Chips are formed due to tearing and shearing. In the chip formation by tear, the work piece material adjacent to the tool face is compressed and crack runs ahead of the cutting tool and towards body of the work-piece. The chip is highly deformed and the work-piece 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 a general movement of the chip over tool face.
The grains of metal ahead of cutting edge of tool start elongating along line AB and continue to do so until they are completely deformed along line CD. The region between the lines AB and CD is called shear zone. After passing over shear zone, the deformed metal slides along the tool face due to the velocity of the cutting tool.
The angle made by plane of shear with the direction of tool travel is known as shear angle. Its value depends on the material being cut and the cutting conditions. If 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.
Types of chips:
Every machining operation involves the formation of chips, the nature of chips differs from operation to operation, properties of work-piece material and cutting condition.
|Types of chips: Discontinuous chips, Continuous chip, Continuous chip with built-up edge
Chips are formed due to cutting tool, which is harder and more wear resistant than the work-piece material, relative motion between tool and work-piece, sufficient force and power to overcome the resistance of work-piece material. The chips are formed by the deformation of the metal lying ahead of cutting tool edge by a process of shear. Basically there are three types of chips
1. Discontinuous chips:
This type of chips is produced during machining of brittle materials like cast-iron and bronze. These chips are produced in the form of small segments.
In machining of such materials, as the tool advances forward, the shear-plane angle gradually reduces until the value of compressive stress acting on the shear plane becomes too low to prevent rupture.
At this stage, any further advancement of the tool results in the fracture of the metal ahead of it, thus producing a chip. With further advancement of the tool, the processes of metal fracture and production of chips goes on repeatedly producing discontinuous chips. Such chips are also sometimes produced in machining of ductile materials, when low cutting speeds are used and adequate lubrication is not provided.
This causes excessive friction between the chip and tool face, leading to fracture of chip in small segments. This will also result in excessive wear on the tool and poor surface finish on the work-piece. Other factors responsible for production of discontinuous chips are smaller rake angle on the tool and too much depth of cut.
2. Continuous chip:
This type of chip is produced while machining a ductile material, like mild steel and copper at very high cutting speed and minimum friction between the chip and the tool face.
The friction at the chip-tool inter face can be minimized by polishing the tool face and adequate use of coolant. The basis of production of a continuous chip is the continuous plastic deformation of the metal ahead of the cutting tool, the chip moving smoothly up the tool face.
Other factors responsible are bigger rake angle, finer feed and keen cutting edge of the tool.
3. Continuous chip with built-up edge:
It is very similar to the continuous type and not as smooth as continuous chip. It has a built-up edge adhering on nose of the tool, which 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 chip results in poor surface finish. The normal reaction of the chip on the tool face is quite high, and is maximum at the cutting edge or nose of the tool. This gives rise to an excessively high temperature and the compressed metal adjacent to tool nose gets welded to it. The chip is also sufficiently hot and gets oxidized as it comes off the tool and turns blue in colour. The extra metal welded to tool nose or point of the tool is called built-up edge.
This metal is highly strain hardened and brittle. With the result, as the chip flows up the tool, the built-up edge is broken and carried away with the chip while the rest of it adheres to the surface of the work-piece, making it rough. Due to the built-up edge the rake angle is also altered and so is the cutting force. The common factors responsible for formation of built-up edge are low cutting speed, excessive feed, small rake angle and lack of lubricant.
Adverse effects of built-up edge formation:
a) Rough surface finish on the work-piece.
b)Fluctuating cutting force, causing vibrations in cutting tool.
c) Chances of carrying away some material from the tool by the built-up surface, producing crater on the tool face and causing tool wear.
a) The co-efficient of friction at the chip-tool interface should be minimized by means of polishing the tool face and adequate supply of coolant during the cutting operation.
b) The rake angle should be kept large.
c) High cutting speeds and low feeds should be used, because at high speeds the strength of the weld becomes low. Similarly, at very high temperature also the strength of the weld becomes low.