Introduction to Heat in Machining
Heat has critical influences on machining. To some extent,
it can increase tool wear and then reduce tool life, get
rise to thermal deformation and cause to environmental problems,
etc. But due to the complexity of machining mechanics, it's hard
to predict the intensity and distribution of the heat sources in
an individual machining operation. Especially, because the properties
of materials used in machining vary with temperature, the mechanical
process and the thermal dynamic process are tightly coupled
together. Since early this century, many efforts in theoretical analyses
and experiments have been made to understand this phenomena, but many problems
are still remaining unsolved.
Theoretical Analysis Review
Due to the complexity of heat problem in machining, the following assumptions
are generally imposed:
First, almost all (90%-100%) of the mechanical energy consumed in a machining
operation finally convert into the thermal energy.
Second, There are three major sources of thermal inorthogonal
cutting with a sharp tool: plastic deformation in the so-called
zone and secondary zone, and the frictional dissipation energy
generated at the interface between tool and chip. But if the tool
is with a round tip, part of heat may be generated at the interface
between tool and workpiece due to friction. In pure theoretical analysis,
more assumptions are needed: usually, the plane heat sources at
the shear plane and the tool-chip interface are assumed as being uniformly
Third, even with the above assumptions, the problem of estimating the mean
temperatures on the shear plane and tool face is complex. This is because
part of the thermal energy will convected away by the chip, part will conducted
into the workpiece and tool, i.e., a partition criterion is needed.
In addition, the geometry of the tool, chip and workpiece,as well
as boundary conditions are simplified to some extent.
The pure analytical approches , in general, came out the average temperature
on the shear plane and at the tool/chip interface. The temperature distridution
along the shear plane and the tool/chip interface was also obtained some
of the following approaches:
Since 1920s, many experimental methods were devised to measure the tool,chip
or workpiece temperature and their distribution:
The numerical methods were successfully applied in calculating the temperature
distribution and thermal deformation in tool, chip and workpiece. Especially,the
finite element and boundary element methods can deal with very complicated
geometry in machining, they have great potential to slove the problems
in practice. These methods are listed in the following:
In this class of methods, some information such as chip surface temperature
or temperature distribution in workpiece is first obtained experimentally.
Then the temperature distribution and/or thermal deformation in chip, and
sometimes in the tool and workpiece as well are calculated analytically.
The inverse heat transfer problem in machining is an example of these methods.
Heat Generated in Various Machining Operations
Almost all of the heat generation model were established under orthogonal
cutting condition. But in practice, there are various machining operations
which cannot satisfy this condition, such as oblique turnning, boring,
drilling, milling, grinding, etc.
Generally, the intensity of heat sources in real machining operations
can be determined approximatedly by the external work applied, however,
the distribution of the heat sources are hard to obtained by either theoretical
or experimental methods.
The following listed are the simplified heat source model in real
Boring: A uniform moving ring heat source.
End Milling: An ellipsoidal shape distribution with a distribution
of uniform heat flux at milling area.(Heat source not defined by its intensity)
Grinding: A circular heat source moving on the surface of workpiece.
Types of Heat Sources
There are several types of heat source in machining:
Plastic work converted to heat.
Viscous dissipation transformed into heat if the cut material are viscoplastic.
Work done by friction converted to heat.
Ambient heat source sometimes need be considered if thermal deformation
In non-traditional machining, other types of heat sources exist.
Heat Generated in Primary Zone
Heat generated in this zone is mainly due to plastic deformation and viscous
dissipation. But in classical machining theory, the rate of heat generated
is the product of the shear plane component, Fs, of the resultant force
and the shear velocity, Vs, i.e., the shear energy is completedly converted
If heat source is uniformly distributed along the shear plane,
the intesity of shear plane heat source, Ip, satisfies the following relation:
Ip = ---------------
where b is the cutting width and t1 the uncut depth.
Heat Generated in Secondary Zone
In this region, because of the complexity of plastic deformation, this
part of heat was ignored in many prevoius theoretical research.
Boothroyd has shown that the secondary plastic zone is roughly triangular
in shape and that strain rate, E., in this region varies linearly from
an approximatedly constant value along the tool/chip interface given by
E. = --------------
Where Vc is the chip velocity, dt the maximum thickness of the zone.
