Ultrasonic-Machining ProcessFIGURE 9.19 (a) Schematic illustration of the ultrasonic-machining process by whichmaterial is process by which material is removed through microchipping and erosion.(b) and (c) typical examples of holes produced by ultrasonic machining. Note thedimensions of cut and the types of workpiece materials.
Ultrasonic waves: Magnetostrictive transducersMagnetostrictive transducers use theinverse magnetostrictive effect toconvert magnetic energy into ultrasonicenergy.This is accomplished by applying astrong alternating magnetic field tocertain metals, alloys and ferrites. Ultrasonic waves: Piezoelectric Transducers Piezoelectric transducers employ the inverse piezoelectric effect using natural or synthetic single crystals (such as quartz) or ceramics (such as barium titanate) which have strong piezoelectric behavior. Ceramics have the advantage over crystals in that they are easier to shape by casting, pressing and extruding.
Principle of Ultrasonic MachiningIn the process of Ultrasonic Machining,material is removed by micro-chippingor erosion with abrasive particles.In USM process, the tool, made of softermaterial than that of the workpiece, isoscillated by the Booster andSonotrode at a frequency of about 20kHz with an amplitude of about 25.4 um(0.001 in).The tool forces the abrasive grits, in thegap between the tool and theworkpiece, to impact normally andsuccessively on the work surface,thereby machining the work surface.
Principle of Ultrasonic Machiningstandard mechanism used in most of the universal Ultrasonic machines - 1
Principle of Ultrasonic Machining During one strike, the tool moves down from its most upper remote position with a starting speed at zero, then it speeds up to finally reach the maximum speed at the mean position. Then the tool slows down its speed and eventually reaches zero again at the lowest position. When the grit size is close to the mean position, the tool hits the grit with its full speed. The smaller the grit size, the lesser the momentum it receives from the tool. Therefore, there is an effective speed zone for the tool and, correspondingly there is an effective size range for the grits.
Principle of Ultrasonic MachiningIn the machining process, the tool, at some point, impacts onthe largest grits, which are forced into the tool and workpiece.As the tool continues to move downwards, the force acting onthese grits increases rapidly, therefore some of the grits maybe fractured.As the tool moves further down, more grits with smaller sizescome in contact with the tool, the force acting on each gritbecomes less.Eventually, the tool comes to the end of its strike, the numberof grits under impact force from both the tool and theworkpiece becomes maximum.Grits with size larger than the minimum gap will penetrate intothe tool and work surface to different extents according to theirdiameters and the hardness of both surfaces.
MechanismPiezoelectric TransducerPiezoelectric transducers utilizecrystals like quartz whose dimensionsalter when being subjected toelectrostatic fields.The charge is directionally proportionalto the applied voltage.To obtain high amplitude vibrations thelength of the crystal must be matchedto the frequency of the generator whichproduces resonant conditions.
MechanismMagnetostictive transducerMagnetostictive transducers work onthe principle that if a piece of Ferro-magnetic material (like nickel) ismagnetized, then a change indimension occurs.The transducer has solenoid typewinding of wire over a stack of nickellaminations (which has rapiddimensional change when placed inmagnetic fields) and is fed with an A.Csupply with frequencies up to 25,000c/s.
MechanismAbrasive SlurryThe abrasive slurry contains fineabrasive grains. The grains are usuallyboron carbide, aluminum oxide, orsilicon carbide ranging in grain sizefrom 100 for roughing to 1000 forfinishing.It is used to microchip or erode thework piece surface and it is also used tocarry debris away from the cutting area.
Mechanism Tool holder The shape of the tool holder is cylindrical or conical, or a modified cone which helps in magnifying the tool tip vibrations. In order to reduce the fatigue failures, it should be free from nicks, scratches and tool marks and polished smooth.
MechanismToolTool material should be tough andductile. Low carbon steels and stainlesssteels give good performance.Tools are usually 25 mm long ; its sizeis equal to the hole size minus twice thesize of abrasives.Mass of tool should be minimumpossible so that it does not absorb theultrasonic energy.
Materials that can be machined on USMHard materials like stainless steel,glass, ceramics, carbide, quatz andsemi-conductors are machined by thisprocess.It has been efficiently applied tomachine glass, ceramics, precisionminerals stones, tungsten.Brittle materialsApplications It is mainly used for (1) drilling (2) grinding, (3) Profiling (4) coining (5) piercing of dies (6) welding operations on all materials which can be treated suitably by abrasives.
Advantages of USM Disadvantages of USMMachining any materials regardless oftheir conductivity USM has low material removal rate.USM apply to machining semi-conductor such as silicon, germanium Tool wears fast in USM.etc. Machining area and depth isUSM is suitable to precise machining restraint in USM.brittle material.USM does not produce electric, thermal,chemical abnormal surface.Can drill circular or non-circular holesin very hard materialsLess stress because of its non-thermalcharacteristics