SEM722 Advanced Manufacturing Technology

Machining is the process of removal or cutting away part of a material. Usually referred to as a workpiece. It includes grinding and other advanced forms of machining processes which include electrical, use of lasers, hydraulic and chemical processes. Drilling on the other hand refers to majorly boring of holes on a material. It involves introducing or enlarging of existing holes by means of drilling tools which are referred to as drill bits. Drilling is an important metal cutting operation and accounts for about 33% of the works performed on a metal.

Fig 1: Diagram of Cutting Process   Fig 2: Diagram of Drilling Process

Aluminum is a naturally occurring metal that is extracted from the earth’s core from Bauxite. Which is an ore of Aluminum. As a metal it is ductile however the properties can be changed by reacting it with different elements to achieve better properties. Aluminum 1045 is a classification of metal that contains at least 99% aluminum and other metal alloys. When machining metals the limitation of the cutting tools is dependent on work piece strength, the heat properties of the tool, the speed of cutting, ductility etc. Aluminum alloys have relatively low shear strength compared to steel alloys. Steel is an alloy of iron and primarily carbon. Steel is known to be one of the materials with high tensile strength and low cost of production (Mário C. Santos Jr1 & Alisson R. Machado2, 2016). This properties make it preferable as a material widely used as a raw material in buildings. Due to the different properties of both of the materials they exhibit different reaction to drilling and milling processes. The differences are notable in chip formation, pressure applied when drilling and use of different feed rate. (Elso Kuljanic, 2010)

Machining can be good when production of some items and useful as it archives a relatively nice surface finish depending on the size of the material feed. It can also be used for archiving accurate dimensions of final products for example screw threads, accurate or round holes. However, machining has various limitations which will be discussed for both of the selected work pieces. The process can be wasteful as the chips formed after machining are largely unusable. The process of machining can also be time consuming, generally as noted the process is not as time efficient as powder metallurgy or forming processes. Machining operations on a workpiece are mostly determined by the hardness of a material (Committee, 1989). Materials that exhibit properties of being harder require a lesser feed while machining and will generally take a longer time to machine than softer materials. The feeding of the workpiece is normally on an axial direction. Before working on the machine it is important to ensure the various parts of the machines are well familiarized with. This will not only ensure safety while working but also be vital in completion of the task successfully. The following itemized steps are taken for working on drilling the two types of materials. Figure 3 below is an example of a milling tool. This is a lathe machine. The cutting tool is held on the tool base while the work piece is secured in the chuck which rotates. The feed shaft is then used to move the tool post along the lead screw. To control the feed angle the tool post is adjustable and calibrated to incline the angle of the tool.

 

Fig 3: Basic Parts of a lathe machine

In the analysis below the machinability of two materials Aluminum and Steel is discussed.

Machinability is a term referred to mean the ease of a given material to be worked on or cut using a cutting tool. According to American Iron and Steel Institute (AISI) many metal alloys were tested using normal cutting speeds, tool life and surface finish was compared to that obtained when machining material B1112 which is steel with 100% machinability. (Marinov, 2005)

The experiment was set up by first securing the tool in the tool post. The tool was then inspected to determine whether there was significant wear. Usin Allen keys the work piece was secured in the chuck. Care was taken to ensure the tool was fixed secure and parallel to the lead screw. The tail stock was used during the aligning. The next step was to adjust the angle of the tool relative to the work piece. This would form the rake angle. The experiment was done for aluminum and steel.

Figure 4: Graph of Milling Aluminum 1

Figure 5: Graph of Drilling Aluminum 6061

Figure 6: Graph of Drilling Steel 1045

Figure 7: Graph of Milling Steel 1045

Materials which scored more than 1 had excellent machinability and materials with lesser than one had lower machinability ratio. In this study most aluminum alloys scored 1.5 – 2.0 which showed high machinability. Steel alloys scored below the 1. This is due to steel having a high Brinells hardness number. If tool life is objective in the analysis of machinability of a material then a material which is good in machinability should result in a long tool life when it’s being worked on (Irvin Paul, 2015). This is crucial for economic purposes, most cutting tools are expensive and constant replacement would not be ideal.

