Chapter 28: Nontraditional Manufacturing Processes DeGarmo’s Materials and Processes in Manufacturing

28.1 Introduction Non-traditional machining (NTM) processes have several advantages Complex geometries are possible Extreme surface finish Tight tolerances Delicate components Little or no burring or residual stresses Brittle materials with high hardness can be machined Microelectronic or integrated circuits are possible to mass produce

NTM Processes Four basic groups of material removal using NTM processes Chemical Electrochemical Thermal Mechanical

Disadvantages of Machining Processes Machining processes that involve chip formation have a number of limitations Large amounts of energy Unwanted distortion Residual stresses Burrs Delicate or complex geometries may be difficult or impossible

Conventional End Milling vs. NTM Typical machining parameters Feed rate Surface finish Dimensional accuracy Workpiece/feature size NTM processes typically have lower feed rates and require more power consumption The feed rate in NTM is independent of the material being processed

28.2 Chemical Machining Processes Typically involves metals, but ceramics and glasses may be etched Material is removed from a workpiece by selectively exposing it to a chemical reagant or etchant Gel milling- gel is applied to the workpiece Maskant- selected areas are covered and the remaining surfaces are exposed to the etchant

Masking Several different methods Cut-and-peel Scribe-and-peel Screen printing Etch rates are slow in comparison to other NTM processes Figure 28-1 Steps required to produce a stepped contour by chemical machining.

Defects in Etching If baths are not agitated properly, defects result Figure 28-2 Typical chemical milling defects: (a) overhang: deep cuts with improper agitation; (b) islands: isolated high spots from dirt, residual maskant, or work material inhomogeneity; (c) dishing: thinning in center due to improper agitation or stacking of parts in tank. If baths are not agitated properly, defects result

Advantages and Disadvantages of Chemical Machining Process is relatively simple Does not require highly skilled labor Induces no stress or cold working in the metal Can be applied to almost any metal Large areas Virtually unlimited shape Thin sections Disadvantages Requires the handling of dangerous chemicals Disposal of potentially harmful byproducts Metal removal rate is slow

Photochemical Machining Figure 28-4 Basic steps in photochemical machining (PCM).

Design Factors in Chemical Machining If artwork is used, dimensional variations can occur through size changes in the artwork of phototool film due to temperature and humidity changes Etch factor (E)- describes the undercutting of the maskant Areas that are exposed longer will have more metal removed from them E=U/d d- depth U- undercutting Anisotropy (A)- directionality of the cut, A=d/U

Etch Rates

28.3 Electrochemical Machining Process Electrochemical machining (ECM) removes material by anodic dissolution with a rapidly flowing electrolyte The tool is the cathode and the workpiece is the electrolyte Figure 28-6 Schematic diagram of electrochemical machining process (ECM).

Electrochemical Processing Pulsed-current ECM (PECM) Pulsed on and off for durations of approximately 1ms Pulsed currents are also used in electrochemical machining (EMM) Electrochemical polishing is a modification of the ECM process Much slower penetration rate

Other Electrochemical Processing Electrochemical hole machining Used to drill small holes with high aspect ratios Electrostream drilling High velocity stream of charged acidic, electrolyte Shaped-tube elecrolytic machining (STEM) Capable of drilling small holes in difficult to machine materials Electrochemical grinding (ECG) Low voltage, high-current variant of ECM

Figure 28-8 The shaped-tube electrolytic machining (STEM) cell process is a specialized ECM technique for drilling small holes using a metal tube electrode or metal tube electrode with dielectric coating.

Figure 28-9 Equipment setup and electrical circuit for electrochemical grinding.

Other Electrochemical Processes Electrochemical deburring Electrolysis is accelerated in areas with small interelectrode gaps and prevented in areas with insulation between electrodes Design factors in electrochemical machining Current densities tend to concentrate at sharp edges or features Control of electrolyte flow can be difficult Parts may have lower fatigue resistance

Advantages and Disadvantages of Electrochemical Machining ECM is well suited for the machining of complex two-dimensional shapes Delicate parts may be made Difficult-to machine geometries Poorly machinable materials may be processed Little or no tool wear Disadvantages Initial tooling can be timely and costly Environmentally harmful by-products

28.4 Electrical Discharge Machining Electrical discharge machining (EDM) removes metal by discharging electric current from a pulsating DC power supply across a thin interelectrode gap The gap is filled by a dielectric fluid, which becomes locally ionized Two different types of EDM exist based on the shape of the tool electrode Ram EDM/ sinker EDM Wire EDM

Figure 28-10 EDM or spark erosion machining of metal, using high-frequency spark discharges in a dielectric, between the shaped tool (cathode) and the work (anode). The table can make X-Y movements.

EDM Processes Slow compared to conventional machining Produce a matte surface Complex geometries are possible Often used in tool and die making Figure 28-11 Schematic diagram of equipment for wire EDM using a moving wire electrode.

EDM Processes Figure 28-12 (left) Examples of wire EDM workpieces made on NC machine (Hatachi). Figure 28-13 (above) SEM micrograph of EDM surface (right) on top of a ground surface in steel. The spherical nature of debris on the surface is in evidence around the craters (300 x).

Figure 28-14 The principles of metal removal for EDM.

Considerations for EDM Graphite is the most widely used tool electrode The choice of electrode material depends on its machinability and coast as well as the desired MRR, surface finish, and tool wear The dielectric fluid has four main functions Electrical insulation Spark conductor Flushing medium Coolant

Advantages and Disadvantages of EDM Applicable to all materials that are fairly good electrical conductors Hardness, toughness, or brittleness of the material imposes no limitations Fragile and delicate parts Disadvantages Produces a hard recast surface Surface may contain fine cracks caused by thermal stress Fumes can be toxic

Electron and Ion Machining Electron beam machining (EBM) is a thermal process that uses a beam of high-energy electrons focused on the workpiece to melt and vaporize a metal Ion beam machining (IBM) is a nano-scale machining technology used in the microelectronics industry to cleave defective wafers for characterization and failure analysis Figure 28-15 Electron-beam machining uses a high-energy electron beam (109 W/in.2)

Laser-Beam Machining Laser-beam machining (LBM) uses an intensely focused coherent stream of light to vaporize or chemically ablate materials Figure 28-16 Schematic diagram of a laser-beam machine, a thermal NTM process that can micromachine any material.

Plasma Arc Cutting (PAC) Uses a superheated stream of electrically ionized gas to melt and remove material The process can be used on almost any conductive material PAC can be used on exotic materials at high rates Figure 28-18 Plasma arc machining or cutting.

Thermal Deburring Used to remove burrs and fins by exposing the workpiece to hot corrosive gases for a short period of time Thermal deburring can remove burrs or fins from almost any material but is especially effective with materials of low thermal conductivity Figure 28-20 Thermochemical machining process for the removal of burrs and fins.