Introduction to Heat in Machining

    Introduction

    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

    Assumptions

    Due to the complexity of heat problem in machining, the following assumptions are generally imposed:
     
     

    Approaches

    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:
     
     

    Experiment Retrospection

    Since 1920s, many experimental methods were devised to measure the tool,chip or workpiece temperature and their distribution:
     
     

    Numerical Simulation

    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:
     
     

    Semi-Analysis

    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 Generation

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 operations:
 
 

Types of Heat Sources

There are several types of heat source in machining:
 
 

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 into heat.

 If heat source is uniformly distributed along the shear plane, the intesity of shear plane heat source, Ip, satisfies the following relation:
 
 

Fs Vs
Ip = ---------------
b t1

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
 
 

Vc
E. = --------------
dt

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
 
 

F Vx
Ic = ------------
h b

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:
 
 

Heat Transfer

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, 1984)
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:
 
 

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 Effects

Heat Influences on Cutting Forces

Heat influence on the cutting forces is mainly because that:
 
 

Heat Effects on Tool Life

Heat has great influence on tool life. The following diagram verify this point:

 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, 1984)

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 various operations
     
     

    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.
     
     

    Heat Effects

    Other than research issues mentioned above, there are still some areas listed here:
     
      More issues, please refer to the above section: Heat Effects

Links to Other Web Sites

CYBERMAN Education Page

Machine-Tool Agile Manufacturing Research Institute at MTU

Machine-Tool Agile Manufacturing Research Institute at UIUC