The following were extracted from the experiment when comparing the two materials.

1. Aluminum required relative low cutting force compared to stainless steel.
2. Metal removal rate was fairly even as the rake angle was set equal while working on both work pieces.
3. While working on aluminum the tool wear was lower, the cutting tool was observed under a microscope. See Fig. 4
4. The finish surface of aluminum was poorer than that of steel.
5. The geometric characteristics of the aluminum work piece was un-even. The finish of steel was more consistent.
6. Steel gave a good curl and break out than aluminum. This was due to ductility of aluminum.

Given the observations from the experiment the conclusion was that based on the requirement of the finish, which could be an important factor to consider. Steel 6000 lead to shearing of the cutting tool (Producers, 2010 ). This could be attributed to the material being hard. On machining this material one would incur high costs in replacement of the cutting tool. Aluminum has better machinability.(Tawela, 2003)

Aluminum 1045 falls under the category of mechanically workable aluminum alloys. This means machining, drilling and other mechanical processes can be applied to the alloy. The surface finish of aluminum improves with increase of the rotating speed of the work piece, this is because the contact area reduces when shearing.

Cutting Speed: Cutting speed has a direct impact on the tool life. When using high cutting speeds it could lead to higher flank wear due to presence of hard particles in the alloy. To minimize this problem application of minimal lubricant can greatly reduce this. A decrease in cutting force also causes a decrease in material roughness.

 

 

Fig 8: Machine Tool Wear x100              Fig 9:Machine Tool Wear x350

Feed Rate: This is another Limitation to machining. Increasing the feed rate leads to increase in the shear forces of the tool. This also generates more heat as the friction between the two surfaces increase. Consequently the tool becomes blunt and reduces the quality of finish of the surface of the work piece. Also feed rate determines the finish of the surface with higher feed rate attributed to better surface finish. As seen in figure 4 and 5 the surface of the tool is worn out on continuous use of the cutting tool. (Aliso Machado, 2016)

Material hardness: This is limitation to machinability is the hardness of the material in that it could lead to reduction in material tool life. As seen in the experiment there was considerable damage to the work piece. Steel 6000 has a high Brinells hardness number this property of steel being hard lead to the damage of the work piece. The solution to this would be use of extra hard material which can be expensive to maintain or replace when damaged. Figure 6 shows the damaged drill bit due to the nature of the work piece being hard, the tool wear is significant.

Fig 10: Sheered milling tool      Fig 11: Damaged drill bit

Workpiece: The micro structure of the metal, chemical composition and physical properties of the workpiece will majorly influence its machinability. Variations in the micro structure can greatly change the final result when machining. (J. T. Lin, 1996)

Work Hardening Properties; Mechanical processes such as heat treatment, rolling and drawing has a large influence on the final structure of the metal and therefore its physical characteristics.

Tool Material: This can be factors such as hardness, resistance to wear etc. as seen in the experiment the work piece was damaged while working on steel.

Analysis of chip formation is crucial in machining processes in a study by Tresca (1978) it was observed that finer chips formed were responsible for more plastic deformation on the surface than deeper cuts. This lead to the conclusion that deeper cuts were more suitable hence the development of stiffer tools used for machining. (J, 1956)

The quick stop method is a type of analysis used that involves suddenly stopping of the machine and removal of the tool to analyze the chip formation at a certain point. Preferably the speed of removal of the tool and work piece should be greater than the cutting speeds involved. The principle of the squick stop is use of a shearing pin which provides explosive movement to separate the tool from the work piece. The challenge in this is constant breakage of the chip when separating. (Azmi, 2007)

