What is ultrasonic machining process?
In ultrasonic machining of a workpiece the material is removed by micro chipping or erosion through high-velocity bombardment by abrasive particles, in the form of a slurry, through the action of an ultrasonic transducer.
Fig.1 Schematic diagram of ultrasonic machining process
The ultrasonic machine is used for machining hard and brittle materials and can produce very small and accurate holes 0.015 in. (0.4 mm)
What is a Sonotrode?
The tip of the tool used in ultrasonic machining is called as a sonotrode which vibrates at a frequency of 20 kHz and a low amplitude of 0.0125 to 0.075 mm.
A high velocity is produced due to the vibrations on abrasive grains present between the tool and the workpiece.
The stress generated by the impact of abrasive particles on the workpiece surface is high because:
(a) the contact time between the particle and the surface is very short (10 to 100 /zs).
(b) the area of contact is very small. In brittle materials, micro chipping and erosion of the workpiece surface takes place due to high impact stresses.
Which are the best suited materials for ultrasonic machining?
Ultrasonic machining process is best suited for materials that are hard and brittle, such as ceramics, carbides, precious stones, and hardened steels.
Fig. 2 Parts made by ultrasonic machining process
In this process, a tool is required for each shape to be produced; hence it is also called a form tool.
The tip of the tool (which is attached to a transducer through the tool holder) usually is made of mild steel. Boron carbide is the abrasive grain which is mostly used, although aluminum oxide or silicon carbide grains are also used, with sizes ranging from grit number 100 for roughing to grit number 1000 for finishing operations.
The grains are carried in a water slurry with concentrations of 20 to 60% by volume; the slurry also carries the debris away from the cutting zone.
What is rotary ultrasonic machining?
In this process, the abrasive slurry is replaced by a tool with metal-bonded diamond abrasives either impregnated or electroplated on the tool surface.
The tool is vibrated ultrasonically and rotated at the same time while being pressed against the workpiece surface at a constant pressure.
The process is similar to a face-milling operation, but with the inserts being
replaced by abrasives.
The chips produced are washed away by a coolant that is pumped through the core of the rotating tool.
Rotary ultrasonic machining (RUM) is particularly effective in producing deep holes and high material removal rate in ceramic parts.
RUM substantially increases the drilling efficiency.
What are the design considerations for ultrasonic machining?
The basic design guidelines for USM include the following:
° Avoid sharp profiles, corners, and radii, because these are eroded by the abrasive slurry.
° Accept that holes produced will have some taper.
° Note that, because of the tendency for the chipping of brittle materials at the exit end of holes machined, the bottom of the parts should have a backup plate.
What are the advantages and disadvantages of this process?
1. Reduction in touch temperature.
2. Ease in machining of complex hard materials.
3. Greater surface finish.
1. Skilled labour required.
2. Defects due to higher grain size.