ABSTRACT
In machining processes such as milling, turning, planning, grinding, honing and lapping, the phenomenon of chip formation is similar at the point where the tool meets the work. In these cases, it is crucial that chips are produced due to the desired machining processes. The study and analysis of chip formation is very important in order to determine the solutions for the speed rate, feed rate and other related parameters. This is necessary when we would like to consider the tool life, surface finish, machining time and many other criteria which may improve the manufacturing cost and efficiency. In this particular lab of chip formation studies, samples of chips were obtained by a machining process of a lathe machine using cylindrical mild steel as the work piece. Various parameters such as feed rate, feed speed and coolants were taken into account in order to analyze the relations between these factors with the type of chip being produced.
INTRODUCTION
In order to determine any desired dimension on a work piece, it has to be machined. The machining process can be present in many methods and in this case, it is determined by a machining process of a lathe machine. The process involved the reduction in the cross sectional area to see the relevancy between several parameters which have been selected, with the type of chips being produced. Mainly, there are three types of chips that often being produced and they are continuous, discontinuous and build up edge chips.
Continuous Chip
This particular type of chip is usually being produced due to machining of ductile materials such as Mild Steel, Aluminium and Copper. The problem faced due to continuous chips is that the chips are long and may become hazardous to the machine operator and also might affect the work piece that is being machined. However, this matter can be overcome by using chip breaker, where continuous chips will break into shorter length and thus reducing the risk of injury and also will not affect the work piece. Other than that, the surface finish that is being produced due to continuous chip is usually good.
Discontinuous Chip
Commonly, this type of chip is formed due to cutting hard and brittle materials as brittle materials cannot withstand high shear forces. Therefore, the chips that is being formed sheared away cleanly. The effect of discontinuous chips is that the chips being produced are in small pieces and may spread easily to the surroundings. That is why machine operator must wear safety goggle to avoid injuries of eyes. Other than that, the formation of discontinuous chip will cause tool chatter; due to the pressure at the tool tip increasing during chip formation. The surface finish of discontinuous chips is often poor and further machining is needed to make it better.
Built Up Edge
Built up edge or B.U.E. is produced when particle of the work piece is welded together to the rake face of the tool during cutting process take place. When B.U.E. is produced, this will significantly change the geometry of the cutting edge and the shear plane angle. This matter will lead to residual stresses in the material below the depth of cut and the more B.U.E. is being formed, it can deposit work hardened particles making it embedded in the finished surface. However, a thin, stable B.U.E. is generally considered desirable as this can tend to reduce frictional wear on the rake face of the tool.
PROCEDURES
- Clamp the work piece material securely in a chuck. The work piece material should not extend more that three times its diameter.
- Then install a cutting tool (Tungsten Carbide).
- Move the cutting tool off the part by backing the carriage up with the carriage hand wheel and then use the cross feed to set the desired depth of cut.
- Start the machine to cutting tool cut the work piece material. The different feed rates apply to produce different chip with cutting or without cutting fluid.
RESULTS
| NO. | Speed (rpm) | Depth of Cut (mm) | Usage of Coolants | Type of Chips | Chip Formation |
| 1. | 90 | 0.5 | No | Continuous |
|
| 2. | 90 | 0.5 | Yes | Continuous |
|
| 3. | 300 | 1.0 | No | Continuous |
|
| 4. | 300 | 1.0 | Yes | Continuous |
|
| 5. | 300 | 2.0 | No | Continuous |
|
| 6. | 300 | 2.0 | Yes | Continuous |
|
| 7. | 300 | 3.0 | No | Discontinuous |
|
| 8. | 300 | 3.0 | Yes | Continuous |
|
| 9. | 540 | 2.0 | No | Discontinuous |
|
| 10. | 540 | 2.0 | Yes | Discontinuous |
|
| 11. | 540 | 3.0 | No | Discontinuous |
|
| 12. | 540 | 3.0 | Yes | Discontinuous |
|
| 13. | 540 | 4.0 | No | Built Up Edge |
|
| 14. | 540 | 4.0 | Yes | Built Up Edge |
|
Table 1.0 The results of the chip formation.
DISCUSSION
1. RESULTS OF THE DEMONSTRATION
From the demonstration, it is obtained that 3 types of chips were being produced due to the parameters of cutting speed, feed rate and the usage of coolants. However, the resulted chips were unsatisfactory whereby the shapes were mostly the same and it is hard to indicate the types of chips that were being produced. As for the early samples of chips, it is easy to determine the type of chips being formed but later on when the feed rate and cutting speed is being increased, it is hard to notice the differences between those chips. The most obvious differences was that, the type of chip do vary in terms of coolants usage where chips produced with coolants flow showed smoother line and bright in colour while chips produced without the coolant flow seemed to have built up edge and the colour were sort of darker due to the high heat during the shearing process.
