Technical Research Paper On Machinability Evaluation Of 316 Stainless Steel And Aluminium Using Drilling And Milling Operations

In Practical 1 you performed a set of experiments related to evaluating the machinability between two alloys under different machining operations, including an experiment to examine the surface roughness of the samples. Using the template provided, you will create a technical research paper based around these two experiments. Please read the carefully template in terms of how to write and structure your technical paper. Your paper will discuss the following elements.

• Machinability ? the focus of your paper will be based on assessing the machinability of two different materials, stainless steel and aluminium. Using the experimental data collected from the practical by any means necessary, you will identify and report and discuss in your paper, which material has the better machinability and discuss why?   
• Limitations to machinability ? based on your observations from the practical and from the experimental data collected, you will discuss what is ‘machinability’ and why this so difficult to quantify?  
• Surface Roughness Analysis – based on the obtained surface profile results for each of the machined surfaces, you will draw inferences as the machinability of the respective material based on the surface roughness profile. You will discuss why this measurement is important and what insights it can provide with respect to the machine tooling tracks and overall finish of the material. You will also discuss the differences that result from changes in material and machining parameters on the surface roughness.

Machinability and its importance

The term was originated around 1920 when life of tools is being considered as importance for economic viability, the engineers have found certain relation with these tools life with suitability of material and its ability to extended life. One of the procedure which is very common and readily adopted procedure is machining by chip removal, which holds the meaning of operation of metals with several lathe machine tools such as drilling, cutting and milling etc.

As discussed earlier, the potential advantage of cutting tools to perform different operation on metals with ability of metals to be machined is known as machinability. The decision making of machinability for material is not as simple as definition, it requires in depth study of material as well as theoretical part. There are several methods applied for deciding the parameters of machinability for certain material, some of these includes geometry of instrument, properties of apparatus, type of coolant during machining, the condition given for metals to be machined, and several others parameter. Most of the common operation done for the machining is drilling and milling, and especially this report we will include these two operations, so that we can decide the machinability of 316 stainless steel and aluminium. The speed of cutting and material of the device are the two imperious constraints while evaluating the machinability of a material. Materials having great machinability can be cut with less power and speed and surface finish will also good, makes less wear the instrument called as free machining (Eugene Thiele 2014).

The important reasons to study the 316 stainless steel and aluminium is it’s a very common form of metals which is being used for variety of application. By changing certain amount of composition in these two metals change the properties according to application and need of the Job. There are several techniques to analyse machinability. Some of these are by chip removal, quick stop technique, CDA’s Universal machinability index, classical standard technique etc. we are going through the procedure Quicks top techniques. The reason to choose these techniques is because of its simplicity, we can see the wear happened to material and tools with the help of magnifying glasses, and despite we have multipurpose CNC machine, which can perform different operation and provide accurate data with the help of dynamometer which is attached with it. The data obtained from these dynamometers will help in deciding the machinability of given metals easily.

Methods for determining machinability

As discussed earlier universal machinability test is more prominent in analysing machinability of steel and aluminium, but due to its requirement of enormous amount of resources it is limited scientific level. This is calculated from maximum production rate with giving constant time for tools and it is around 8 hours, this includes several workpiece materials this technique is out of scope at present (S. Yua 2016).

Another thing we would like to discuss about the machinability itself is complex process, it is difficult to clearly define the machinability and quantify it. The complexity and dependability of machining are based upon number of factors, most similar behaviour in material is being changed if its consideration is changed from tool life to cutting power or the quality of surface is good or bad.

For instance, the material for cutting tool is more important than material to be machined, both physical and geometry, that often machinability is expressed as “effective features of the work-tool amalgamation”. In this condition where number of parts being produced is large and not costly then protecting cutting tools is become important aspect of machinability, which significantly influences productivity and economy in machining, the extent of cutting forces which affects power feeding and dimensional accuracy surface finish the life of cutting tools also take part in performance of the industries, if appropriate material is given for cutting. For cutting tools point of view, the temperature arises due to cutting and formation of chip is important aspect of machinability.

