Unconventional Machining Unit 6 Unconventional Machining

The requirements that lead to the development of nontraditional machining Very high hardness and strength of the material. (above 400 HB.) The work piece is too flexible or slender to support the cutting or grinding forces. The shape of the part is complex, such as internal and external profiles, or small diameter holes. Surface finish or tolerance better than those obtainable conventional process. Temperature rise or residual stress in the work piece are undesirable.

What is Non-conventional machining ? Non-conventional manufacturing processes is defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tools as it needs to be used for traditional manufacturing processes. The major characteristics of Non-conventional machining are: Material removal may occur with chip formation or even no chip formation may take place. For example in AJM, chips are of microscopic size and in case of Electrochemical machining material removal occurs due to electrochemical dissolution at atomic level.

2. In NTM, there may not be a physical tool present 2.In NTM, there may not be a physical tool present. For example in laser jet machining, machining is carried out by laser beam. However in Electrochemical Machining there is a physical tool that is very much required for machining 3. In NTM, the tool need not be harder than the work piece material. For example, in EDM, copper is used as the tool material to machine hardened steels. 4. Mostly NTM processes do not necessarily use mechanical energy to provide material removal. They use different energy domains to provide machining. For example, in USM, AJM, WJM mechanical energy is used to machine material, whereas in ECM electrochemical dissolution constitutes material removal.

Ultrasonic Machining

Ultrasonic machining is a subtraction manufacturing process that removes material from the surface of a part through high frequency, low amplitude vibrations of a tool against the material surface in the presence of fine abrasive particles. The tool travels vertically or orthogonal to the surface of the part at amplitudes of 0.05 to 0.125 mm (0.002 to 0.005 in.). The fine abrasive grains are mixed with water to form a slurry that is distributed across the part and the tip of the tool. Ultrasonic machining is typically used on brittle materials as well as materials with a high hardness due to the micro cracking mechanics.

Principle & Process An ultrasonic mill consists of two major components, a transducer and a sonotrode. An electronic oscillator in the control unit produces an alternating current oscillating at a high frequency, usually between 18 and 40 kHz in the ultrasonic range. The transducer usually consists of a cylinder made of piezoelectric ceramic. The oscillating voltage is applied to electrodes attached to the transducer, which converts the electrical energy into mechanical vibrations. The transducer then vibrates the sonotrode at low amplitudes and high frequencies.  The sonotrode is usually made of low carbon steel. A constant stream of abrasive slurry flows between the sonotrode and work piece. This flow of slurry allows debris to flow away from the cutting area. The slurry usually consists of water (20 to 60% by volume) and boron carbide, aluminum oxide and silicon carbide particles. The sonotrode removes material from the work piece by abrasion where it contacts it, so the result of machining is to cut a perfect negative of the sonotrode profile into the work piece.

Advantages Disadvantages Extremely hard & brittle material can easily be machined. Highly accurate profile and good surface finish. The machined workpiece are free of stress. Metal removal cost is low. The operation is noiseless. Operation of equipment is quite safe. Disadvantages The metal removal rate is low. The initial equipment cost is higher for setup. This process does not suit heavy metal removal. The cost of tooling is also high. Power consumption is quite high. Slurry may need to be replaced periodically.

Application of USM 1. Tool and die making, specially wire drawing and extrusion dies. 2. Several machining operations like turning, threading, grinding, trepanning, milling etc. 3. Machining of hard to machine and brittle surface. 4. Producing holes of round or such other shapes for cutting tool. 5. Dentistry work – to drill fine holes of desired shape in teeth.

Electrochemical Machining

Electrochemical machining is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional methods. A shaped tool or electrode is used in this process, which forms cathode. The workpiece forms anode. A small gap is maintained between the tool and the workpiece and an electrolyte is pumped through it. Low voltage direct current is employed, which in presence of electrolyte, enables a controlled removal of metal from the workpiece by anodic dissolution. Most widely used electrolyte in this process is sodium nitrate solution. Sodium chloride solution in water is a god alternative but is more corrosive than the former. Practically all metals can be machined by this method. However, its application suits best only to mass production work because of excessively high tooling and equipment cost. Normally a dimensional tolerance of 0.125mm can be expected through this process.

Principle The principle of ECM process is based on faraday's law of electrolysis. The tool and workpiece are held close to each other with a very small gap (say 0.5mm) between them. A dc voltage of 3 to 30V is applied between the two and an electrolyte is continuously pumped. Due to the applied voltage, the current flows through the electrolyte with +vely charged ions being attracted towards the tool(cathode) and negatively charged ones towards workpiece(anode). The electrochemical reaction, taking place due to this flow of ion, results in the removal of metal from the workpiece in the form of sludge. This sludge is taken away from the gap by the flowing electrolyte with it. The area where the tool and workpiece are closer experiences flow of higher current due to low resistance.

Advantages Disadvantages Any good electrically conducting material can be machined. Intricate & Complex shape can be machined easily. Metal removal rate is quite high. Wear on tool is insignificant or non existent. No cutting forces are involved. High surface finish of the order of 0.1 to 2.0 microns can be obtained. Disadvantages Non conducting materials can not be machined. Power consumption is very high. Corrosion & rusting of workpiece is frequent. Designing and fabrication of tool is difficult. Large floor space is required. Extremely fine corner radii, say less than 0.2 mm, can not be produced.

Application of ECM 1. Machining of hard to machine and heat resistant material. 2. Machining of complicated profile, such as of jet engine blades, turbine blades, turbine wheels etc. 3. Drilling small deep holes, such as in nozzles. 4. Machining of cavities and holes of irregular shape. 5. Deburring of parts.

Thank You……