29. Electrical Contact Machining

29.1 Electric Discharge Machining (EDM)

• Physical Principle,

1. charge up an electrode

2. bring the electrode near a metal workpiece (oppositely charged).

3. as the two conductors get close enough a spark will arc across a dielectric fluid. This spark will “burn” a small hole in the electrode and workpiece.

4. continue steps 1-3 until a hole the shape of the electrode is formed.

• The process is based on melting temperature, not hardness, so some very hard materials can be machined this way.

• The arc that jumps heats the metal, and about 1 to 10% of the molten metal goes into the fluid. The melted then recast layer is about 1 to 30 micro m thick, and is generally hard and rough.

• typical electrode materials are,

copper,

tungsten

graphite

• The user can select the following parameters

Electrode material

Electrode polarity +/-

pulse current If (A)

pulse duration ti (micro s)

pulse off time to (micro s)

average voltage U (V)

Average current I (A)

working current density Id (A/cm2)

open gap voltage Vo (V)

Dielectric

flushing mode

• These in turn effect,

metal removal rate Vw (mm3/min)

relative electrode wear theta (% or a fraction)

surface finish R (peak to valley micro m)

thickness of recast layer

gap between electrode and workpiece

corner and edge radii

• Fluid

fluid is used to act as a dielectric, and to help carry away debris.

if the fluid is pumped through and out the end of the electrode, particles will push out, and mainly collect at the edges. They will lower the dielectric resistance, resulting in more arcs. As a result the holes will be conical.

if fluid is vacuum pumped into the electrode tip, straight holes will result.

quite often kerosene-based oil.

• The electrode workpiece gap is in the range of <10 micro m to <100 micro m.

• Uses a voltage discharge of 60 to 300 V to give a transient arc lasting from 0.1 micro s to 8 ms.

• Typical cycle time is 20 ms or less, up to millions of cycles may be required for completion of the part.

• Electrode materials are high temperature, but easy to machine, thus allowing easy manufacture of complex shapes.

• When the energy density is higher (machining faster), the results are,

energy density (lower to higher)

amount machined (less to more)

machining speed (slower to faster)

clearance (less to more)

surface roughness (fine to rough)

• Keep in mind the power is given by P=V I t

• Basic process,

 

 

 

 

• Rotating the wire in an orbital direction will,

increase accuracy in form and surface finish

decrease electrode wear

• Typical machine parameters are,

 

29.1.1 Wire EDM

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• A thin wire of brass, tungsten, or copper is used as an electrode.

• Deionized water is used as the dielectric.

• The process is similar to standard EDM,

 

 

• Slowly cuts groove in shape of wire.

• Wire is consumed and is slowly fed.

• This process is much faster than electrode EDM.

• Machine speed is,

 

• Higher currents, and lower rest times increase the speed of this process.

• Relations between groove width and speed are shown in the graph below.

 

• This process is well suited to production of dies for plastic molding, progressive dies, etc.

• Summary of EDM characteristics,

mechanics of material removal: melting and evaporation aided by cavitation

medium: dielectric fluid

tool materials: Cu, Brass, Cu-W alloy, Ag-W alloy, graphite

material/tool wear = 0.1 to 10

gap = 10 to 125 micro m

maximum mrr = 5*103 mm3/min

specific power consumption 1.8 W/mm3/min

critical parameters: voltage, capacitance, spark gap, dielectric circulation, melting temperature

materials application: all conducting metals and alloys

shape application: blind complex cavities, microholes for nozzles, through cutting of non-circular holes, narrow slots

limitations: high specific energy consumption (about 50 times that in conventional machining); when forced circulation of dielectric is not possible, removal rate is quite low; surface tends to be rough for larger removal rates; not applicable to nonconducting materials

29.1.2 Problems

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Problem 29.1 We try an EDM process where the copper tool has a mass of 200g before beginning and 180g after. The iron workpiece drops from 3.125kg to 3.096kg, but has rounded corners.

a) What is the tool wear factor?

b) If the tool was cylindrical to begin with, draw sketches of the electrode before and after.

Problem 29.2 What are the selection criteria for choosing between machining and EDM?

Answer 29.2 EDM is particularly useful when dealing with internal cuts that are hard to get tools into. Machining tends to work best with external cuts. EDM is suitable for removal of smaller amounts of material at a much slower rate.