When machining the geometry of the feed mark depends on factors such as the side cutting edge angle, end cutting and nose angle of the tool. The speed of cutting also determines the size of chips formed as seen in the experiment. When machining aluminum the diameter of the chip formed was seen to increase with increase of the feed. At lower feed rate the chip formed is fragmented. This is as the chip approaches the work piece the tool chip formed is compressed and deforms plastically (Vinayak Neelakanth Gaitondea, 2015). The material behaves like a brittle material hence the observation. When the speed is increased the curls in the chips formed increases considerably. (Radhika N, 2014)

 

Fig 12: Sample chip formation high speed[3]   Fig 13: Sample chip formation low speed [3]

Fig 14: Sample chip formation high feed[3]    Fig 15: Sample chip formation low feed[3]

The geometry of the tool and the feed speed are two independent items that affect the chip formation which has an effect on the surface roughness of the work piece (Smolenicki, Boos, Kuster, & Wegener, 2012).  When performing the experiment chips that were ‘saw’ shaped were also tied to a better surface finish. This was largely the case although increasing the speed caused the surface to increase in roughness. At low speeds the chips formed were segmented and discontinuous (Usama Umerl, 2008).  Figure 9 shows the chip profile of high speed cutting. Consequently figure 11 shows the chips formed when a higher feed rate used. 

To understand machinability of materials numerous tests and observation of the workpiece results need to be undertaken. In the technical research paper the different elements of machining were discussed including cutting speed, material feed, finish, chip formation and limitations to machining processes. Aluminum is largely machinable but the surface finish could be improved. Steel on the other hand is seen to have a large effect on tool wear.

1. When machining aluminum the tool life is longer than when machining steel.
2. The surface finish is improved at medium speeds
3. Higher feed rates improve finish however it could shear the tool
4. Machining has limitations and only some materials can be machined
5. The chips formed depend largely on speed and tool angle

[1]Aliso Machado, W. F. (2016). Machinig of Aluminum Alloys: A Review . International Journal Of Advanced Manufacturing Technology .

[2]Azmi, A. I. (2007). Chip Formation Studies In Machinign Fiber Reinforced Polymer Composites . Perlis: University Malaysia Palis.

[3]Committee, A. H. (1989). Machining of Aluminum and Aluminum Alloys . ASM International®.

[4]Department, M. E. (2011). Standard Operating Proceedures For Manual Milling Machines . Califonia: University of Califonia Riverside.

[5]Elso Kuljanic, M. S. (2010). MACHINABILITY OF DIFFICULT MACHINING MATERIALS . Trends in the Development of Machinery and Associated Technology , 2-8.

[6]Irvin Paul, S. D. (2015). Machine Tools for Machining. Auckland : Springer-Verlag London Ltd.

[7]J, A. F. (1956). Chip Formation Fundermentals. New York.

[8]J. T. Lin, ”. D. (1996). CHIP FORMATION IN THE MACHINING OF SiC- PARTICLEREINFORCED. Auckland: Elm&r Science Limited.

[9]Marinov, V. (2005). MACHINABILITY. In V. Marinov, Manufacturing Technology (pp. 5-6).

[10]Mário C. Santos Jr1 & Alisson R. Machado2, 3. &. (2016). Machining of aluminum alloys. International Journal of Manufacturing Technology , 1-9.

[11]Producers, C. o. (2010 ). Stainless Steel For Machining . Washington D.C.

[12]Radhika N, S. ,. (2014). Analysis of Chip Formation in Machining Aluminum Hybrid Composites . Journal Of Scientific Research , 10-18.

[13]Smolenicki, D., Boos, J., Kuster, F., & Wegener, K. (2012). Analysis of The Chip Formation of Bainitic Steel in Drilling. Zurich.

[14]Tawela, A. (2003). Machinability Studies of High Strength Materials. Dublin.

[15]Usama Umerl, L. X. (2008). FINITE ELEMENT CHIP FORMATION ANALYSIS. Karachi.

[16]Vinayak Neelakanth Gaitondea, S. R. (2015). Machinability Evaluation in Hard Milling. Rio .