Solved problems on Material Selection - 1
Solved problems on Material Selection - 1
In the design of a jet engine part, the designer has a choice of specifying either an aluminium alloy casting or a steel casting. Either material will provide equal service, but the aluminium casting will weigh 1.2 kg as compared with 1.35 kg for the steel casting.
The aluminium can be cast for Rs. 80.00 per kg, and the steel one for Rs. 35.00 per kg. The cost of machining per unit is Rs. 150.00 for aluminium and Rs. 170.00 for steel. Every kilogram of excess weight is associated with a penalty of Rs. 1,300 due to increased fuel consumption. Which material should be specified and what is the economic advantage of the selection per unit?
Given Data:
Weight of aluminium casting/unit = 1.2 kg
Cost of making aluminium casting per kg = Rs. 80.00
Cost of machining aluminium casting per unit = Rs. 150.00
Weight of steel casting/unit = 1.35 kg
Cost of making steel casting per kg = Rs. 35.00
Cost of machining steel casting per unit = Rs. 170.00
Penalty of excess weight of steel casting per kg = Rs. 1,300
Solution:
(a) Cost of using aluminium metal for the jet engine part:
Total cost = Total Cost of making aluminium casting per unit + Cost of machining aluminium casting per unit
Total Cost of making aluminium casting per unit = Total weight of aluminium per unit * Cost of making aluminium casting
= 1.2 * 80
= Rs. 96
Cost of machining aluminium casting per unit = Rs. 150
Total cost = 96 + 150
= Rs. 246
(b) Cost of jet engine part made of steel/unit:
Total cost = Total Cost of making steel casting per unit
+ Cost of machining steel casting per unit
+ Total Penalty for excess weight of steel casting
Total Cost of making steel casting per unit = Total weight of steel casting per unit * Cost of making steel casting
= 35 x 1.35
= Rs. 47.25
Cost of machining steel casting per unit = Rs. 170
Total Penalty for excess weight of steel casting = Difference in weight * Penalty for excess weight of steel casting
= (1.35 - 1.2)*1,300
= Rs. 195
Total cost = 47.25 + 170 + 195
= Rs. 412.25
Decision:
The total cost per unit of a jet engine part made of aluminium is less than that for an engine made of steel. Hence, aluminium is suggested for making the jet engine part.
The economic advantage of using aluminium over steel per unit is Rs. 412.25 - Rs. 246 = Rs. 166.25