For practical point of view all those aspects are not considered at a time, we are selecting some important aspect and categorise the material with few important aspects of machinability. For example, aluminium is soft, can be used where tough work in not required, whereas stainless steel is tough and can be used for longevity of life at rough places.  Most of the time the machinability criteria are selected according to material to be used for milling drilling purposes. The response given by the material is also important for material section, some of the material behave unacceptable after getting heated during process, therefore coolant is used very carefully for that material,

The computer numerical control most widely used machine for cutting operation has acronym (CNC). In this machine material removed from workpiece with the help of different cutting tools which can be fixed on CNC machine, almost all kind of material cutting is possible through CNC machine. All the data for cutting and feeding is numerically controlled through amplifying signal of the dynamometer which is attached to it. That is it is largely used for analysing the machinability of material, because we can record all the data from the dynamometer. The figure of CNC machine with different part is given below.

Choosing the right cutting tools and materials

There are commonly two type of CNC machine is available one is for drilling and another one is for milling. The determination of machining parameter by the operator of the machine is doing through G-Code, due to high level of tolerance and accuracy it is regarded as one of most manufacturing equipment in industries. Most of the cutting tools available for use in CNC machine includes Mill tolls, drill, tap face milling cutters etc. we can produce high level of geometry through this CNC machine.

For machinability test we must use two kind of machining operation, the first one is drilling and second one is milling. These operations will be conducted with the help of suitable carbide material cutting tools. The water tap is regularly flown to use is as coolant for the operation. This coolant helps to prevent the distortion in the tools, and prevent the property change in operating material, in case of out it is aluminium and steel. The ration of coolant in the water is about 20:1 or around 10 litre /min. The parameter for machine trial is described below.

The dimeter used for tools for milling operation is 12 mm, we have decided to keep the mill speed at 120 /min with the feed rate of 0.1 mm /tooth. The depth of cut given will be 1 mm to the workpiece and length of the cut will be 100 mm.

Diameter of tools is around 8 mm, with a feed rate of 0.1 mm/revolution, the depth of the cut given will be 10 mm and drilling speed will be 50 mm / min. As with the above given parameter we can perform the operation in controlled environment. All the operation is similar for both pieces.

Setting direction is one of the important things in CNC machine. For milling purposes, the axis of considered along horizonal direction i.e. in X and Y direction, but in case of drilling operation the direction will be downward i.e. Z direction. The movement of tools in drilling operation is unidirectional and it is upward or downward. The dynamometer of is of Kistler made amplifies the frequency generated during the operation and sent to record in the computer.

During operation we must keep an eye on chip generation on the workpiece. The shapes of pieces generated is continuous chips, lamellar chips, segmented chips and discontinuous chip. The method especially adopted as quick stop method, consists of increasing the speed of cutting tool operation suddenly with the help of captive bolt gun. When contact between tools and workpiece happen, the shear pin gets tipped off and frozen chip is being collected from the workpiece. The frozen chip is analysed, and tool wear has been decided.

Quicks top techniques for machinability evaluation

The analysis of tools with respect to wear and tear is one of the important primary analysis for machinability. This helps to determine toughness of material which is being passed recently. The observed tools in this regard is having flank wear and crater wear. This signifies that the tools contact was happened is with finished parts and crater wear is due to cutting failure of tools during the operation.  This means that the second material is tougher that first one.  

Optical surface profile is nothing but a kind of microscope which is used to measure the crest and trough on the surface of material which is to be tested. The basic principle of this microscope is that it is uses wavelength of light to look for the contrast of the wave and in comparison, with test surface and reference surface. The major parts of this profile consist of a shaft which is known as light shaft, a reflector which is used for reflecting the pillar like shape of from the test material which is passed through the objecting lens which magnifies before sending to the eyepiece. The other portion of the split bar is reflected from the surface which is known as reflector.

As per the analytical result obtained from frozen chip in quick stop method, we can say that the surface roughness value of steel and aluminium is as given below

Table 1 - Surface roughness as per chip

As per given data in the table the average Rc for aluminium (Al) = 0.5474

And for the average for steel = 0.49003

Rake angle for cutting tool  = 12^o

The average cutting force for Al = 45 N

The average cutting force for steel = 155 N

Shear angle   Putting the all the relevant value in this equation.

