The following points highlight the four main types of chips formed during metal cutting. The types are: 1. Discontinuous Chip 2. Continuous Chip 3. Continuous Chip with Built Up Edge 4. Inhomogeneous Strain-Chip.
Type # 1. Discontinuous Chip:
These chips are small segments which adhere loosely to each other and form slightly larger length. Discontinuous chips are formed when the amount of deformation which the chips undergo is limited by repeated fracturing.
It has been found that segments are regularly formed due to rupture of the metal ahead of the tool. Due to rupture taking place when the material directly above the tool face is compressed to such an extent that the deformed metal starts sliding along the face and the magnitude of compression force reaches the fracture limit of the metal.
This type of chip is obtained by machining hard and brittle metals like bronze, brass and cast-iron. Sometimes, cutting of ductile metals at very low feeds with small rake angle of the cutting tool and high speeds and high friction forces at the chip tool interface also result in the production of discontinuous chips.
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Discontinuous chips in ductile materials are formed when the hydrostatic pressure near the cutting edge is tensile or the shear energy reaches a critical value. The formation of this type of chip in brittle materials imparts good finish, increases tool life and consumes less power. Presence of discontinuous chips in ductile-materials results in poor-finish and excessive tool-wear. Smaller chips are easier to dispose off.
If discontinuous chips are produced from the brittle materials, then surface finish is fair, power consumption is low and tool life is reasonable. However when these are produced with ductile materials, then finish is poor and tool wear is excessive.
Type # 2. Continuous Chip:
In continuous chip formation, the pressure of the work-piece builds until the material fails by slip along the slip plane. The inside of the chip displays steps produced by the intermittent slip, but outside of the chip is burnished smooth by chip rubbing on tool surface.
It has its elements bonded together in the form of long coils and is formed by the continuous plastic deformation of metal without fracture ahead of the cutting edge of tool and is followed by the smooth flow of chip up the tool face.
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The chips so obtained have same thickness throughout. This type of chip is obtained by machining ductile materials at very high cutting speed. Mild-steel and copper are considered to be most desirable for obtaining continuous chips.
Sometimes, continuous chips are produced at low cutting speed if effective cutting fluid is used because this type of chip is associated with low friction between the chip and the tool. Since finish is best, power consumption is low and tool life high with this type of chip, this is most preferred type. The only problem is of chip disposal which to some extent can be tackled by the use of chip breakers on the cutting tools.
Type # 3. Continuous Chip with Built Up Edge:
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This type of chip is very similar to that of continuous type, with the difference that it is not as smooth as the previous one. It has a built up edge, adhering on the nose of tool. The built up edge changes the effective geometry of cutting. It is obtained by machining ductile metals with high speed tools at ordinary cutting speeds, thus introducing high friction between the chip and tool face.
The form and size of such an edge depends largely on the cutting speed, being absent at very low and very high cutting speeds. This type of the chip is associated with poor surface finish, but protects the cutting edge from wear due to moving of chips and the action of heat, causing an increase in tool life.
The main factors that are responsible for the formation of built up edge are cutting speed, rake-angle of tool, condition of cutting edge, coarse feed, insufficient cutting fluid, etc. The formation of built up edge can be reduced by machining metals at higher cutting speeds, because it slows down the formation of built up edge due to low friction at the tool chip interface.
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The formation of built up edge can also be reduced by making the tool face smooth, by using a material with a low coefficient of friction with the workpiece material, and by using an efficient cutting fluid. Tendency of welding can be reduced by using non-metallic tool material.
The back of the chip (which is in contact with the tool face) gives a fairly good indication of the built up edge condition. If no built up edge is there, then back of chip should be clean, smooth and highly burnished.
Following points may be noted regarding built up edge:
(i) Built up edge may be formed initially by high friction forces existing on the rake face which may cause adhesion to occur. These friction forces cause the material to reach its shear flow stress along a line inclined to the rake face; a velocity discontinuity occurs and wedge shaped particles are left on the rake surface.
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Built up edge may also be formed by bluntness of the tool edge, i.e. wear at point of the tool which may result in the formation of a dead metal zone. The subsequent growth of built up edge at a given speed depends on the work hardening properties of the metal.
(ii) Adhesion can be inhibited by using a polished tool, or under certain conditions by the application of a cutting lubricant.
(iii) The shape of built up edge is a function of temperature and hence cutting speed.
(iv) Positive rake angles on tools decrease built up edge at low cutting speeds and negative rake angles decrease built up edge at high cutting speeds.
(v) At low cutting speeds, dimensions of build-up edge increase with increase in feed. At higher speeds, build up edge first increases with increase in feed but after a particular value it decreases.
Type # 4. Inhomogeneous Strain-Chip:
This type of chip is produced by machining hard alloys like titanium which suffer a marked decrease in yield strength with increase in temperature.
Chip Breaker:
It is small step or groove ground into face of tool and sometimes separate piece is also fastened to the tool or tool-holder to act as chip breaker. These devices cause the chip to curl and break into pieces of short sections. Continuous chip is not desirable.
