Heat generation in metal cutting and various methods of measuring the cutting temperature

1. Theory of Metal Cutting
Lecture No. (5)
Faculty of Engineering
Prod. & Mech. Design Engineering Department
Dr. Rania Mostafa
Heat in metal cutting

2. INTRODUCTION
 The power consumed in metal cutting is largely converted into heat near the cutting edge
of the tool, and many of the economic and technical problems of machining are caused
directly or indirectly by this heating action.
 The heat generated can cause temperatures to be as high as 6000C at tool chip interface.
 The cost of machining is very strongly dependent on the rate of metal removal, and costs
may be reduced by increasing the cutting speed and/or the feed rate, but there are limits to
the speed and feed above which the life of the tool is shortened excessively.
 This may not be a major constraint when machining aluminum and magnesium and
certain of their alloys, in the cutting of which other problems, such as the ability to handle
large quantities of fast moving chips, may limit the rate of metal removal.

3.  The bulk of cutting, however, is carried out on steel and cast iron, and it is in the cutting of
these, together with the nickel based alloys, that the most serious technical and economic
problems occur.
 With these higher melting point metals and alloys, the tools are heated to high temperatures
as metal removal rate increases and, above certain critical speeds, the tools tend to collapse
after a very short cutting time under the influence of stress and temperature.
“There is little doubt that when the laws of variation of the temperature of the shaving and
tool with different cutting angles, sizes and shapes of cut, and of the rate of abrasion are
definitely determined, it will be possible to indicate how a tool should be ground in order to
meet with the best efficiency and the various conditions to be found in practice.
However, determination of temperatures and temperature distribution in the important
region near the cutting edge is very important and vital.
INTRODUCTION

4. Cutting Temperatures
Elastic deformation- Energy required for the operation is stored in the material as strain
energy and no heat is generated.
Plastic deformation – Most of the energy used is converted as heat.
Cutting temperatures are important because high temperatures,
1. Reduce tool life.
2. Produce hot chips that pose safety hazards to the machine operator.
3. Can cause inaccuracies in work part dimensions due to thermal
expansion of work piece material.

5. The effect of cutting temperature, particularly when it is high is mostly detrimental to both
the tool and the job.
The major portion of the heat is taken away by the chips. But it does not matter because
chips are thrown out.
So attempts should be made such that the chips take away more and more amount of heat
leaving small amount of heat to harm the tool and the job.
1- Effect of cutting temperature on tool
The possible detrimental effects of the high cutting temperature on cutting tool (edge) are
 Rapid tool wear which reduces tool life
 plastic deformation of the cutting edges if the tool material is not enough hot-hard and
hot-strong
 thermal flaking and fracturing of the cutting edges due to thermal shocks.
 Built up Edge formation.

6. 2- Effect of cutting temperature on Job
The possible detrimental effects of the high cutting temperature on machined
job are:
 Dimensional inaccuracy of the job due to thermal distortion and
expansion-contraction during and after machining
 surface damage by oxidation, rapid corrosion, burning etc.
 induction of tensile residual stresses and micro cracks at the surface /
subsurface.

7. Positive effect of Cutting Temperature (Hot machining)
However, often the high cutting temperature helps in reducing the magnitude of the
cutting forces and cutting power consumption to some extent by softening or
reducing the shear strength, τs of the work material ahead the cutting edge.
To attain or enhance such benefit the work material ahead the cutting zone is often
additionally heated externally. This technique is known as Hot Machining and is
beneficially applicable for the work materials which are very hard and hardenable
like high manganese steel, Hadfield steel, Ni-hard, Nimonic etc.

8. Factors Affecting Temperature

10. Sources and Causes of heat generation in Machining
During machining, heat is generated at the cutting point from three sources,
 Primary shear zone (1) where the major part of the energy is converted into heat.
 Secondary deformation zone (2) at the chip – tool interface where further heat is
generated due to rubbing and / or shear.
 At the worn out flanks (3) due to rubbing between the tool and the finished surfaces.

11. Thermal Aspects of Machining
The heat generated is shared by the chip, cutting tool
and the blank.
The apportionment of sharing the heat depends upon
the configuration, size and thermal conductivity of the
tool – work material and the cutting condition.
The following figure visualizes that maximum amount
of heat is carried away by the flowing chip.
From 10 to 20% of the total heat goes into the tool
and some heat is absorbed in the blank.
With the increase in cutting velocity, the chip shares
heat increasingly.

