Lec11 Flashcards
disadv of traditional machining
1 large amount of energy goes into producing unwanted chips
2 a lot of energy ends up was undesirable heat
3 unwanted residual stress burs req further processing
4 some geometries are too delicate
Adv of traditional machining
ability to machine
1 Complex geometries beyond simple planar or cylindrical features
2 extreme surface finish and tolerance
3 components that cannot withstand large cutting forces
4 no burs or residual stresses
5 brittle materials or very high hardness
Types of nontraditional machining
chemical
electrochemical
mechanical
thermal
Chemical non traditional machining
a chemical reaction between liquid reagent and the workpiece results in etching
Electro chemical
an electrolytic reaction at the
workpiece surface is responsible for material removal
Mechanical
high velocity abrasives or liquids are used to remove materials e.g. Ultrasound, water jet
etc
Thermal
high temp. in very localised region evaporates materials
General chemical machining steps
cleaning
masking
etching
stripping
Cleaning for chemical machining
contaminants on surface are removed to permit uniform etching
pickling rinsing degreasing
Masking for chemical machining
etch resistant maskant to cover a certain region of a workpiece - uncovered is exposed to etching
Etching for chemical machining
Spraying etchant onto a workpiece or immersing workpiece in etchant
chemical reaction stopped by rinsing
Stripping for chemical machining
maskant is removed from the workpiece and its surface is cleaned
Etch rate proportional to
etchant concentraction adjadcent to the region of machining
disadv of chemical machining x2
1 MRR is very slow in terms of unit area exposed
2 handling of dangerous chemicals and disposal of harmful products
Photochemical machining is a type of
chemical machining
How does photochemical machining work
Photoresist used as mask
CAD used to develop design
develop photographic negative (phototool)
bring phototool in contanct with workpiece using vacuum frame
expose workpiece to intense UV light
expose regions more resistant to solvents
dip or spray in etchant
remove mask
Types of photoresists
dry film - use a hot roller lamination
lquid - dipping flowing rolling electrophoresis
adv of chemical machining x6
1 simple no highly skilled operator req
2 no induced stress in material
3 large areas can be machined
4 virtually any shape part
5 thin section can be machined as no forces
6 materials that cannot sustain large cutting forces
Electro chemical maching removes material by
anodic dissolution with rapidly flowing electrolyte
deplating process tool is the cathode workpiece is the anode
Electro chemical process x3
1 tool is the cathode
workpiece is the anode
2 electrolyte pumped through hole in tool
3 electrolyte in the tool work piece gap washes away metal ions on workpiece away
Electro chemical machining what happens to metal particles
washed metal particles are flushed away by the flow of electrolyte prevent them from plating onto the tool
electro chemical machining tool advance rate
constant penetration rate that matches deplating rate of the workpiece
What happens to heat and waste product
sweeped away by electrolyte, sludge is caputred and removed from electrolyte through filtration
adv of ECM x4
1 complex shapes and conductive materials that cannot withstant high machining forces
2 good surface finish
3 no residual stress or thermal damage
4 no tool wear
Disadv of ECM x2
preparation of tool electrode can by time consuming and costly
disposal of environmental harmful by products
How is material removed by water jet cutting WJC
removed by impact of high velocity water jet
Speed and pressure of water in WJC and kerf
high pressure 400 MPa
velocity greater than speed of sounds
small nozzle (0.1 - 0.4 mm dia) produce very narrow width of cut (kerf)
Adv of WJC
cut materials without burning or crushing
inexpensive
disadv of WJC
noisy - operators need ear protection
cannot cut brittle materials (will crack)
limited to thin sheets of metal
abrasive in WJC (abrasive water jet cutting AWC)
abrasives added to waterjet in mixing chamber
required for most metal working process
How are abrasives mixed
abrasives fed into centre of the nozzle with converging set of angled water jets imparting momentum to the abrasives
(better mixing and increases nozzle life)
Type of material that can be cut
flat sheet but any material with right choice of abrasive jet pressure and feed rate
adv of AWC
cut through composite without delaminating the layers - airplane industry to cut CF sections of airfract
thick materials up to 8 inch
disadv of AWC
more expensive than normal WJC
Remove method for abrasive jet machining (AJM)
removes material by a focused jet of abrasives - similar to AWC but momentum transferred by jet of inert gas no water
abrasive producing micro scale chipping action on workpiece
Applications of AJC
processing hard brittle materials glass silicon tungsten
not suitable for soft and elastic materials
Abrasive flow machining AFM
involves
use of semisolid liquid or gel laden with abrasives to flow over workpiece
AFM setup
workpiece mounted in machines and pistons drive the abrsive back and forces
applications of AFM
edge finishing deburring radiusing polishing or minor surfacing machining
features normally inaccessibly in internal passageways
AFM should be considered if
workpiece req laborious hand finishing of complex shapes
internal edges difficult to reach
Non tradition machining produes
exceptionally smooth surfaces and complex shapes
allows machining of brittle materials
chemical machining
req a lengthy process of masking protecting etching and washing
abrasives can be
added to water or other media to create useful machining methods
