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”
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.
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
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
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)
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)
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)
8
Q
ECM - Process Characteristics
A
-Feed rate (fr) primarily a function of the current density.
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.
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).
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.
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.
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).
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.
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
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.
