Introduction to Heat in Machining - Mechanical engineering
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Introduction to Heat in Machining

Saturday, April 18, 2009

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:

  • 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 in orthogonal cutting with a sharp tool: plastic deformation in the so-called primary 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 distributed.
  • 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.


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:


  1. Moving Heat Source Method

    Here is the idealized diagram of shear plane moving heat source:

    The temperature distribution along the shear plane was assumed as the same as that along a uniform band source of heat moving obliquely through an infinite solid. Similarly, the temperature distribution or average temperature at the chip/tool interface also can be approximatedly obtained.


  2. Image Source Method

    To avoid giving rise to two different temperature on either side of a same plane due to partition principle, Chao and Trigger approximated the uniform plane heat source at the interface by a grid of point sources. Assuming that the angle between the rake and flank faces was right and that the tool surfaces were insulated, the grid of real and fictious image point sources is shown as follows:

    Then they used an iterative procedure to obtain the final interfacial temperature distribution and later extended their work to include the frictional heat source at the tool/work interface.


  3. Dimensional Analysis

    Kronenberg postulated that the temperature at the tool/chip interface was dependent on the following important variables: chip area, cutting speed, a specific cutting force, a thermal conductivity, and the product of density and specific heat. Applying the principle of dimensional analysis, two dimensionless groups of variables were derived. Then the experimental results were fitted in the following:


Experiment Retrospection

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

  1. Tool-Chip Thermocouple Technique

    Here is the schematic of tool-chip thermocouple set-up:


    Generally, only the average temperature at the tool/chip interface can be obtained. Sometimes, a tool-work thermocouple was also used.

    The calibration of the tool-chip thermocouple is shown as:

    Some limitations of this methods are mainly:

    1. The elimination of parasitic e.m.f.'s which could affect the thermocouple output.
    2. The calibration of the thermocouple output.


  2. Embedded Thermocouple Technique

    The following is a typical embedded thermocouple setup:


  3. Infrared Radiation Technique

    The first use of this technique was reported by Schwerd who developed a total Radiation Pyrometer for determining the temperature distribution at the surfaces of tool and workpiece.

    Several researchers attempted to access to chip/tool interface by scanning through holes drilled either in the work or tool. Prins' pyrometer arrangement is shown as follows:

    Limitation of the above measurements is that there is considerable interference of the contact zone which must influence the heat flow and the resulting temperature of either the chip or tool surface.

    Infrared Photography technique proposed by Boothroyd can obtain a full temperature field in the chip and workpiece. A calibration strip, which was electrocally heated from one end,was simply used to calculate the real temperature. The following setup was used at that time:

    Today, many disavantages of this technique are overcomed.


  4. Metal Microstructure and Microhardness Variation Measurement

    Under suitable cutting conditions, the metallographic method can be used to determining the temperature gradients in high speed steel cutting tools. The following picture is the etched rake face of tool used to cut nickel in normal dry atmosphere at 46 m/min, 0.25 mm/rev feed for 30 seconds(After E. F. Smart and E. M. Trent):

  5. However, the microstructure, microhardness and other properties change to some extent dependent on temperature and time.


  6. Thermosensitive Painting Technique

    Using the thermosensitive paints for estimating cutting temperature is because that these paints change colors at different temperature.

    Limitations are that there is a relatively long time lag for the colors to change and that small changes in temperature are not easy to be detected.

  7. Temper Color Technique

    This technique is based on the fact that the temper colors of some metals are different at different temperatures. And it wasn't widely used.


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:


  1. Finite Difference Method

    Generally, the pure theoretical methods only gave us very approximate results, such as average temperature and temperature distribution along the shear plane and the tool/chip interface.

    Finite difference method can be used to calculate the temperature distributions in the chip, tool and workpiece. And better results can be expected because the geometry and boundary conditions of chip, tool and workpiece, as well as the shape of distributed heat sources can be descripted well.

  2. Finite Element Method

    Finite element method(FEM) has great potential to calculate the temperature distributions in the chip, tool and workpiece if the geometry, boundary conditions and the shape of distributed heat sources become very complicate.The following diagram is the mesh used by O. A. Tay:

    In addition, FEM can be used to calculate the temerature distribution in either toolholder or machined parts and then to obtain the thermal deformation.

    Finally, because an accurate distributed heat source model is needed in order to obtain a better result in the temperature distribution. FEM can be naturally coupled with some mechanics model, therefore, predicts the intensity and distribution of the heat sources.

  3. Boundary Element Method

    Boundary Element Method(BEM) was used in calculating the temperature distribution in the tool by O. A. Tay. BEM has great potential in reducing solid modeling to surface modeling. A wide application in this field is undoubtable.


  1. 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


  1. 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:

    • 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.

  2. 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 is concerned.
    • In non-traditional machining, other types of heat sources exist.

  3. 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.

  4. 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.

  5. 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.

  6. program to Calculate Heat Generation

  7. More on Heat Generation

    Heat generation is not well investigated in the following areas:

    • Non-Coulumb Friction
    • Plastic Deformation Work in the Second Zone
    • Temperature Influence on Heat Generation
    • Heat in the Practical Operations

Heat Transfer


  1. 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.

  2. 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.

  3. 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 edge.

  4. 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


  1. 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.

  2. 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)

  3. Heat Influences on Surface Toughness

    Heat gives rise to thermal deformatiom in the workpiece, which finally takes on the form of surface toughness.

  4. Heat Influences on Thermal Deformation in Lathe

    Thermal deformation in the lathe is the so-called thermal error in precision machining.

  5. Heat Effects on Mass Transfer in Coolant Circulation System

    Interesting? please take a Health issue in Enviromentally Conscious Machining.


Heat Related Research Issues


  1. Heat Generation Model

    Predictive heat generation models in either orthogonal cutting or other various operations

  2. 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.

  3. 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.

  4. Heat Effects

    Other than research issues mentioned above, there are still some areas listed here:

    • 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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