Electrical Machining Flashcards

1
Q

Electrochemical Machining Principle

A
  • Based on the chemical (or anodic) dissolution of metal by electrolysis
  • Also known as the “reverse of electroplating”
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2
Q

Further Method

A
  • Shaped tool (electrode) is brought close to an electrically conductive workpiece at a constant rate maintaining a gap while submerged in an electrolyte solution.
  • Tool & workpiece are connected to a DC supply and a high density current passes through the gap and the rapidly flowing electrolyte.
  • Electrochemical reaction deplates the metal from the anode and gets washed away by the electrolyte before plating on the cathode (tool) can take place.
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3
Q

ECM - Process Mechanism

A
  • Electrode is fed into the workpiece at a rate equal to the material removal rate.
  • Feed rate is CNC controlled and varies from from 0.25mm/min to 20mm/min (control less critical than EDM as no tool wear) maintaining a close gap of 0.05-0.75mm.
  • Electrolyte pumped fast through the gap for a quick removal of metal particles avoiding their deposit onto the cathode; typically 1ltr/min or 15-50m/sec
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4
Q

ECM - Process Mechanism

A
  • DC power of 10-25V; kept low to minimise the risk of an arc forming across the gap.
  • High current of up to 40,000A giving a current density of 8-450A/cm2 to achieve material removal rates of 1-4mm3/A-min
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5
Q

ECM - Process Characteristics

A
  • Material Removal Rate (MRR):
  • Ranges between 1-4mm3/A-min

MRR= V/t

V = C x I x t

V= Volume of metal removed
C = Specific removal rate
I = Current
t = Time (sec)
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6
Q

ECM - Process Characteristics

A

-Current flow & resistance of the ECM process:

I= E/R; R= gxr/A; I = ExA/gxr

I= Current
E= Voltage
R= Resistance
g= Gap between electrode & workpiece in mm
r= Resistivity of electrolyte in Ohm-mm.
A = Surface area between workpiece & tool in the frontal gap (mm2); (direction of feed into the work)
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7
Q

ECM - Process Characteristics

A
  • Feed rate (fr):
  • Removal of volume V over area and time (Axt) for a linear travel rate
  • V=CxExAxt/gxr
  • V/Axt = CxE/gxr =fr
  • fr = CXI/A
fr= feed rate
V= Volume of metal removed (mm3)
C = Specific removal rate (based on atomic weight, valence, density of the material mm3/A-sec).
A = Frontal area of the electrode (mm2)
t= Time (sec)
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8
Q

ECM - Process Characteristics

A

-Feed rate (fr) primarily a function of the current density.

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9
Q

ECM - Process Characteristics

A
  • Surface finish & accuracy:
  • Hard & soft materials can be machined with equal speed & precision (stainless steel, titanium, nickel alloy).
  • Stress- and burr-free surfaces
  • Precision accuracy of 0.03mm on one dimensional cuts & 0.1mm on contours.
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10
Q

Electrolyte solution for ECM

A
  • Function & properties:
  • Conductor - allow electric current to flow between the tool & the workpiece.
  • Coolant - to keep temperature of tooling and workpiece constant as the conductivity of the fluid depends on it’s temperature.
  • Flushing agent - to carry off deplated material (microscopic particles) from the gap and remove hydrogen bubbles.
  • Medium:
  • Sodium chloride solution, sodium nitrate with a water base (typical 10% solution).
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11
Q

Electrodes for ECM

A
  • Materials:
  • Copper, brass, stainless steel, bronze, titanium.
  • Desired properties:
  • Good strength not to deform by flow pressure from the electrolyte bath.
  • Minimal electrical resistance
  • High chemical resistance.

Tool wear:
*Very little; only caused by flowing electrolyte.

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12
Q

Electrodes for ECM

A

-Generally produced by traditional NC machining methods

  • Surface of the workpiece does not reproduce exactly the surface of the tool:
  • For best accuracy the outer surface of the tool is insulated with a thin silicon carbide or silicon nitrate coating.
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13
Q

ECM - variations

A

Electrochemical Grinding:
*Material removal is a combination of electrochemcial decomposition and action of a diamond abrasive particles contained in the grinding wheel (95% deplating).

