In this article we will discuss about the meaning and geometry of chip formation from metals in the industries.
Meaning of Chip-Formation:
In any machining operation, the material is removed from the workpiece in the form of chips, the nature of which differs from operation to operation. As the form and dimensions of a chip from any process can reveal lot of information about the nature and loyalty of process, the analysis of chip formation is very important. Chips are formed due to tearing and shearing.
In the process of chip formation by tear, the workpiece material adjacent to tool face is compressed and a crack runs ahead of cutting tool and towards the body of the workpiece. The chip is highly deformed and workpiece material is relatively under-formed. Cutting takes place intermittently and there is no movement of the work-piece material over the tool face. In chip formation by shear, there is general movement of the chip over tool face.
As the tool advances into the work-piece, the metal ahead of the tool is severely stressed. The cutting tool causes internal shearing action in the metal, such that the metal below the cutting edge yields and flows plastically in the form of chip. Firstly compression of the metal under the tool edge takes place and then follows separation of metal, when compression limit of that metal has been exceeded.
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Plastic flow takes place in a localized region called shear plane, which extends from the cutting edge obliquely upto the uncut surface ahead of the tool. When the metal is sheared the crystals are elongated, the direction of elongation being different than from that of the shear.
It may be mentioned that the deformation of metal in the process of separation of chip, does not occur sharply across the shear-plane. The grains of the metal ahead of cutting edge of tool start elongating along the line AB and continue to do so until they are completely deformed along the line CD.
The region between the lines AB and CD is called shear-zone. After passing out the shear-zone, the deformed metal slides along the tool face due to the velocity of the cutting tool. For all mathematical analysis this shear zone is treated as a plane and is called a shear-plane.
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The angle made by plane of shear with the direction of tool travel is known as shear angle (f). Its value depends on the material being cut and the cutting conditions. If (f) is small, path of shear will be long, chips will be thick, and the force required to remove the layer of metal of given thickness will be high and vice versa.
Geometry of Chip Formation:
When a wedge shaped tool is pressed against the workpiece, chip is produced by deformation of material ahead of cutting edge because of shearing action taking place in a zone (treated as single plane) known as shear plane. Shear plane separates the deformed and under-formed material.
When the tool moves with the velocities V against the work, it shears the metal along the shear plane AB. The depth of cut t which is actually the feed in turning operation changes into the chip thickness tc. This experiences two velocity components Vc and Vs (Velocity of the chip relative to the tool, and velocity of the chip relative to the workpiece along the shear plane). The former is acting along the tool face and the latter along the shear plane.
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In accordance with the principle of kinematics, these three velocity vectors form a closed velocity triangle ABD as shown in Fig. 22.13 (b). It may be noted that the vector sum of cutting velocity V and the chin velocity Vc is equal to the shear velocity vector Vs.
The relationship between chip thickness ratio (r) and the shear angle can be obtained from the Fig. 22.13 (a). This chip thickness ratio is defined as the ratio of the depth of cut (t) to the chip thickness (tc). It may be noted that in Fig. 22.13 (a), AF which is perpendicular to tool chip interface represents tc, i.e. chip thickness.
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From the right-angle triangle ABG in Fig. 22.13 (a), AB = (t/sin f) and from the right-angle triangle ABF, AB = tc /sin (f – α).
Comparing these two equations we have chip- thickness ratio:
(1/r is termed chip reduction coefficient or chip compression factor and is denoted by K) and shear angle 0 is given by the equation given below:
The cutting ratio or chip thickness ratio is always less than unity and can be evaluated by measuring chip thickness and depth of cut. But actually it is very difficult to measure chip thickness precisely due to the roughness of the back surface of chip.
Since the volume of metal removed = volume of chip.
We have tbl ρ = tcbclcρc
(t, b, I, ρ being thickness or depth, width, length, and density of metal cut and c standing suffix for chip).
It is found that width of chip is same as of workpiece and also density of both is same.
Chip thickness ratio or cutting ratio ‘r’ = t/tc = lc/l
It is easier to measure the length of chip than thickness of work.
Cutting ratio can also be defined as the ratio of the chip velocity Vc to the cutting speed V. This ratio i.e. (Vc/V) can be determined mathematically by finding the kinetic forces acting on the chip. The forces acting on chip (Refer Fig. 22.15) are static normal force N and tangential force F.
Normal force N is balanced by centrifugal kinetic force m (v2/R) = m V (dθ/dt), and tangential force F is balanced by the
The shear angle can be measured by measuring chip- thickness, depth of cut and rake angle of the tool. This can be most conveniently solved with the aid of monogram shown in Fig. 22.14 for determining shear angles.
When f is small, tc is large in comparison to t which gives low cutting ratio and a long shear-plane. On the contrary when f is large, tc is small and hence high cutting ratio. In other words it approaches unity. It is always desirable to obtain a shortest possible shear plane, because for a fixed shear strength, reduction in shear plane area reduces the shearing forces required to produce sufficient stress. It has been observed that with carbide tools better cutting conditions can be achieved with negative rake-tools and high cutting speeds.