29.1.3 References

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29.1 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

29.2 Lascoe, O.D., Handbook of Fabrication Processes, American Society of Materials, Metals Park, Ohio, 1988.

29.2 Electrochemical Machining (ECM)

• The physics: an electrode and workpiece (conductor) are placed in an electrolyte, and a potential voltage is applied. On the anode (+ve) side the metal molecules ionize (lose electrons) break free of the workpiece, and travel through the electrolyte to the electrode (a cathode; has a -ve charge; a surplus of electrons).

• NOTE: in EDM an arc was used to heat metal, here the metal dissolves chemically.

 

• Variation in the current density will result in work taking the electrodes shape.

• The electrode is fed with a constant velocity, and the electrolyte is fed through the tool. The tool is designed to eliminate deposition of the ionized metal on the electrode.

 

• Supply V = 8 to 20V, I = >1000A.

• Electrode gap is typically 0.1 to 0.2 mm.

• mrr is about 1600mm3/min. per 1000A, OR 3KWhr for 16000 mm3 (not very efficient, 30 times more than standard machining techniques).

• mrr is independent of material hardness.

• Good for low machinability, or complicated shapes.

• Very little tool wear,

• Forces are large with this method because of fluid pumping forces.

• Faraday’s laws state that,

 

• The basic principle is shown below

 

• The chemical reaction between an electrode and the electrolyte leads to electrons being added, or removed from the electrode metal. This addition/subtraction leads to a voltage potential.

 

• To make a battery.

 

 

• To do electrolysis.

 

• The mrr is,

 

• e.g.

 

• Actual rates may vary from theory as other factors come into effect.

 

• The table below shows various materials and relevant properties,

 

• e.g.

 

• While the current required is related to the metal removed, the voltage required depends upon,

electrode potential.

the current flow in and about the electrodes will disturb the normal distribution of voltage. Extra potential is required to overcome the effects.

Ion collect near electrodes and impede ion transfer from the electrode to the electrolyte, also adding a potential.

Some solid film forms on the surface of the electrode, also increasing resistance.

electrolyte resistance,

 

 

• The feed of the electrodes has the following effects

 

 

 

• The ECM process will erode material in a radial direction, so care must be made in tooling design.

 

• As current flows through the electrolyte, it is heated, and conductivity decreases.

• Surface finish is affected by,

selective dissolution

sporadic breakdown of the anodic film

flow separation and formation of eddies

evolution of hydrogen

• Typical electrolytes are,

 

• Summary of ECM characteristics,

mechanics of material removal: electrolysis

medium: conducting electrolyte

tool material: Cu, brass, steel

material/tool wear: infinite

gap 50 to 300 μm

maximum mrr 15*103 mm3/min

specific power consumption 7W/mm3/min

critical parameters: voltage, current, feed rate, electrolyte, electrolyte conductivity

materials application: all conducting metals and alloys

shape application: blind complex cavities, curved surfaces, through cutting, large through cavities.

limitations: high specific energy consumption (about 150 times that required for conventional processes), not applicable with electrically non-conducting materials and jobs with very small dimensions, expensive machines.

surface finishes down to 25 μin.

• This technique has been combined with a metal grinding wheel in a process called Electrolytic drilling. The wheel does not touch the work, and gives a surface finish from 8 to 20 μin.

29.2.1 References

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29.3 Ghosh, A., Manufacturing Science, Ellis Horwood Ltd., Chichester, UK, 1986.

29.4 Krar,

29.3 Problems

Problem 29.3 a) ECM is used to remove metal from an iron workpiece. The feature being cut is a square hole 1cm by 0.5 cm. To cut down 1cm/min, what current is generally required?

b) How would the current change if the part was 3% carbon?

c) What advantages does ECM have over EDM?

Problem 29.4 We have a metal alloy that is a combination of Zinc and Silicon, but we are unsure what the ratio of the metals is. But an ECM machine can cut a 5mm square hole 2mm deep at a current of ___________ in one minute. What percentage of the alloy is zinc?

Problem 29.5 Describe good applications for EDM.

Problem 29.6 Explain how EDM is best suited to producing complex internal features, such as sharp inside corners.

Problem 29.7 Wire EDM produces parts with a profile that is vertical. What types of applications is this well suited to? Are there particular dies that benefit from this technique?

Problem 29.8 A hole that is 70mm deep and 10mm in diameter. What is the machining time using a) EDM, b) ECM?

Problem 29.9 Which of the following materials are suitable for EDM, EBM, ECM? State Why? a) stainless steel, b) ceramics, c) quartz, d) thermoset plastic, d) copper, e) diamond.

Problem 29.10 Describe the ability of EDM, EBM, ECM to produce sharp corners.

Problem 29.11 Describe the basic operation of an EDM machine using figures. Show why corners are rounded and why flushing is important.

 

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