12. Temperature distribution in Metal Cutting
Fig. shows temperature distribution in work piece and chip
during orthogonal cutting (obtained from an infrared
photograph, for free-cutting mild steel where cutting speed is
0.38m/s, the width of cut is 6.35mm, the normal rake is 300,
and work piece temperature is 6110C)

14. Experimental methods of determination of cutting temperature
 Amongst θS, θi, and θf , θi is obviously the highest one and its value is maximum almost at the middle of the
chip – tool contact length. Experimental methods generally provide the average or maximum value of θi.
 Some techniques also enable get even distribution of temperature in the chip, tool and job at the cutting zone.
The temperatures which are of major interests are:
θs : average shear zone temperature
θi : average (and maximum) temperature at the chip-tool interface
θf : temperature at the work-tool interface (tool flanks)
θavg : average cutting temperature
Cutting temperature can be determined by two ways :
•Experimentally – this method is more accurate, precise and reliable.
•analytically – using mathematical models (equations) if available or can be developed. This method is
simple, quick and inexpensive but less accurate and precise.

16. • Pioneering work in this area was done by Benjamin Thompson,
who in 1798 investigated that the heat generated in the boring of a
cannon and developed the concept of mechanical equivalent of
heat.
• The total heat was measured by performing the drilling operation
with the work piece, the chips, and the tool submerged in water.
• Three different calorimetric setups were used for determining
(i) the total heat generated in drilling,
(ii) heat in the tool after the cut,
(iii) heat in the chips.
Calorimetric Method

17. • The heat in the tool was determined by
cutting an sample test bar dry and dropping
the tool into the calorimeter immediately
upon the completion of cutting. Heat in the
chips was obtained by noting the temperature
rise of the calorimeter and water into which
only chips were permitted to fall.

18. Major points:
Quite simple
Low cost
Inaccurate
Only grand average value
 Much of the heat generated in cutting was carried out by the chips ( 70–80%) with 10% entering the
work piece, and the remainder into the tool.

19. Decolorizing Agent
• Paints or Tape are placed on the tool or job near
cutting point.
• Variation of temperature causes change in color.
• In steels color of chips may also indicate
temperature.

20. Tool work Thermocouple Technique
In a thermocouple two dissimilar but
electrically conductive metals are connected at
two junctions.
Whenever one of the junctions is heated, the
difference in temperature at the hot and cold
junctions produce a proportional current which
is detected and measured by a milli-voltmeter.
In machining like turning, the tool and the job
constitute the two dissimilar metals and the
cutting zone functions as the hot junction.
Then the average cutting temperature is
evaluated from the mV after thorough
calibration for establishing the exact relation
between mV and the cutting temperature.

22. • Advantages of Thermocouples include the following:
 simple in construction,
 ease of remote measurement,
 flexibility in construction,
 simplicity in operation and signal processing,
 low cost.

23. 2-Moving thermocouple technique

24. 3- Embedded thermocouple technique
• In operations like milling, grinding etc.
• The standard thermocouple monitors the job
temperature at a certain depth, hi from the cutting zone.
The temperature recorded in oscilloscope or strip chart
recorder becomes maximum when the thermocouple
bead comes nearest (slightly offset) to the grinding
zone.
• With the progress of grinding the depth, hi gradually
decreases after each grinding pass and the value of
temperature, θm also rises as has been indicated in Fig.
• For getting the temperature exactly at the surface i.e.,
grinding zone, hi has to be zero, which is not possible.
So the θm vs hi curve has to be extrapolated up to hi = 0
to get the actual grinding zone temperature. Log – log
plot helps such extrapolation more easily and
accurately.

25. • The limitations of the embedded thermocouples include the following:
 difficulty in placing thermocouple close to tool-chip contact & further causes
interference in flow of heat.
 the technique is difficult to implement as it involves the use of fine holes.
 thermocouples have limited transient response due to their mass and distance from
the points of intimate contact.
plotting of the temperature isotherms using embedded thermocouples in the tool can
be difficult.

26. 4- Chip-tool interface temperature by compound tool
In this method a conducting tool piece (carbide) is embedded in a non-conducting tool
(ceramic). The conducting piece and the job form the tool-work thermocouple as shown in
Fig. 8 which detects temperature θi at the location (Li) of the carbide strip. Thus θi can be
measured along the entire chip-tool contact length by gradually reducing Li by grinding the
tool flank. Before that calibration has to be done as usual.

27. 5- Photo-cell technique
• This unique technique enables accurate measurement of
the temperature along the shear zone and tool flank as can
be seen in Fig. The electrical resistance of the cell, like
PbS cell, changes when it is exposed to any heat radiation.
• The amount of change in the resistance depends upon the
temperature of the heat radiating source and is measured
in terms of voltage, which is calibrated with the source
temperature.
• It is evident from Fig. that the cell starts receiving
radiation through the small hole only when it enters the
shear zone where the hole at the upper end faces a hot
surface. Receiving radiation and measurement of
temperature continues until the hole passes through the
entire shear zone and then the tool flank.

28. 6- Infrared photographic technique
• This modern and powerful method is based
on taking infra-red photograph of the hot
surfaces of the tool, chip, and/or job and get
temperature distribution at those surfaces.
• Proper calibration is to be done before that.
This way the temperature profiles can be
recorded as indicated in Fig.
• The fringe pattern readily changes with the
change in any machining parameter which
affect cutting temperature.

29. Infra-red photographic technique