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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.
Advantages and Disadvantages of Chemical Machining Advantages
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
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
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.
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 Advantages
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 28-13 (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 28-12 (left) Examples of wire EDM workpieces made on NC machine (Hatachi).
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
Advantages 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.
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