Ultrasonic machining (USM)

Non-traditional machiningUltrasonic machining (USM)Ultrasonic machiningNon traditional means of uniform stock removalIs also known as ultrasonic collision grinding or impact grindingDiffers from most other machining operations since very little amount of heat is producedHas been around since the 50s. Primarily used as surface finishing process for parts made by electro-discharge machiningUltrasonic machiningCan cut any material. Most effective on very hard and brittle materials, where it offers better surface finish, a lower degree of surface degradation and greater complexity of geometrySome examples of such materials are glass, quartz,sapphire,ferrite,aluminium oxide,silicon,silicon carbide,silicon nitride,ruby, diamond,fibre opticsandceramics

Working principleUltrasonic machining is the elimination of material by the abrading action of micro stones-loaded liquid slurry available between the work piece and a tool vibrating at a 90 degree angle to the work surface at a frequency more than audible range (ultrasound).The cutting tool oscillates at the frequency range of 20kHz to 40kHzThe vibration amplitude of the workpiece ranges from 15 20 microns

Working principleAbrasive grains are actuated by the high speed reciprocate motions across the small gap, in between the tool and the work pieceThere is no contact between the tool and the workpiece. As a result, the grinding pressure is rarely more than 2 pounds, which is perfect for machining hard and brittle materialsThe impact of abrasive particles is the energy source which is mainly responsible in removal of the material, through the form of small wear particles which are carried away by the abrasive slurry

Abrasive slurryProvides constant source of abrasive particlesCarries away machining debris and fractured particlesCirculated by a refrigerated pump systemRemoves heat from the cutting process, preventing boiling between the tool and workpieceCutting action comes from accelerated particles being repeatedly forced against the workpiece surfaceAbrasive slurryThe abrasive grit in the slurry medium is typically of low cost and easily available. Examples:Aluminium oxide or silicon carbide is used for glassBoron carbide is used for die steel and gems Boron silicarbide Diamond

Abrasive slurryTypical grit size for the abrasive slurry range from 100 to 800Selection of abrasive particles depends on:Type of material being machinedHardness of materialDesired removal rate of materialSurface finish neededLarger grit rougher cutSmaller grit finer finished surfaceAbrasive slurryWater is typically used for ultrasonic machiningOther fluid mediums are:BenzeneGlycerol OilsConcentration of abrasive grains or grit in water slurry ranges from 20% - 60% by volumeBest average results are achieved around 30% concentrationMaterial removal rate (MRR)MRR = 5.9 f (s/H) 0.5R 0.5yWheref = frequency of oscillations, HzH = surface fracture strength, BHNs = static stress in tool, kg/mm^2R = mean radius of grit, mmy = amplitude of vibration, mmMaterial removal rateSome examples of different rate per material:

Work materialRelative MRRGlass100.0Brass6.6Tungsten4.8Titanium4.0Steel3.9Chrome steel1.4Ultrasonic machine partsUltrasonic machining consists of :High power sine wave generatorMagnetostriction transducerTool holderTool

Magnetostriction transducerGenerates vibrationWorks as an ultrasonic generator converting electrical energy into mechanical energy

ToolThe tool is made of an easily shaped, softer material than the workpieceCommonly used tool materials are nickel, mild steel, carbide, tool steel or brassThe tool suffers very little deformation during the processTool material is subject to wear, therefore it is compulsory that proper care is given to the selection of work-to-tool combinations

ToolTool wear varies depending on the tool materialWear ratios are in the range of 1:1 to 100:1 (material removed vs. tool wear)Ductile tool materials allow tool surface hardening in some processes, thus increasing wear ratiosAt the same time it provides ease of manufacture of tools by traditional processes and thus create cost reduction in the creation of tools making the entire process more affordable

AdvantagesWorkpiece is free from burns and distortionsFree from stress and damagesThe process is non-thermal, non-chemical, and non-electrical, leaving the chemical and physical properties of the workpiece unchangedMultiple features can be machined on the workpiece simultaneously, and the process is scalableGood surface finish and structural integritySuitable for machining brittle materials

Disadvantages Very poor material removal rateUnable to make deep holesRelatively high tool wearApplications Machining auto-engine componentsMachining hard and brittle alloys, semiconductors, glass, fiber material, ceramics, carbides, etcDrilling very fine holes in helicopter power transmission shafts and gearsReferenceshttp://www.mechscience.com/4270-ultrasonic-machining-process-usm-concept-of-ultrasonic-machining-process-usm/http://www.eng.morgan.edu/~mahmud/IEGR563/ultra.html