22-02-2013, 10:11 AM
Ultrasonic Machining (USM)
Ultrasonic Machining.ppt (Size: 1.56 MB / Downloads: 373)
Introduction
Ultrasonic machining (USM) is the removal of hard and brittle materials using an axially oscillating tool at ultrasonic frequencies [18–20 kHz]
During that oscillation, the abrasive slurry of B4C or SiC is continuously fed into the machining zone between a soft tool (brass or steel) and the workpiece.
The abrasive particles are, therefore, hammered into the workpiece surface and cause chipping of fine particles from it.
The oscillating tool, at amplitudes ranging from 10 to 40 μm, imposes a static pressure on the abrasive grains and feeds down as the material is removed to form the required tool shape.
Balamuth first discovered USM in 1945 during ultrasonic grinding of abrasive powders.
The industrial applications began in the 1950s when the new machine tools appeared.
USM is characterized by the absence of any deleterious effect on the metallic structure of the workpiece material.
Machining System
The machining system of USM is composed mainly from the magnetostrictor, concentrator, tool and slurry feeding arrangement.
The magnetostrictor is energized at the ultrasonic frequency and produces small-amplitude vibrations.
Such a small vibration is amplified using the constrictor (mechanical amplifier) that holds the tool.
The abrasive slurry is pumped between the oscillating tool and the brittle workpiece.
A static pressure is applied in the tool-workpiece interface that maintains the abrasive slurry.
Magnetostrictor
It has a high-frequency winding wound on a magnetostrictor core and a special polarizing winding around an armature.
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization.
The variation of material's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ.
The effect was first identified in 1842 by James Joule at Manchester when observing a sample of nickel.
Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and are used to build actuators and sensors.
Magnetic Amplifier
The choice of the shape of the acoustic horn controls the final amplitude.
Five acoustic horns (cylindrical, stepped, exponential, hyperbolic cosine, and conical horns) have been reported by Youssef (1976).
Exponential and stepped types are frequently used.
Because they are easily designed and produced compared to the conical and hyperbolic horns.
Aluminum bronze and marine bronze are cheap with high fatigue strengths of 185 and 150 MN/m2respectively.
Drawbacks of magnetostrictive transducer: (a) high losses (b) low efficiency (55 %) © heating up and the need for cooling.
Higher efficiencies (90–95 %) are possible by using piezoelectric transformers in modern USM machines.
Abrasive Slurry
Abrasive slurry is usually composed of 50 vol. % of fine abrasive grains and 50 vol.% of water.
Abrasive grain sizes: 100 – 800 grit number.
Abrasive particles used: (a) Boron carbide (B4C) (b) Aluminum oxide (Al2O3) or © Silicon carbide (SiC).
The abrasive slurry is circulated between the oscillating tool and workpiece.
Under the effect of the static feed force and the ultrasonic vibration, the abrasive particles are hammered into the workpiece surface causing mechanical chipping of minute particles.
The slurry is pumped through a nozzle close to the tool-workpiece interface at a rate of 25 L/min.
As machining progresses, the slurry becomes less effective as the particles wear and break down.
The expected life ranges from 150 to 200 h of ultrasonic exposure.
Material Removal Process
The relative contribution of the cavitation effect is reported to be less than 5 percent of the total material removed.
The dominant mechanism involved in USM of all materials is direct hammering.
Soft and elastic materials like mild steel are often plastically deformed first and are later removed at a lower rate.
In case of hard and brittle materials such as glass, the machining rate is high and the role played by free impact can also be noticed.
When machining porous materials such as graphite, the mechanism of erosion is introduced.
The rate of material removal, in USM, depends, on the frequency of tool vibration, static pressure, the size of the machined area, and the abrasive and workpiece material.
MRR and machinability by USM depends on the brittleness criterion which is the ratio of shearing to breaking strength of a material.