2.2 SHAPING MATERIALS UNCONVENTIONALLY 42 What is the Material Removal Rate (MRR)? The selection of abrasive particles depends on the hardness and the MRR of the workpiece. Most commonly, aluminium oxide or silicon carbide particles are used. MECHANICAL ENERGY-BASED PROCESSES The advent of hard materials has made conventional machining methods very difficult and time consuming , so other processes, known as unconventional , have been developed and classified according to the energy they employ: mechanical, electrical, thermal, chemical, and Hard materials can be cut if abrasives are introduced into water, transforming the process into Abrasive Water Jet Machining (AWJM). The grains used in the abrasive fluid are usually boron carbide or silicon carbide, while the fluid medium can be either water, benzene, glycerol or oil. electrochemical. Mechanical energy-based processes With mechanical energy-based unconventional processes, unwanted material in the workpiece is removed by mechanical erosion. The most common ones are: Abrasive Jet Machining (AJM), Water Jet Machining (WJM) and Ultrasonic Machining (USM). AJM The Abrasive Jet Machining process is used for: providing a smooth surface finish to hard metallic workpieces; boron carbide: carburo di boro burr: bava carbon dioxide: anidride carbonica feed: fornitura, aggiunta fine: sottile nitrogen: azoto nozzle: ugello silicon carbide: carburo di silicio slot: fessura, apertura slurry: fango abrasivo smooth: liscio, levigato time consuming: che richiede tempo to undergo: subire 88 WJM The Water Jet Machining process uses water to produce a high velocity jet to cut a soft workpiece. The water jet strikes the workpiece and its kinetic energy is converted into pressure energy; this causes a high stress on the workpiece until the unwanted particles are automatically removed. WJM is a relatively fast and very precise process which prevents thermal damage to the workpiece because no heat is generated, and which does not produce any hazardous gas (it is eco-friendly), but only soft materials, such as rubber, wood, ceramics, plastics and aluminium can be machined. USM The Ultrasonic Machining is a process in which material is removed from the surface of a workpiece by high frequency vibrations of a tool against the surface of the material. Abrasive particles are present: the slurry is formed by mixing fine abrasive grains in water. This method is used to machine both brittle and hard materials with high precision, leaving no burr in the process. cutting, cleaning and machining semi-conductors (e.g.: silicon, gallium or germanium); making holes and slots in glass, quartz, mica and ceramics. Compressed air or gas is passed into a mixing chamber and a vibrator controls the feed of some abrasive powder. The two elements are mixed in the chamber, then the mixture is sent to a nozzle which produces a fine abrasive jet used to remove unwanted material from the workpiece. The gas used can be either carbon dioxide or nitrogen. Waterjet machining Shaping and joining materialS MechPower.indb 88 30/01/24 17:07
