29.1.1 Wire EDM

29.1.2 Problems

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 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.

29.3 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.