Hence the maximum intensity of heat source in this zone is proportional
to the strain rate.
Heat Generated at Interface between Tool & Chip
Heat is generated at the tool/chip interface by friction. The intensity,Ic,
of the frictional heat source is approximatedly by
Ic = ------------
where F is the friction force, Vx the sliding velocity of the chip along
the interface, and h is the plastic contact length.
Program to Calculate Heat Generation
More on Heat Generation
Heat generation is not well investigated in the following areas:
Plastic Deformation Work in the Second Zone
Temperature Influence on Heat Generation
Heat in the Practical Operations
Conduction, Convection & Radiation in Ordinary Cutting Operations
The three types of heat transfer, conduction, convection and radiation,
all exist in the machining operations.
Heat transfer inside the chip and workpiece, the tool and toolholder
is by conduction.
Heat transfer between coolant/air and the chip/tool/workpiece
is by convection.
Radiation is rarely investigated in traditonal machining operations.
But radiation techniques are widely applied in measuring the temperature
distribution in various machining operations.
Temperature Distribution near Cutting Zone
The typical temperature distributions are shown as follows: Here is the
isothermal lines for dry orthogonal cutting of free machining steel with
a carbide tool.
(From: Milton C. Shaw, Metal Cutting Principles, Clarendon Press, Oxford,
For more plots of temperature distrbutions, please click here.
Cutting Fluids' Effects on Heat Transfer
Cutting fluids' effects on heat transfer are, in gerneral, classified as:
Cutting fluids may reduce the cutting force, such as friction, therefore,
heat generation is reduced to some extent.
Using cutting fluids, heat generated in machining can be rapidly removed
away by convection.
Generally, using cutting fluid cannot reduce the maximum temperature at
the tool/chip interface, but increase the temperature gradient in both
the chip and the tool because cutting fluid is not easy to access the cutting
More About Heat Transfer
In practice, there are other types of heat source involved in machining,
such as ambient heat sources. They may cause some thermal deformation in
the lathe and so on.
Heat Influences on Cutting Forces
Heat influence on the cutting forces is mainly because that:
The friction coeffient is tightly dependent upon temperature.
The properties of cut material also depend on temperature.
Heat Effects on Tool Life
Heat has great influence on tool life. The following diagram verify this
Variations of tool life with workpiece bulk temperature when milling
Cr-Ni-Mo steel at speeds of (1) 150 fpm and (2) 200 fpm. (After krabacher
and Merchant 1951)
(From: Milton C. Shaw, Metal Cutting Principles, Clarendon Press, Oxford,
Heat Influences on Surface Toughness
Heat gives rise to thermal deformatiom in the workpiece, which finally
takes on the form of surface toughness.
Heat Influences on Thermal Deformation in Lathe
Thermal deformation in the lathe is the so-called thermal error in precision machining.
Heat Effects on Mass Transfer in Coolant Circulation System
Interesting? please take a look.
Heat Effects on Enviroment
When heat generated in machining finally flow into the coolant circulation
system, it may degrade the used coolant and cause vaporization and atomiziton
of coolant. This is also the Health Issue in Enviromentally Conscious Machining.
Heat Related Research Issues
Heat Generation Model
Predictive heat generation models in either orthogonal cutting or other
Convection by Coolant
Because convection of coolant varies with many factors, such properties
of coolant, application conditions, state of coolant flow, and operation
conditions, etc, it's required to investigated these corresponding issues.
A Heat Transfer Performance Module, which can predict the convective heat transfer
coeffients of several kinds of coolants used in some typical machining
operations, can be accessible.
Simulation of Open Cutting Fluid Circulation System
A energy and mass flow model of cutting fluid circulation system is a very
important issue in environmentally conscious machining. Sometimes, the
disposal of chips and coolants needs much more energy than that in real
cutting operations. Developing an effective way to utilize energy should
be under consideration.
Other than research issues mentioned above, there are still some areas
More issues, please refer to the above section: Heat
Thermal softening on shear banding formation in the chip
Heat influence on chip morphology
Heat effect on the carry-off capacity of coolant
Thermal and mechanical coupled machining theory
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