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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 ProcessesNTM ProcessesFour 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 Limitations of Conventional Machining ProcessesMachining 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. NTMConventional End Milling vs. NTMTypical machining parameters
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 ProcessesTable 19-1 Summary of NTM Processes
19.2 Chemical Machining 19.2 Chemical Machining ProcessesProcessesTypically involves metals, but ceramics
and glasses may be etchedMaterial 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.
MaskingMasking
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 EtchingDefects 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 Advantages and Disadvantages of Chemical Machiningof Chemical MachiningAdvantages
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 ProcessesEDM 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 ProcessesEDM 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 Effect of Current on-time and Discharge Current on Crater SizeDischarge 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 Effect of Current on-time and Discharge Current on Crater SizeDischarge 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 EDMConsiderations for EDMGraphite is the most widely used tool
electrodeThe choice of electrode material depends
on its machinability and coast as well as the desired MRR, surface finish, and tool wear
Four main functions of dielectric fluid:1) Electrical insulation2) Spark conductor3) Flushing medium4) Coolant
Advantages and Disadvantages Advantages and Disadvantages of EDMof EDM
AdvantagesApplicable to all
materials that are fairly good electrical conductors
Hardness, toughness, or brittleness of the material imposes no limitations
Fragile and delicate parts
DisadvantagesProduces a hard
recast surfaceSurface may
contain fine cracks caused by thermal stress
Fumes can be toxic
Electron and Ion MachiningElectron 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 MachiningLaser-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)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.
HW for Chapter 19HW for Chapter 19Review Questions:17, 19, 20 (page 521)