There are 3 regions/sources of heat generation in machining;
- Shear or primary deformation zone
- Tool chip interface or secondary deformation zone
- Tool workpiece interface zone
I have explained these sources of heat generation below in detail.
But before that, let me give you some introduction regarding this.
Machining is a complex mechanism and so it is hard to predict the intensity of heat source and distribution of heat sources in different machining processes.
This prediction of heat generation becomes difficult because the properties of metals vary with temperature.
In simple words, the mechanical process and the thermal dynamic properties are closely related to each other.
In the elastic deformation of metals, the energy required for the process is stored in the material as a strain energy and hence no heat is generated during the elastic deformation process.
But in the plastic deformation process, the applied energy gets converted into heat energy.
In case of metal cutting, the workpiece is subjected to very high strains and most of the energy is converted into heat.
Sources of Heat Generation in Machining
There are 3 regions/sources of heat generation in machining process.
#1) Shear or primary deformation zone
The heat generated in primary deformation zone is because of two reasons;
- Plastic deformation
- Viscous dissipation
Around 60% of the heat generated in the machining process is in the primary deformation zone.
Mathematically, the heat generated (Qp) is the product of shear plane component (Fs) of the resultant force and shear velocity (Vs), since the shear energy is converted into heat.
Qp = (Fs × Vs) / (b × t1)
b = width of cutting,
t1 = thickness of uncut chip.
#2) Tool chip interface or secondary deformation zone
The heat generation in secondary deformation zone is due to chip distortion.
The separation of chip from the workpiece takes place in the direction perpendicular to the tool face.
So when the chip flows along the rake surface, it has to bend at the right angle.
During this chip flow, the inside layer of chip bends and the outer layer of the chip contracts.
Because of this, the grain distortion takes place in the chip that leads to friction between the heated chip and tool face.
This friction generates heat and it’s about 30% of the total heat generated.
#3) Tool workpiece interface zone
The heat generated in this region is due to friction between the tool flank and workpiece surface.
Around 10% of heat generation takes place in this region.
Mathematically, the rate of heat generated (Qr) in this region is given by the formula;
Qr = (F × Vc) / (b × h)
F = frictional force,
Vc = velocity of chip along the interface,
h = plastic contact length.
Most of the heat generated in machining process is carried away by the chip (around 70%).
Remaining heat is absorbed by the workpiece (around 15%) and the cutting tool (around 15%).
If the cutting fluid is used during machining operation, then some portion of heat will be carried away by it.