Chapter 18 Powder Metallurgy EIN 3390 Manufacturing Processes Spring, 2012.
Ein 3390 chap 19 nontraditional machining fall 2011
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Transcript of Ein 3390 chap 19 nontraditional machining fall 2011
Chapter 19
Electronic Electrochemical Chemical
and Thermal Machining Processes
EIN 3390 Manufacturing ProcessesFall, 2011
19.1 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 (IC) are possible to mass produce
NTM Processes Four basic groups of material removal using NTM
processes
◦ Chemical:
Chemical reaction between a liquid reagent and workpiece results in etching
◦ Electrochemical
An electrolytic reaction at workpiece surface for removal of material
◦ Thermal
High temperature in very localized regions evaporate materials, for example, EDM
◦ Mechanical
High-velocity abrasives or liquids remove materials
Limitations of Conventional 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 (5 – 200 in./min.)
◦ Surface finish (60 – 150 min) AA – Arithmetic Average
◦ Dimensional accuracy (0.001 – 0.002 in.)
◦ Workpiece/feature size (25 x 24 in.); 1 in. deep
NTM processes typically have lower feed rates and require more power consumption
The feed rate in NTM is independent of the material being processed
Table 19-1 Summary of NTM Processes
19.2 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 reagent or etchant◦ Gel milling- gel is applied to the workpiece in gel form.
◦ Maskant- selected areas are covered and the remaining surfaces are exposed to the etchant. This is the most common method of CHM.
Masking
Several different methods◦ Cut-and-peel
◦ Scribe-and-peel
◦ Screen printing
Etch rates are slow in comparison to other NTM processes
Figure 19-1 Steps required to produce a stepped contour
by chemical machining.
Defects in Etching
If baths are not agitated properly, defects result
Figure 19-2 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 rateis 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, where d- depth, U- undercutting
Anisotropy (A)- directionality of the cut, A=d/U, and Wf = Wm + (E d), or
Wm = Wf - (E d) where Wf is final desired width of cut
19.3 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 19-17 Schematic diagram of
electrochemical machining process
(ECM).
19.3 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 19-17 Schematic diagram of
electrochemical machining process
(ECM).
Table 19-3 Material Removal Rates for ECM Alloys Assuming 100% Current Efficiency
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 19-19 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 19-20 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
Table 19-4 Metal Removal Rates for ECG for Various Metals (Electrochemical Grinding – ECG)
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
19.4 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 19-21 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.
Figure 19-21 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 19-22 Schematic diagram of equipment
for wire EDM using a moving wire electrode.
EDM Processes
Figure 19-24 (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 19-23 (left) Examples of wire EDM
workpieces made on NC machine (Hatachi).
Effect of Current on-time and Discharge Current on Crater SizeMRR = (C I)/(Tm
1.23),Where MRR – material removal rate in in.3/min.; C –constant of proportionality equal to 5.08 in US customary units; I – discharge current in amps; Tm – melting temperature of workpiece material, 0F.
Example:
A certain alloy whose melting point = 2,000 0F is to be machined in EDM. If a discharge current = 25A, what is the expected metal removal rate?
MRR = (C I)/(Tm1.23) = (5.08 x 25)/(2,0001.23)
= 0.011 in.3/min.
Figure 19-25 The principles of
metal removal for EDM.
Effect of Current on-time and Discharge Current on Crater Size
From Fig 19 – 25: we have the conclusions:◦ Generally higher duty cycles with higher currents and lower frequencies are used to maximize MRR.◦ Higher frequencies and lower discharge currents are used to improve surface finish while reducing MRR.◦ Higher frequencies generally cause increased tool wear.
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
Table 19-5 Melting Temperatures for Selected EDM Workpiece Materials
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 19-26 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 19-27 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 19-29 Plasma arc machining or cutting.
Thermal Deburring Used to remove
burrs and fins by exposing the workpiece to hot corrosive gasesfor 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 19-31 Thermochemical machining
process for the removal of burrs and fins.
HW for Chapter 19
Review Questions:7, 17(page 521)