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Cutting fluid
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Functions: Cooling of the job and the tool Lubrication at the
chiptool interface and the tool flanks Cleaning the machining zone
by washing away the chip particles and Protection of the finished
surface Essential properties of cutting fluids For cooling :
high specific heat, thermal conductivity and film coefficient
for heat transfer spreading and wetting ability
For lubrication : high lubricity without gumming and foaming
wetting and spreading high film boiling point friction reduction at
extreme pressure (EP) and temperature
Chemical stability less volatile and high flash point high
resistance to bacterial growth odourless and also preferably
colourless non toxic in both liquid and gaseous stage easily
available and low cost.
Cutting Fluid
L4: cutting fluid
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Basic principle of lubrication The cutting fluid make a layer in
between two surface ( hydrodynamic film) At high temperature, low
pressure and low sliding velocity hydrodynamic layer is not
maintained However, a few molecule thick layer of fluid is
localized absorbed on the surface known
as boundary layer Frictional resistance (Ff) is combination of
viscous shearing of fluid and asperity
shearing
Cutting Fluid
L4: cutting fluid
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Basic principle of lubrication
Cutting Fluid
L4: cutting fluid
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Lubrication in metal cutting
Cutting Fluid
L4: cutting fluid
Commonly used lubricant ( used in mechanical systems) may be
used for lubrication in metal cutting, but not always
Some of the materials not used as lubricant gives very good
result in machining????? Example CCl4
Why cutting fluid is not effective at higher velocity?????
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Lubrication in metal cutting
Cutting Fluid
L4: cutting fluid
Mechanism of lubrication
Cp: contact length of plastic zone Ce: contact length of elastic
zone
Fluid enters in elastic zone and not in plastic zone
Thus presence of fluid reduces secondary deformation (elastic
zone)
Chemical activeness of fluid helps to penetrate
Thus,
Lubricant must have: A small molecular size to allow rapid
penetration Suitable reactive ingredient that forms a
compound of lower shear strength as in case of machining of Cu
in presence of CCl4 creates CuCl2
Can be sheared below temperature and pressure existing at the
chip-tool interface
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Lubrication in metal cutting Mechanism of lubrication Mechanism
of lubrication
Cutting Fluid
L4: cutting fluid
(1) contamination of the rake face at A providing lubricating
between the chip and the rake face,
(2) contamination of the two chip elements sliding over each
other at B
(3) cooling the zone of plastic deformation at C and thus
limiting the flow shear stress in this zone,
(4) Lubrication and cooling of the flank-workpiece interface at
D.
(1) Similar. (2) Cooling of the free surface of the
partially
format chip at B, (3) cooling the zones of plastic deformation
at C
and E that increases the flow shear stress of the work material
in these zones
(4) Similar.
(1) Similar. (2) cooling of the free surface of the
partially
format chip at B reducing its plastic deformation and thus the
chip compression ratio
(3) cooling the zones of plastic deformation at C and E
promoting propagation of cracks.
(4) Similar.
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Some types of cutting fluids
Air blast : used if any cutting fluid in liquid form is
difficult to use eg grey cast iron
Straight oils are non-emulsifiable : composed of a base mineral
or petroleum oil ;
provide the best lubrication and the poorest cooling
Synthetic Fluids : formulated from alkaline inorganic and
organic compounds; best
cooling performance among all cutting fluids.
Soluble Oil : emulsion when mixed with water; provide good
lubrication and heat transfer
performance.
Semi-synthetic fluids : combination of synthetic and soluble oil
fluids
Paste and Solid Lubricants : Waxes, pastes, soaps, graphite
etc.
Cryogenic cutting fluid: Extremely cold (cryogenic) fluids
(often in the form of gases)
like liquid CO2 or N2
Cutting Fluid
L4: cutting fluid
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Methods of application of cutting fluid
(a) Manual application: operator using a brush
(b) Flood Application : continuous fluid flow at low pressure;
effectiveness depends upon
geometry
(c) Mist application : cutting fluid mixed with air; Used for
high speed application where
uncut chip thickness is smaller
(d) Hole in tool :
Cutting Fluid
L4: cutting fluid
(a) (b) (c)
