Lec12 Flashcards
Types of thermal machining x4
1 electrical discharge machining (EDM)
2 Particle beam machining (PBM)
3 Laser beam machining (LBM)
4 Plasma arc cutting (PAC)
EDM removes metal by
discharing electrical cutten from dc power supply across a thin inter electrode gap between a tool a workpiece
removal is based on spark erosion process
EDM process
1 dielectric fluid in gap
2 fluid locally ionised at narrowest gap
3 ionisation creates conduction path - spark produced
4 spark produces tiny crater by vaporisation and melting
5 spherical chips produced by resolidifcation of melted workpiece
What causes tool wear in EDM
high temp spark melts or vaporises the tool
How are chips removed in EDM
dielectric fluid pumped through theinterelectrodge gap to flush out chips and bubbles while confining the sparks
two types of EDM
Ram EDM
Wire EDM
Ram EDM
electrode is a die shape of the negative of the cavity made from copper or copper alloy
by feeding die into the workpiece shape of the die is produced
Wire EDM
electrode used is wire
used only once made of brass copper tungsten
travels along prescribed path cutting workpiece with discharge sparks acting like teeth
should have sufficient tensile strength fracture toughes and high electrical conductivity
Wire EDM material vs wire
wire does not contact workpiece therefore tool does not need to be harder than material
applications of EDM
manufacture of punches and dies
low volume high performance components
adv of EDM
complex geometries (especially hardened tool materials) no burrs on edges able to cut intricate openings tight ridius contours and corners
disadv of EDM
slow compared to conventional
thin recast and heat affected layers
hazardous fumes
PBM or electron beam welding
high energy electrons focused on workpiece to melt and vaporise it
Where is PBM performed
vacuum chamber
PBM how is electron beam produced
electron gun via thermionic emission
tungsten filament heated to 2000 C electrons emitted and focused using electrostatic optics
Adv of PBM
recast material and depth of heat damage very small
small features - micoron level precision
disadv of PBM
intercations of beam wtih surface produce exrays - need shielding
needs vacuum
LBM uses
intensly focused coherent stream of light to remove material from workpiece surface
material removal process dependent upon type of laser used and how it is operated
Two types of material removal mechanism
thermal evaporation - material vaporised
chemical ablation bonds bronze by excessive energy (no HAZ)
Adv of LBM
increadibly precise and accurate
no mechanical contact - no contamination good clean edge and surface finish and no tool wear
small or no HAZ
disadv of LBM
safety implications of lasers
applications
drilling microholes in turbine engines
machining complex shapes in sheet metal
PAC uses
superheated stream of electrically ionised gas to melt and remove materials
plasma
10,000 25,000 C created inside a water cooled nozzle
mixture of free electrons and positively charged ions and neutral atoms
adv of PAC
high cutting speed one electrode can be used to machine a wide variety of materials and thicknesses
disadv
poor tolerances tapered cuts and double arcing leading to premature wear on nozzle
What sustains the plasmc arc
high voltage dc power
high velocity gases melt and blow away molten metal chips
applications of plasma arc cutting
cutting structural materials
either handheld or CNC machines
economics of non traditional machining factors to consider x 5
1 cost of tool and equipment (chemicals and acids)
2 MRR (slow for many)
3 Operational cost
4 operator skill req (high for some and have to be protected)
necessary secondary and 5 finishing operations (req for some
Chemical machining economics
chemicals maskants disposal and cleaning of chemical
EDM economics
cost of consumables electrodes/wire
laser and plasma machining
economics
main cost is investment of equipment
production rate feasibility of other methods
economics of machinig non traditional processes
choose them are they only feasible method - as traditional methods probably faster
