Electrochemical Machining Flashcards
What is ECM
Electrochemical Machining
- Based on the chemical (anodic) dissolution of metal by electrolysis
- known as the “reverse of electroplating)”
- shaped tool(electrod) is brought close to an electrically conductive workpiece at a constant rate maintaining a gap while submerged in an electrolyte solution
- tool and 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(workpiece) and gets washed away by the electrolyte before plating on the cathode(tool) can take place
ECM process mechanism
-electrode is fed into the workpiece at a rate equal to the material removal rate
-feed rate is cnc controlled and vaires 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
-dc power of 10-25v kept low to minimise risk of arc forming across the gap
-high current of 40,000A giving the current density of 8-450A/cm^2 to achieve MRR of 1-4mm^3/A-min
ECM - MRR
-material removal rate
=ranges between 1-4mm^3/A-min
MRR=V/t
V=CIt
where
V=volume of metal removed (mm^3)
C=specific removal rate (based on atomic weight, valence, density of the material
current flow and resitance of ECM process
-I=E/R where I= Current E= voltage R= resistance
-R=(gr)/A
goes to
I=(EA)/(g*r)
where
g= gap between electrode and workpiece in mm
-r= resistivity of electrolyte in Ohm-mm
A=surface area between workpiece and tool in the working frontal gap (mm^2)
ECM- Feed rate
-removal of volume V over area and time (A*t) for a linear travel rate
V=(C*E*A*t)/(g*r) goes to V/(A*t)=(C*E)/(g*r)=fr goes to fr=(C*I)/A
where fr=feed rate(mm/sec) V=volume of metal removed (mm^3) C=specific removal rate( based on atomic weight, valence, density of a material mm^3/A-sec) A=frontal area of the electrode(mm^2) t= time (sec
-feed rate is primarily a function of the current density
ECM- surface finish and accuracy
- ranges from Ra 0.4-0.8micrometers depending on electrode and workpiece material and MRR
- hard and soft materials can be machined with equal speed and precision(stainless steel, titanium, nickle alloy)
- stress and burr-free surfaces
- precision accuracy of 0.03mm on one dimensional cuts and 0.1mm on contours
properties of electrolyte solution for ECM
-function and properties
conductor- allow electric current to flow between the tool and the workpiece
-coolant- to keep temperature of tooling and workpiece constant as teh conductivity of the fluid depends on its temperature
-flushing agent- to carry off deplated material (microscopic particles) from the gap and remove hydrogen bubbles
Medium of electrolyte solution for ECM
sodium chloride solution, sodium nitrate with a water base (typically 10%)
electrodes for ECM- materials
- copper
- brass-
- stainless steel
- bronze
- titanium
desired properties for electrodes (ECM)
- good strength not to deform by flow pressure from the electrolyte bath
- minimal electrical resistance
- high chemical resistance
tool wear for ECM
very little; only caused by flowing electrolyte
electrodes for ECM
-generally produced by traditional NC machining methods
-surface of the workpiece does not reproduce exactly the surface of the tool (overcut)
=for best accuracy on the outer surface of t he tool is insulated with thin silicon carbide or coating
Electrochemical grinding
-material is a combination of electrochemical decomposition and action of diamond abrasive particals contained in the grinding wheel
Pros of electrochemical grinding
- high grinding ratio with dramatically reduced wheel wear(10x)
- comparable MRR with normal grinding
- no heat distortion or danger of burning
- little mechanical force eliminating burrs or distortion
- surface finish of Ra 0.2micrometers possible
Cons of electrochemical grinding
- 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
- Sharpening of sintered carbide tools or high strength alloys
- surgical needles, thin wall tubes
- fragile parts
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/mm^2)
Electrolytic in-process dressing (ELID) mirror-surface grinding
- grind mirror-quality surfaces(Ra 2-10micrometers), no requirement for secondary operation such as lapping
- use of conventional super-abrasive, metal-bonding grinding wheel and conventional coolant
- electrolytic action is between a negative copper electrode and the grinding wheel as an anode
- for optical lenses, mirrors or slicing and surface grind silicon wafers
Electrchemical deburring
- removal of burrs or to round sharp corners by anodic dissolution
- best results when used with a shaped cathode; partially insulated
shaped tube electrolytic machining (STEM)
- electrochemical drilling of small diameter, deep holes in a super-alloy materials (diameter of 0.5mm, ratio 300:1)
- drilling of round or shaped holes using tube (usually titanium) as cathode
- acid is used as electrolyte and fed through a tube to keep the metal in solution
- multiple holes can be drilled simulatinuously
- used for air cooling and weight reduction holes in jet engine blades
- drilling feeds 2-4mm/min
ECM applications
- 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
pros of ECM
- any electrically conducting material regardless of hardness can be machined; workpiece can 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
- little surface damage to the workpiece
- no burrs
- little tool wear (only from flowing electrolyte)
Cons of ECM (part 1)
- conductive materials (tooling and workpiece) can be machined
- not suited for sharp profiles (sharp square corners; internal and external)
- large forces on electrode and workpiece due to small gap and high pressure flow of electrolyte
- workpiece must be cleaned and oiled directly after machining to avoid corrosion
- environmental impact with the disposal of electrolytic sludge
Cons of ECM (part 2)
-expensive equipment and 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 and filtration system including electrolyte regeneration
=safe removal of hydrogen gas required
- significant power consumption
- high maintainance to keep equipment clean from electrolyte residue to avoid corrosion