2. IMPORTANCE OF CHIP FORMATION KNOWLEDGE
In metal cutting process, knowledge of chip formation is very crucial whereby it can help to improve many factors of the machining process itself. By identifying the type of chips being produced in any metal cutting process, we are able to determine the affects and effects of the chip formation towards the work piece. Parameters such as feed rate, speed rate, rake angle and coolants must be taken into consideration in order to determine the desired dimension with optimum machining process. Besides that, this knowledge will also help us to tackle the problem raised due to the machining process and making us able to overcome them in order to obtain good quality end product with maximum production rate, longer tool life, and also with optimum production cost.
3. DRY MACHINING
Nowadays, there is a new method of machining work piece known as Dry Machining. This term refers to machining process without the presence of coolants fluids. There are many reasons that makes dry machining is more desirable than wet machining; machining process with coolants. One of the main factors is that coolants usage consumes the cost of more than 5 times compared to the cost of cutting tools. In engineering field, this is a tremendously expensive. Furthermore, chemical substances contained in coolant are very harmful to environment and machine shop workers. Besides that, by applying wet machining, a cutting fluid can stain the part of product or contaminate it. Considering an example of a medical implant, such as a ball joint for a hip where fluids are undesirable as there is a tendency of contamination. Since there is no coolant being used in the process, the cutting tool must be able to withstand the high temperature. Thus, Both ordinary tools and coated tools are used in dry machining. Ordinary high speed steel tools, aluminum ceramic tools, Cubic Boron Nitride (CBN), and ceramic can be used in machining various kinds of materials without coolant presence. Coating tools usually have longer tool life than non coated tools if coating is done properly to be effective. Coating provides insulation and lubrication in cutting. Materials used for coating cutting tools include TiAlN (Titanium Aluminum Nitride), TiN [7], and diamond. Diamond coated tools generally outperform other coated tools. Recently, multilayered coating tools have also been demonstrated to provide multiple functions. One layer is several nanometers thick. Each layer provides a specific function such as, heat insulation, anti wear, or lubrication. These three functions are the major functions provided by coated materials.
4. CHUCKS IN LATHE MACHINE
Chucks which functions as fixtures is located at the spindle axis of the lathe machine. This arrangement provides convenience in that parts can be mounted and dismounted quickly. There are many types of chucks being used in lathe machine, each has specific purpose and method of clamping work piece.
Three-Jaw Chuck
This type of chuck has three jaw chucks which moves dependently and able to hold cylindrical shaped work piece. These three jaw chucks able to hold both internal and external diameter of a work piece. A spiral gear meshes with cog teeth on the jaws to move all three jaws in or out simultaneously.
Four-Jaw Chuck
In four jaw chucks, there are 4 independent jaws present and it does not only limited to clamp cylindrical work piece, but also square, rectangular or odd shaped work pieces.
Figure 1.0 The three jaw chuck. Figure 1.1 The four jaw chuck.
Power Chuck
The power chuck is powered or actuated either hydraulically or pneumatically and often being used in automated manufacturing process, high production rates, and also robotic work piece handling. 
Figure 1.2 The power chuck.
5. IMPORTANT ASPECTS IN METAL CUTTING
Overall, important aspects in metal cutting can be divided into several parts. Firstly, the technique of machining or metal cutting process must be understood both theoretically and practically. This is important as in real life hands on, theory itself will not be adequate in order to execute metal cutting process. Secondly in terms of cutting tool selection, proper selection must be done considering the type of material to be machine and also the type of cutting tool required. Thirdly, basic parameters such as feed rate, speed rate, rake angle and coolants usage must also be taken into account. Without these aspects, metal cutting process will not be successful.
6. EFFICIENT CHIP MANAGEMENT
In other to execute metal cutting process with desirable chip formation, the steps as below must be taken into consideration.
- The usage of cutting tool such as inserts with chip breakers may be an advantage in order to obtain longer tool life and avoiding continuous chips.
- Selection of a minimum depth of cut about two thirds of the nose radius and maximum of one third about the cutting edge length. As for the finishing process, selection of cutting depths must be less than one third of the nose radius.
- The usage of coolants will also help to reduce the temperature due to the shearing process between the tool and the work piece. However, in dry machining, a stronger and more durable cutting tool is required as no coolants is present.
CONCLUSION
From the laboratory work, it is determined that various factor can produced variety types of chip formation. The factors can be cutting speed, feed rate and also the usage of coolants, not forgetting the rake angle of the cutting. However, it can be concluded that this particular laboratory work is not successful as the chips that were being obtained do not show significant differences from one to another. Inexperienced machine operator was being suspected to be the main factor whereby the ability to produced different types of chips is a failure. Due to this factor, the students were not able to identify and clarify clearly about the characteristics of the chip formation. Therefore, it is assumed that the samples were not accurate and bringing difficulties to be examined. In the other hand, even this laboratory in not successful, the knowledge that being obtained through observation of the chip formation process, plus additional information from reference books and also from the internet, made us students able to understand better about the study of chip formation.
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