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

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 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).

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

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.