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14
Q

ECM - Variations

A

-Electrochemical Grinding - Advantages:

  • High grinding ratio with dramatically reduced wheel wear (10x)
  • Comparable Material Removal Rates with normal grinding.
  • No heat distortion or danger of burning.
  • Little mechanical force eliminating burrs or distortion.
  • Surface finish of Ra 0.2 micrometres possible.
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15
Q

ECM - Variations

A
  • Electrochemical Grinding - Limitations:
  • High initial equipment costs
  • Large power consumption, only recommended for hardened material or difficult to machining ones.
  • Only for electrical conductive material
  • Corrosive environment.

-Electrochemical Grinding - Applications:

  • Sharpeneing of sintered carbide tools or high strength alloys.
  • Surgical needles, thin wall tubes.
  • Fragile parts
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16
Q

ECM - Variations

A
  • Electrochemical Honing:
  • MRR several times higher than traditional honing especially on hard material.
  • Up to 5x faster than traditional honing.
  • Higher accuracy as temperatures are cooler as lower honing pressure resulting in lower tool wear (10x less)
  • Very costly
  • Pulsed Electrochemical Machining (PECM):
  • Pulsed rather than direct current to eliminate high electrolyte flow rates
  • Very high current densities (1A/mm2)
17
Q

ECM - Variations

A
  • Electrolytic In-process Dressing (ELID) Mirror-Surface Grinding:
  • Grind mirror-quality surfaces, no requirement for secondary operation such as lapping.
  • Use of conventional super-abrasive, metal-bonded grinding wheel & conventional coolant.
  • Electrolytic action is between a negative copper electrode and the grinding wheel as an anode.
  • For optical lenses, mirrors or slicing & surface grind silicon wafers.
18
Q

ECM - Variations

A
  • Electrochemical Deburring:
  • Removal of burrs or to round sharp corners by anodic dissolution.
  • Nest results when used with a shaped cathode; partially insulated.
19
Q

ECM - Variations

A

-Shaped Tube Electrolytic Machining (STEM):

*Electrochemical drilling of small diameter, deep holes in super-alloy materials
(Ø0.5mm; ratio 300:1)

  • Drilling of round or shaped holes using a tube (usually titanium) as cathode.
  • Acid is used as electrolyte and fed through a tube to keep the metal in solution.
20
Q

ECM - Variations

A
  • Shaped Tube Electrolytic Machining (STEM):
  • Multiple holes can be drilled simultaneously
  • Used for air cooling & weight reduction holes in jet engine blades.
  • Drills feeds 2-4mm/min
21
Q

ECM - Applications

A
  • Hard or difficult to machine metal.
  • Difficult geometry or impossible to manufacture with conventional techniques.
  • Die sinking, forging dies, and other shaping tools with irregular contours.
  • Multiple hole drilling.
  • Deburring
  • Micromachining.
22
Q

ECM - Advantages

A
  • Any electrically conducting material regardless of hardness can be machined; work-piece can already be hardened.
  • Complex shapes and contours can be machined in one process step with excellent surface finish.
  • No distortion as no thermal or mechanical stress.
  • No burrs
  • Little tool wear (only from flowing electrolyte)
23
Q

ECM - Limitations

A
  • Only conductive materials (tooling & work-piece)
  • Not suited for sharp profiles (sharp square corners; internal & external).
  • Large forces on electrode & work-piece due to small gap and high pressure flow of electrolyte.
  • Work-piece must be cleaned and oiled directly after machining to avoid corrosion.
  • Environmental impact with the disposal of electrolytic sludge.
24
Q

ECM - Limitations (continued)

A
  • Expensive equipment & tooling cost:
  • Stainless steel and other corrosion resistant components.
  • Tool making more complex due to insulation for correct conductive paths; make provision for taper (overcut).
  • Complex plumbing & filtration system including electrolyte regeneration.
  • Safe removal of hydrogen gas (explosive!) required.
  • Significant power consumption
  • High maintenance to keep equipment clean form electrolyte residue to avoid corrosion.