Experimental Study On Drilling Of 6061 Aluminium Alloy Biology Essay

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Drilling is a widely used fabrication process in all industries. Aluminium 6061 is a hardened and extremely versatile heat treatable alloy. It has good mechanical properties and widely used in automobile, aerospace, manufacturing and construction sectors. A set of optimum parameters for the drilling on 6061 aluminium alloy can prevent the failure of products to be happened.

The objective of this research is to investigate the optimum parameters for dry drilling process through response surface methodology (RSM). In RSM, central composite design (CCD) approach is chosen and it allowed the determination of levels of various parameters to be carried out with the interrelation between each parameter evolved simultaneously. The parameters selected in this research are feed rate, cutting speed and drill bit diameter. Surface roughness is an important factor to assess the quality of manufacturing process. Therefore, the drilling characteristics were evaluated in terms of surface roughness. This drilling experimental study was conducted using computer numerical control (CNC) machining centre. Profilometer with stylus method was used to evaluate the surface finished. Besides, profile projector was used to measure the roundness of each surface finished. It was found that bigger diameter with lowest cutting speed and feed rate, the surface roughness (Ra) had the highest value.

Methodology

Machine

The Experiment was conducted in computer numerical control (Mazak) machining centre. The experimental parameters selected based on the machine specification and they are feed rate, cutting speed and drill bit diameter.

Material

The material used is aluminium alloy 6061. The primary reasons for selecting 6061 are its acceptable mechanical properties coupled with its relative ease with which it can be cast, extruded, rolled, machined, etc., market acceptance and relative ease of development [1]. Aluminium alloy 6061 of 170 mm x 100 mm x 20 mm is used in this experiment.

Statitical analysis software

Minitab is statistical analysis software. Central Composite Design (CCD) approach is chosen to generate the optimum parameters. The use of CCD allowed determination of levels of various parameters to be carried out with the interrelation between each parameter evolved simultaneously [2].

Drill

High speed steel (HSS) drill bit is used in this experiment. Twist drill is chosen for the drilling process. Drill bits diameter in this experiment are 10mm, 12mm and 14mm.

Surface roughness, roundness measuring instrument

Profilometer with stylus method is used in the surface roughness measurement. Surface roughness, Ra is measured for each surface finished. Profile projector is used for the roundness measurement. Each hole is projected on the screen. The diameter of each hole is measured.

Experiement Procedure

1. The specimen is clamped tighly in the machine.

2. Drill bit is clamped in the chuck.

3. The reference point of the specimen is set .

3. Feed rate, cutting speed and drill position is set.

4. Make sure the drilling process is in enclosure

condition.

5. Eighteen holes are drilled in different cutting speed, feed rate and drill bit diameter according to the parameters obtained from the Minitab.

6. Surface roughness, Ra for each hole is measured through profilometer with stylus method.

7. Roundness of each hole is measured through the profile projector.

Results and discussion

Tables 1 and 2 shows experiemntal parameters coded values and experiemntal table and response values.

Table 1: Coded Values of Parameters

Code

Feed (mm/min)

Speed (rpm)

Diameter (mm)

