Types of chips formed in the machining process
There are three main types Of cutting chips that Formed due to machining a workpiece by using a single-point cutting tool either on the Lathe machine Or Milling machine.
Different types of chips of various shape, size, color, etc. are produced by machining depending upon:
- Type of cut, i.e., continuous (turning, boring etc.) or intermittent cut (milling).
- Work material (brittle or ductile etc.).
- Cutting tool geometry (rake, cutting angles etc.).
- Levels of the cutting velocity and feed (low, medium, or high).
- Cutting fluid (type of fluid and method of application).
Whatever the cutting conditions can be, the chips produced may belong to one of the following three types.
1. Discontinuous chips:
2. Continuous chips.
3. Continuous chips with build-up edge (BUE).
4. Non-homogeneous chip
Types of chips in Metal Cutting:
The three common types of chips from a single point tool are.
1: Discontinuous or segmental chip:
These types of chips are usually produced when cutting more brittle materials like grey cast iron, bronze, and hard brass. These materials lack the ductility necessary for appreciable plastic chips formation. The material ahead of the tool edge fails in a brittle fracture manner along the shear zone.
This produces small fragments of discontinuous chips. Since the chips break up into small segments, the friction between the tool and the chips reduces, resulting in better surface finish. These chips are convenient to collect, handle and dispose off.
Discontinuous chips are also produced when cutting more ductile materials under the following conditions:
(i) large chip thickness.
(ii) low cutting speed.
(iii) cutting with the use of a cutting fluid.
(iv) small rake angle of the tool.
2: Continuous Chips:
These types of chips are produced when, machining more ductile materials. Due to large plastic deformations possible with ductile materials, longer continuous chips are produced. This type of chip is the most desirable, since it is stable cutting, resulting in generally good surface finish. On the other hand, these chips are difficult to handle and dispose off. The chips coil in a helix (chip curl) and curl around the work and the tool and may injure the operator when break loose. Also, this type of chip remains in contact with the tool face for a longer period, resulting in more frictional heat. These difficulties are usually avoided by attaching to the tool face or machine on the tool face, a ‘chip breaker’, The function of the chip breaker is to reduce 1 curvature of the chip and thus break it.
The following cutting conditions also help in the production of continuous chips :
- Small chip thickness.
- High cutting speed.
- Large rake angle of the cutting tool.
- Reducing the friction of the chip along the tool face, by: imparting high surface finish to the tool face, use of tool material with a low coefficient of friction, and use of good cutting fluid.
3: Continuous Chip with Built-up Edge:
- When machining ductile materials, conditions of high local temperature and extreme pressure in the cutting zone and also high friction in the tool-chip interface, may cause the work material to adhere or weld to the cutting edge of then tool forming the built-up edge.
- Successive layers of work material are then added to the built-up edge. When this edge becomes larger and unstable, it breaks up and part of it is carried up the face of the tool along with the chip while the remaining is left over the surface being machined, which contributes to the roughness of the surface.
- The built-up edge changes its size during the cutting operation. It first increases, then decreases, then again increases, etc. This cycle is a source of vibration and poor surface finish.
- Although the built-up edge protects the cutting edge of the tool, it changes the geometry of the cutting tool. Low cutting speed also contributes to the formation of the built-up edge.
- Increasing the cutting speed, increasing the rake angle, and using a cutting fluid contribute to the reduction or elimination of the built-up edge.
- Benefits of Continuous chip- After generation of continuous chips during machining the quality of the machined surface is quiet better (low surface roughness value) than discontinuous chips generation.
4. Non-homogeneous chip(Serrated chip)
- The temperatures generated in the machining zone control the rate of tool wear, the practical cutting speed, and the MRR.
- It is important to understand the factors which influence the generation of heat, the flow of heat, and the temperature distribution in the tool and work material near the cutting edge.
• Temperature is developed in three types of zones as
1. Shear zone
2. Tool-chip interface
3. Tool-work interface
Conditions in which above chips are generated –
Sr. no. Types of chips Conditions 1. Segmental or discontinuous chip Produced in brittle material machining and while cutting ductile material at low speed with high DOC. 2. Continuous chip Produced ductile material machining at normal cutting speeds with lower DOC. 3. Continuous chip with built-up edge Cutting ductile materials with high-speed steel tools at low cutting speeds. 4. Non-homogeneous chip Due to the higher temperature at the shear plane a large strain is developed at the tool chip interface at medium cutting speeds.
Some Questions and Answers :
1. Explain with sketch chip formed during machining cast iron.
Discontinuous or segmental chips
Machining of brittle materials like cast iron produces these types of chips. Small fragments are produced because of a lack in ductility of the material. Friction between tool and chip reduces, resulting in better surface finish.
2. Write the type of chip formed with the following factors.
(i) High rake angle
(ii) High cutting speed
(iii) Small depth of cut
(iv) Low cutting speed
(v) Large depth of cut
(vi) Low rake angle
(i) High rake angle: Continuous chips
(ii) High cutting speed: Continuous chips
(iii) Small depth of cut: Continuous chips
(iv) Low cutting speed: Continuous chips with built-up edge
(v) Large depth of cut: Continuous chips with built-up edge
(vi) Low rake angle: Continuous chips with built-up edge
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