, the machining zone is 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 is brittle, due to crack initiation, propagation and brittle fracture of the material. Hence, USM is 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).
Process Parameters and their Effects.
During discussion and analysis as presented in the previous section, the process parameters which govern the ultrasonic machining process have been identified and the same are listed below along with material parameters
• 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
• 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
The basic mechanical structure of an USM is very similar to a drill press. However, it has additional features to carry out USM of brittle work material. The workpiece is mounted on a vice, which can be located at the desired position under the tool using a 2 axis table. The table can further be lowered or raised to accommodate work of different thickness.
The typical elements of an USM are • 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 is driven by suitable signal generator followed by power amplifier. The transducer for USM works on the following principle
• Piezoelectric effect
• Magneto strictive effect
• Electro strictive effect
• 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.