-1

200

200

10

0

550

550

12

1

900

900

14

"-1" lower value; "0" middle value; "+1" higher value

Table 2: Experiemnt Parameters and Response (Suface Roughness) data

Coded

Response

Block

Feed

Speed

Diameter

Ra

1

1

-1

-1

-1

0.822

2

1

1

-1

-1

5.486

3

1

-1

1

-1

8.075

4

1

1

1

-1

4.730

5

1

-1

-1

1

10.600

6

1

1

-1

1

8.200

7

1

-1

1

1

6.816

8

1

1

1

1

9.913

9

1

-1

0

0

5.971

10

1

1

0

0

4.416

11

1

0

-1

0

9.278

12

1

0

1

0

5.659

13

1

0

0

-1

8.169

14

1

0

0

1

7.813

15

1

0

0

0

6.380

16

1

0

0

0

7.101

17

1

0

0

0

5.890

18

1

0

0

0

8.004

Table 3: Analysis of Variance for Ra

Fig. 1. Contour plot of speed and feed diameter of the tool constant

Fig. 2. Surface plot of speed and feed diameter of the tool constant

From the plots above, the lowest surface roughness value, Ra occurs at lower feed (feed of "-1") with lower speed (speed of "-1) or higher speed (speed of "+1") and higher feed rate (feed of "+1"). At higher cutting speed (speed of "+1") and middle range of feed rate (feed of "0"), Ra increases. At high cutting speed, there was occurred poor surface because of high vibration [3]. When cutting parameters increases, the surface roughness can be high (poor final shape of hole) because of drilling vibrations and the chatter which is an important factor for quality of produce holes[4]. Ra value is decreasing at higher feed (feed of "+1) and lower feed (feed of "-1"). However, Ra reaches higher value at higher speed (speed of "+1") and middle range of feed (feed of "0"). The maximum value of R is reached when the cutting speed is higher speed (speed of "+1") and the feed is middle range (feed of "0").

Fig. 3. Contour plot of diameter and speed, feed constant

Fig. 4. Surface plot of diameter and speed, feed constant

From the Figs. 3 and 4, Ra is increasing when the diameter of the tool and cutting speed are increasing. The Ra give the highest value when lower cutting speed (speed of "-1") with bigger diameter of the tool (diameter of "+1"). At bigger diameter of the tool (diameter of "+1"), Ra is increasing when the cutting speed is decreasing. At middle range of diameter (diameter of "0") and lower range of diameter (diameter of "-1"), Ra increases when the cutting speed is increasing.

Fig. 5. Contour plot of diameter and feed, speed constant

Fig. 6. Surface plot of diameter and feed, speed constant

From the Figs. 5 and 6 above, Ra is reducing at lower feed rate with smaller diameter of the tool and higher feed rate (feed of "+1") with smaller diameter of the tool(diameter of "-1"). The Ra reaches maximum value at higher diameter of the tool and lower (diameter of "+1") and middle range feed rate (feed of "0"). Ra is increasing at the middle range feed rate (feed of "0").

Table 4: Roundness of Hole

Diameter of the tool (mm)

Diameter of the hole drilled (mm)

Difference (mm)

1

10

9.992

-0.008

2

10

9.954

-0.046

3

10

10.012

+0.012

4

10

10.042

+0.042

5

14

14.036

+0.036

6

14

7

14

14.168

+0.168

8

14

14.076

+0.076

9

12

12.024

+0.024

10

12

12.006

+0.006

11

12

12.46

+0.46

12

12

12.15

+0.15

13

10

10.044

+0.044

14

14

14.068

+0.068

15

12

12.022

+0.022

16

12

11.074

-0.026

17

12

12.036

+0.036

18

12

12.118

+0.118

From the 18 holes drilled, there are found that the sixth hole is unable to be drilled 100%. When the drilling process is running, the process was stopped emergency after 1 minute. This hole was unable to be drilled after some tries. Due to safety, this hole was stopped to be drilled. Therefore, it was unable to be projected out through the profile projector.

From this measurement, the diameter of the eleventh hole had the largest difference with the diameter of the tool. Average range of tool diameter (diameter of "0") with low cutting speed (speed of "-1") and average range of feed rate (feed of "0") contribute to this difference.

However, the diameter of the tenth hole had the lowest difference with the diameter of the tool. The difference happened at average range of tool diameter (diameter of "0") with average range of cutting speed (speed of "0") and higher feed rate (feed of "+1").

From the result, most of the diameter of the holes are bigger than the drill bits diameter.

Conclusion

Through this experiment, optimum parameters for drilling process are found. Smaller diameter of drill bit with lower feed rate and cutting speed result the lowest value of Ra. Bigger diameter of drill bit with high feed rate and low cutting speed causes the drilling process unable to perform well and result the emergency stop for the drilling process. For the roundness of the hole, average range diameter of drill bit with average range of cutting speed and high feed rate result the closest diameter with the drill bit diameter. Beside, in order to drill a accurate hole, a smaller diameter of drill bit should be chosen.

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