## Introduction to Heat in Machining

1. ### 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.

2. ### 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:

3. ### Experiment Retrospection

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

4. ### 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:

5. ### 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.

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## 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.

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
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## 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.

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.

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## 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)

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.

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## 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

More issues, please refer to the above section: Heat Effects
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## Links to Other Web Sites

• ### Environmentally Conscious Fluid Selection Project at UIUC

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