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ABSRACT
Although micro ultrasonic machining (USM) has been successfully applied in generating micro features in hard and brittle materials such as silicon, the influence of machining parameters on the machining performance is not clearly understood. A new method for micro ultrasonic machining (MUSM) has been developed. In order to obtain high precision tool rotation, the spindle mechanism employed in micro-EDM machines was introduced. Since the mechanism does not allow the vibration of tools, the work piece was vibrated during machining. Using this setup, we have succeeded in machining micro holes as small as 5µm in diameter in quartz glass and silicon. In this machining range, high tool wear posed a problem. To solve this problem, a sintered diamond (SD) tool was tested and was proven to be effective.
1.1 INTRODUCTION
Micro ultrasonic machining (micro USM), is one of the efficient material removal processes especially suitable for the micromachining of hard and brittle materials. The principle of micro USM is shown in Figure 1. In micro USM work piece which is placed on the work piece table vibrates at ultrasonic frequency (40 KHz). Abrasive slurry is injected on the top of the work piece. There is a rotating tool which hits the abrasive particles in the slurry which in turn hit the work piece and chip away the material from it. The vibrations given to the work piece aid in refreshing the slurry so that fresh abrasive particles are in contact with the work piece and also in removing the debris from the tool work piece gap.
The abrasive slurry acts as lubricating agent as well as coolant in reducing the frictional heat generated due to the movement of the abrasive particles on the work piece and heat generated by the vibrations due to the transducer. The slurry also collects the debris from the machined area. In general micro USM is carried out with water as the medium due to its properties of excellent coolant, easy removal of debris from machining zone due to low viscosity, low cost and easy availability. In many conventional machining processes like grinding, milling and broaching processes oil has been successfully used as cutting fluid. These oils can be used either as straight oils, which are pure petroleum based oils or emulsifiers which are water based oils. Use of straight oils have excellent lubricating properties and are used especially for machining process involving low speeds, low clearance requiring high quality surface finish. These oils have more viscosity and good lubricating properties than water and causes less tool wear. Hence, it may be prudent to use oil based slurry in micro USM. This paper reports the experimental investigation of the same.
1.2 Needs in Micromachining
Innovations in the fields of biomedical devices, aerospace, automobile, energy, optics, semiconductors, electronics and communications have led to miniaturization of the parts and devices. Small sized devices and their component parts are desirable to keep things compact and portable. Therefore, material and energy required for manufacturing reduces drastically. As a result the cost of production and environmental pollution is reduced. Small parts have lower inertia because of which production process needs lesser time. Consequently, the productivity increases. Production of small parts requires different processes and systems capable of machining at micro scale. To manufacture functional micro parts and devices, tighter tolerances, higher accuracy and precision, superior surface integrity, improved repeatability and reliability are desirable constantly. These capabilities are limited by the existing technology. Consequently, constant advancement of the micro machining techniques is essential for fabrication of the micro parts and devices. Micro machining is defined as the ability to produce features with the dimensions between 1 μm to 999 μm or when the volume of the material removed is at the micro
level (e.g. micro grinding).
1.3 Micro Rotary Ultrasonic Machining
The advantage of rotary ultrasonic machining for machining hard and brittle materials is clearly evident from the discussion in the previous section. Till date RUM has been developed and well researched only at the macro level. Downscaling of RUM is essential for machining micro parts and micro features in hard and brittle materials. Hence, in the present research feasibility of MRUM has been explored. Major issues involved in the downscaling of RUM are discussed below. Two major requirements for MRUM are the micro sized abrasive bonded tool and a machining system capable of applying very small load on the micro tool with necessary feedback and control mechanisms.
EXPERIMENTAL SETUP
An in-house designed and built micro USM machine was used for performing experiments for MRUM. The system is an assembly of a piezoelectric ultrasonic transducer, a spindle for rotating the tool (Cannon LN 22, 5W, coreless DC motor) and the position of the tool was controlled in the X, Y and Z axes by a precision motion controller (Newport PM500C) with a 25 nm resolution. The work piece was vibrated ultrasonically by mounting it on the free end of the transducer. A double sided duct tape was used to fix the work piece on the transducer. The coolant was injected between the tool and the work piece. To control or monitor the forces developed during machining an electronic balance was used. The response from the balance was given as a feedback to the motion controller through a RS-232 interface to control the position of the tool on the work piece.