Cost Comparison of Manufacturing Methods and Process Sequences
Economic Comparison of Aluminium and Steel Castings
In the design of a jet-engine part, either an aluminium alloy casting or a steel casting can be specified because both materials provide equal service. The aluminium casting weighs 1.2 kg and costs ₹80 per kg for casting, while the steel casting weighs 1.35 kg and costs ₹35 per kg. Machining costs are ₹150 per unit for aluminium and ₹170 per unit for steel. Every kilogram of excess weight carries a penalty of ₹1,300 because of increased fuel consumption. Determine which material should be specified and find the economic advantage per unit.
Also given:
- Weight: Aluminium 1.20 kg, Steel 1.35 kg
- Casting cost: Aluminium ₹80/kg, Steel ₹35/kg
- Machining cost: Aluminium ₹150/unit, Steel ₹170/unit
- Excess-weight penalty: ₹1,300/kg (applies to steel)
Step 1: Calculate Aluminium Casting Cost
Material cost = 1.20 × ₹80 = ₹96
Machining cost = ₹150
Total aluminium cost = ₹96 + ₹150 = ₹246
Aluminium is the lighter material, so no excess-weight penalty is applied to aluminium.
Step 2: Calculate Steel Casting Cost
Material cost = 1.35 × ₹35 = ₹47.25
Machining cost = ₹170
Excess weight = 1.35 − 1.20 = 0.15 kg
Weight penalty = 0.15 × ₹1,300 = ₹195
Total steel cost = ₹47.25 + ₹170 + ₹195 = ₹412.25
Step 3: Economic Advantage
Economic advantage of aluminium = Steel cost − Aluminium cost
= ₹412.25 − ₹246 = ₹166.25 per unit
Conclusion: An aluminium alloy casting should be specified. Economic advantage = ₹166.25 per unit.
Cost Comparison of Dining Table Alternatives
A company manufactures dining tables consisting mainly of a wooden frame and a table top. Alternative A uses a wooden frame with a Sunmica-finished top, while Alternative B uses a wooden frame with a granite top. The quantities, material rates and labour requirements for both alternatives are given in the table. Calculate the manufacturing cost of each alternative, identify the more economical alternative, and determine its economic advantage.
Alternative A: Wood Frame with Sunmica Top
| Item | Quantity | Rate | Amount |
|---|---|---|---|
| Wood for frame and legs | 0.10 m³ | ₹12,000/m³ | ₹1,200 |
| Table top with Sunmica finish | 1 | ₹3,000 | ₹3,000 |
| Leg bushes | 4 | ₹10/bush | ₹40 |
| Nails | 100 g = 0.10 kg | ₹300/kg | ₹30 |
| Total labour | 15 hr | ₹50/hr | ₹750 |
Total cost of Alternative A = 1,200 + 3,000 + 40 + 30 + 750 = ₹5,020
Alternative B: Wood Frame with Granite Top
| Item | Quantity | Rate | Amount |
|---|---|---|---|
| Wood for frame and legs | 0.15 m³ | ₹12,000/m³ | ₹1,800 |
| Granite table top | 1.62 m² | ₹800/m² | ₹1,296 |
| Leg bushes | 4 | ₹25/bush | ₹100 |
| Nails | 50 g = 0.05 kg | ₹300/kg | ₹15 |
| Total labour | 8 hr | ₹50/hr | ₹400 |
Total cost of Alternative B = 1,800 + 1,296 + 100 + 15 + 400 = ₹3,611
Step 3: Compare the Two Alternatives
Economic advantage = Cost of A − Cost of B = ₹5,020 − ₹3,611 = ₹1,409 per table
Conclusion: Alternative B (wood frame with granite top) has the lower manufacturing cost. Economic advantage = ₹1,409 per table.
Selecting the Most Economical Process Sequence
A process-planning engineer has proposed three possible sequences for manufacturing a component: (1) Turning → Milling → Shaping → Drilling, (2) Turning → Milling → Drilling, and (3) performing all operations using a CNC machine. The machine-hour rates and processing times for the alternatives are provided in the table. Determine the most economical process sequence.
Given Data
| Operation | Machine rate (₹/hr) | Seq. 1 (min) | Seq. 2 (min) | Seq. 3 (min) |
|---|---|---|---|---|
| Turning | 200 | 5 | 5 | – |
| Milling | 400 | 8 | 14 | – |
| Shaping | 350 | 10 | – | – |
| Drilling | 300 | 3 | 3 | – |
| CNC operations | 1000 | – | – | 8 |
Operation cost = Machine-hour rate × Processing time (minutes) / 60
Sequence 1: Turning → Milling → Shaping → Drilling
Turning = 200 × 5/60 = ₹16.67
Milling = 400 × 8/60 = ₹53.33
Shaping = 350 × 10/60 = ₹58.33
Drilling = 300 × 3/60 = ₹15.00
Total Sequence 1 = ₹143.33
Sequence 2: Turning → Milling → Drilling
Turning = 200 × 5/60 = ₹16.67
Milling = 400 × 14/60 = ₹93.33
Drilling = 300 × 3/60 = ₹15.00
Total Sequence 2 = ₹125.00
Sequence 3: All Operations on CNC Machine
CNC cost = 1,000 × 8/60 = ₹133.33
Total Sequence 3 = ₹133.33
Comparison
| Process sequence | Total cost per component |
|---|---|
| Sequence 1 | ₹143.33 |
| Sequence 2 | ₹125.00 |
| Sequence 3 | ₹133.33 |
Conclusion: Sequence 2 (Turning → Milling → Drilling) is the most economical, with a cost of ₹125.00 per component.
Revised Process Sequence Cost Analysis
A process-planning engineer has listed three possible operation sequences for producing a component: (1) Turning → Milling → Shaping → Drilling, (2) Turning → Milling → Drilling, and (3) performing all operations using a CNC machine. The table provides the machine-hour rates and processing times for each operation and sequence. Determine the most economical sequence of operations to manufacture the component.
Given Data
| Operation | Rate (₹/hr) | Seq. 1 (min) | Seq. 2 (min) | Seq. 3 (min) |
|---|---|---|---|---|
| Turning | 300 | 8 | 8 | – |
| Milling | 350 | 8 | 14 | – |
| Shaping | 380 | 10 | – | – |
| Drilling | 350 | 5 | 5 | – |
| CNC operation | 1200 | – | – | 10 |
Operation cost = Machine-hour rate × Processing time (minutes) / 60
Sequence 1: Turning → Milling → Shaping → Drilling
Turning = 300 × 8/60 = ₹40.00
Milling = 350 × 8/60 = ₹46.67
Shaping = 380 × 10/60 = ₹63.33
Drilling = 350 × 5/60 = ₹29.17
Total Sequence 1 = ₹179.17
Sequence 2: Turning → Milling → Drilling
Turning = 300 × 8/60 = ₹40.00
Milling = 350 × 14/60 = ₹81.67
Drilling = 350 × 5/60 = ₹29.17
Total Sequence 2 = ₹150.84
Sequence 3: All Operations on CNC Machine
CNC cost = 1,200 × 10/60 = ₹200.00
Total Sequence 3 = ₹200.00
Comparison
| Process sequence | Total cost per component |
|---|---|
| Sequence 1 | ₹179.17 |
| Sequence 2 | ₹150.84 |
| Sequence 3 | ₹200.00 |
Conclusion: Sequence 2 (Turning → Milling → Drilling) is the most economical, with a cost of approximately ₹150.84 per component.