Similarly, for Shear angle for Steel =

We know that,

Shear angle  is given as  (where  = frictional angle)

The calculating  for aluminium = 16^o

And  for steel = 22^o

From Merchant’s circle solution we know that,

tan =  (Coefficient of friction), then for aluminium tan16^o = 0.2876

Similarly,   for steel tan22 = 0.404

Now we have Fc, , α

From Marchant’s circle solution we know that

Again, arranging the above equation, we get

(Putting the respective value in this equation

 = 3.18 N N

Similarly,

Further, from the equation,

Frictional force F = Fc*Sinα+Ft*Cosα (Putting the value for aluminium)

F(al) = 45*Sin12^o + 3.18*Cos12^o = 45*0.208+3.18*0.98 = 12.48 N

Using a computer numerical control (CNC) machine

Similarly, F(SS) = 155*Sin12^o + 27.3204*Cos12^o = 35.36 N

From above calculation the frictional force for aluminium is 12.48 N and for steel is 35.36 N

Aluminium is providing less resistance ass compared to stainless steel

6. Results

The data obtained from dynamometer is being graphed for aluminium and stainless steel, for each of the operation like Drilling and milling. During milling process the force applied along x axis which is pivot and Y axis which is Hub with respect to time is noted, similarly for drilling operation, the force applied along Z axis, and torques applied to rotate the spindle is noted with respect to time. Each operation was conducted for the time of 20 seconds, the results are as follows.

The operation parameter is given as follows for aluminium

Table 2 - Parameter for aluminum

Table 3 - Parameter for steel

The graph for drilling operation for aluminium is given below

As in the figure given above, the force required to  drill the aluminium is quite less than that of steel, second thing we can see that the drilling force for steel is equally distributed whereas force exerted on aluminium is most of the time negatively distributed, this is due to the elastic behaviour of steel, which cannot be seen in aluminium because of plastic behaviour.

The torque required for drilling is also is almost five time that of aluminium, first shows the negative side force due to compression and after middle portion penetration it goes to positive side, it is due to elongation. We can the graph for steel is denser, because there is always resisting force given by the material to the drill due to elastic behaviour of steel.

The force required for milling operation in steel is almost 2.5 time greater than the aluminium the force required for aluminium is in the range of (-180N <fx< -400) and force required for steel is in the range of (-600N <Fx <-800). Most of the time there is negative force exertion this is because of compression is also happening during milling operation.

The milling operation in the Fy direction also show the similar pattern clear gap between 0 line to negative side in both metals force exerted by milling tools for this operation is also similar aluminium (-180N <fx< -400) and force required for steel is in the range of (-600N <Fx <-800) as in Fx direction. The initial and final force is slightly more than that of force exerted in middle time. Based on the above data we can see that we must analyse both the result very minutely, the result is being discussed further which is as follows.

Surface roughness analysis

From the above figure, the force required for different operation for aluminium is quite less than force required for operation in 316 stainless steel. It means that any kind of machining in aluminium metals is easier than that of steel. In this condition we can conclude that aluminium is more machinable than steel. But in the other direction, we can think of why the force required for steel is more than that of aluminium. This is due to the reason that steel offers more resistance during operation. From this point of view, we can think of steel has more wear resistant than aluminium. The quality of steel is quite better in wear and tear machining operation; therefore, steel is superior material than aluminium where hardness and toughness is required we cannot use aluminium as a material in place of steel.

The criteria of machinability of material also includes that which material is going to be used for what purposes, the application of steel and aluminium area not common ground, 316 stainless steel is somewhat anti corrosive and generally used for tougher job like making ships hulls etc. whereas aluminium is fully anticorrosive and applied on the places where comparably less stress is generated.

 In the process of classical standard technique, we are monitoring the wear and tear of tools, and this become one of the criteria of deciding machinability for material. The quick removal technique passes from several changes in last 70 years. Most of them based on time reduction changes (Ravi Shankar 2005).

Conclusion

As from the above result of comparison and analyses of given graph and discussion on two different material one is 316 stainless steel and another one is aluminium. The formation of frozen chips reveals hat aluminium has better machinability than steel, the force of operation on steel keep the steel in disadvantageous position and. The surface roughness occurred in tool after operation also signifies that aluminium is better option for machinability. Less power consumption gives aluminium provide one more plus point. The study of surface roughness indicates that the crest and trough obtained in aluminium surface is even as in case of steel. The aluminium is smoother than steel. Only the hardenability and tough of steel providing more value to steel. On this basis we can say that the machinability property of aluminium is better than that of steel.

As discussed above if we consider according to the point of application for both material then in this condition we must again analyse the entire thing, and we must consider some more parameter with some world-renowned method of signifying machinability. Further analysis can take place as per requirement of material, cost consideration, and life of material.

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