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Chapter 26 Advanced machining processes and Nanofabrication

Nekonvencionalni postupci

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  • Chapter 26Advanced machining processes and Nanofabrication

  • IntroductionChemical machiningElectro chemical machiningElectrical discharge machiningWire EDMLaser beam machiningElectron-beam machining and plasma-arc cuttingWater-jet machiningAbrasive-jet machiningNanofabricationMicro machiningEconomics of advanced machining process

  • Situations where processes are not satisfactory, economical or even possible

    High hardness and the strength of the material Work-piece too flexibleComplex shapeSurface finish and dimensional tolerancesUndesirable Temperature rise and dimensional tolerances

  • Examples of parts made by advanced Machining ProcessesFig : Examples of parts made by advanced machining processes. These parts are made by advanced machining processes and would be difficult or uneconomical to manufacture by conventional processes. (a) Cutting sheet metal with a laser beam.(b) Microscopic gear with a diameter on the order of 100m, made by a special etching process.

  • Chemical machiningChemical attacks metals and etch them by removing small amounts of material from the surface using reagents or etchantsFig : (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to weight ratio of the part. (b) Weight reduction of space launch vehicles by chemical milling aluminum-alloy plates. These panels are chemically milled after the plates have first been formed into shape by processes such as roll forming or stretch forming. The design of the chemically machined rib patterns can be modified readily at minimal cost.

  • Chemical milling:Shallow cavities produced on plates, sheets, forgings, and extrusions

    Procedure for chemical milling Steps :

    1 Residual stresses should relieved in order to prevent warping 2 Surfaces to be thoroughly degreased and cleaned3 - Masking material(tapes,paints,elastomers & plastics ) is applied 4 masking is peeled off by scribe and peel technique5 The exposed surfaces are etched with etchants6 After machining the parts to be thoroughly washed to prevent further reactions with residue etchant7 rest of the masking material is removed and the part is cleaned and inspected8 additional finishing operations are performed on chemically milled parts 9 this sequence is repeated to produce stepped cavities and various contours

  • Process capabilities:Chemical milling used in the aerospace industry Tank capacities for reagents are as large as 3.7m x15m Process also used for micro electronic devicesSurface damage may result due to preferential etching and intergranular attack

    Chemical blanking:Chemical blanking is similar to chemical millingMaterial is removed by chemical dissolution rather than by shearing Burr free etching of printed-circuit boards, decorative panels, thin sheet metal stampings as well as production of small and complex shapes

  • Chemical MachiningFig : (a) Schematic illustration of the chemical machining process. Note that no forces or machine tools are involved in this process. (b) Stages in producing a profiled cavity by machining; not the undercut.

  • Photochemical blanking :Modification of chemical millingMaterial removed from flat thin sheet by photographic techniques

  • Steps for photochemical blankingDesign is prepared at a magnification of 100x

    Photographic negative is reduced to the size of finished part

    Sheet blank is coated with photosensitive material (Emulsion)

    Negative placed over coated blank and exposed to ultra violet light which hardens the exposed area

    Blank is developed which dissolves the exposed areas

    Blank is then immersed into a bath of reagent or sprayed with the reagent which etches away the exposed areas

    The masking material is removed and the part is washed thoroughly to remove all chemical residues

  • Design considerationsDesigns involving sharp corners,deep & narrow cavities, severe tapers or porous work piece should be avoided

    Undercuts may be developed because etchant attacks both in horizontal and vertical direction

    To improve production rate the bulk of the work piece should be shaped by other machining process priorly

    Dimensional variations can occur ,this can be minimized by proper selection of artwork media by controlling the environment

    Many product designs are now made with computer aided design

  • Electro Chemical MachiningFig : Schematic illustration of the electrochemical-machining process. This process is the reverse of electroplating.

  • Electrochemical machiningThis process is reversal of the electro plating

    Electrolyte acts as current carrier

    High rate of electrolyte movement in tool work piece gap washes metal ions away from the work piece ( ANODE)

    This is washed just before they have a chance to plate on the tool ( cathode)

    Shaped tool made of brass , copper , bronze , or stainless steel

    Electrolyte is pumped at a high rate through the passages in the tool

    Machines having current capacities as high as 40,000 A and as low as 5A are available

  • Parts made by Electrochemical MachiningFig : Typical parts made by electrochemical machining. (a) Turbine blade made of a nickel alloy, 360 HB; note the shape of the electrode on the right. (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk.

  • Process capabilities Used to machine complex cavities in high strength materialApplications in aerospace industry,jet engines parts and nozzlesECM process gives a burr free surfaceNo thermal damageLack of tool forces prevents distortion of the partNo tool wear Capable of producing complex shapes and hard materials

  • Biomedical ImplantFig : (a) Two total knee replacement systems showing metal implants (top pieces) with an ultrahigh molecular weight polyethylene insert (bottom pieces) (b) Cross-section of the ECM process as applied to the metal implant.

  • Design considerations for Electrochemical MachiningElectrolyte erodes sharp surfaces and profiles so not suited for sharp edgesIrregular cavities may not be produced to the desired shape with acceptable dimensional accuracyDesigns should make provisions for small taper for holes and cavities to be machined

    Pulsed electro chemical machining(PECM)Refinement of ECMUses pulsed rather than directImproves fatigue life, eliminates recast layer left on die and mold surfaces by electrical discharge machiningVery high current densities, but the current is

  • Electrochemical GrindingCombines electrochemical machining with conventional grindingFig : Schematic illustration of the electrochemical grinding process. (b) Thin slot produced on a round nickel alloy tube by this process.

  • Electrical-Discharge MachiningFig : (a) Schematic illustration of the electrical-discharge machining process. This is one of the most widely used machining processes, particularly for die-sinking operations. (b) Examples of cavities produced by the electrical-discharge machining process, using shaped electrodes. Two round parts (rear) are the set of dies for extruding the aluminum the aluminum piece shown in front. (c) A spiral cavity produced by ECM using a slowly rotating electrode, similar to a screw thread.(a)(b) (c)

  • Examples of EDMFig : Stepped cavities produced with a square electrode by the EDM process. The workpiece moves in the two principal horizontal directions (x-y), and its motion is synchronized with the downward movement of the electrode to produce these cavities. Also shown is a round electrode capable of producing round or elliptical cavities.Fig : Schematic illustration of producing an inner cavity by EDM, using a specially designed electrode with a hinged tip, which is slowly opened and rotated to produce the large cavity.

  • WIRE EDMFig : (a) Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then discarded. (b) Cutting a thick plate with wire EDM. (c) A computer-controlled wire EDM machine.

  • Laser Beam MachiningFig : (a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM.

  • Laser-Beam MachiningFig : Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, so workpiece size is limited to the size is limited to the size of the vacuum chamber.

  • Water Jet MachiningFig : (a) Schematic illustration of water-jet machining. (b) A computer-controlled, water-jet cutting machine cutting a granite plate. (c) Example of various nonmetallic parts produced by the water-jet cutting process.

  • Abrasive Jet MachiningFig : Schematic illustration of Abrasive Jet Machining

  • NanofabricationFig : (a) A scanning electron microscope view of a diamond-tipped (triangular piece at the right) cantilever used with the atomic force microscope. The diamond tip is attached to the end of the cantilever with an adhesive. (b) Scratches produced on a surface by the diamond tip under different forces. Note the extremely small size of the scratches.