I. Common Usage Rate of Plate Thickness
| Sheet Thickness（mm） | 16 | 18 | 20 | 25 | 28 | 30 | 32 | 50 |
| Use Probability（%） | 14 | 9 | 12 | 27 | 9 | 13 | 15 | 1 |
Instruction: According to the usage rate of common sheet thickness in the above table, it is found that the usage rate of 50mm sheet is very low and is mainly used in the frame.
Cutting a 50mm sheet requires a 600A plasma power supply. The 440A plasma equipment costs 630,000 CNY and the 600A plasma equipment costs 850,000 CNY.
Since the price of the 600A plasma is high and its usage rate is low, it is recommended to purchase 440A plasma equipment.
Equipment selection analysis: The riveting and welding workshop currently has two CNC cutting equipment.
One CNC flame cutting equipment was put into use in August 2005, and the other is a CNC plasma/flame cutter with both plasma and two flame cutting torches, which was put into use in December 2013.
At present, the flame cutting machine from 2005 is broken. If it is converted to a 440A plasma and two flame cutting torches, it would cost 550,000 CNY.
If a new 440A plasma and a flame cutting torch are purchased, it would cost 630,000 CNY. Since the price difference is not large, it is recommended to purchase new equipment.
II. Efficiency Analysis
Since the cutting speed varies with sheet thickness, the cutting speed decreases as the sheet thickness increases.
According to the cutting speed analysis of different sheet thicknesses, the average cutting speed of CNC flame cutting is approximately 320mm/min; the CNC plasma cutting speed is approximately 1800mm/min (this speed is for 440A plasma).
III. Quality Analysis
Both CNC flame cutting and plasma cutting can meet the requirements in terms of surface smoothness.
The size accuracy error of flame cutting is around 2mm, and it is very difficult to achieve an error of 1mm. The inclination of the cutting surface is small and meets the requirements; The maximum error size of plasma cutting is about 2mm.
By adjusting the process parameters, it is relatively easy to achieve an error accuracy of 1mm, but the inclination of the plasma cutting surface is relatively large (the inclination of the 50mm sheet does not meet the requirements, the inclination of the 30mm sheet meets the requirements).
IV. Cost Analysis
1. Plasma Cost:
The hourly usage cost in continuous cutting mode:
Electricity usage 55 degrees 60 CNY + main consumable parts cost 120 CNY + other consumable usage costs 2.4 CNY + air 6 cubic meters 6 CNY + oxygen 5 cubic meters 18 CNY + dust removal 2.2 degrees of electricity 2.42 CNY = hourly usage cost 209.42 CNY.
| Project | Electricity Cost | Wear Parts | Air | Oxygen | Others | Dust Removal | Total |
| Amount | 60 | 120 | 6 | 18 | 2.4 | 2.42 | 209.42 |
2. Cost of CNC Flame Cutting
Cost per hour in continuous uninterrupted cutting mode:
Propane CNY30 + Oxygen CNY56 + Electricity usage (3 units) CNY3.3 + Consumable components CNY3 = CNY92.3.
| Project | Electricity Fee | Propane | Oxygen | Fragile Parts | Total |
| Amount | 3.3 | 30 | 56 | 3 | 92.3 |
Note: The plasma cutting speed is 5.62 times that of flame cutting, and the cost is 2.27 times that of the flame.
Considering that most workpieces can be cut with a dual torch using CNC flame cutting, the efficiency of plasma is 2.81 times that of dual-torch flame cutting.
The cost of flame dual-torch cutting for continuous operation for one hour is 184.6 CNY, while plasma is 209.42 CNY.
3. Labor Costs
According to the company’s standard working hour quota, the labor cost for a CNC cutting worker working continuously for one hour is 18 CNY.
4. Equipment Depreciation Expense
| Equipment Price | 750000 （Plasma） | 170000 （Dust Removal Equipment） | |
| Residual Value Rate (0.95) | 712500 | 161500 | |
| Annual Depreciation Cost | 59375 | 13458 | |
| Monthly Depreciation Cost | 1979 | 1122 | |
| Daily Depreciation Cost | 66 | 37 | |
| Hourly Depreciation Cost | 8 hours | 8.25 | 4.67 |
| 16 hours | 4.13 | 2.31 | |
| Equipment Price | 190000（Flame） | |
| Salvage Value Rate (0.95) | 180500 | |
| Annual Depreciation Cost | 15041 | |
| Monthly Depreciation Cost | 1253 | |
| Daily Depreciation Cost | 42 | |
| Hourly Depreciation Cost | 8 hours | 5.2 |
| 16 hours | 2.6 | |
5. Total Cost
| Plasma | Amount | Flame | Amount |
| Electricity Cost | 60 | Electricity cost | 3.3 |
| Fragile Parts | 120 | Vulnerable parts | 3 |
| Air | 6 | Propane | 30 |
| Oxygen | 18 | Oxygen | 56 |
| Others | 2.4 | Labor | 18 |
| Dust Removal | 2.42 | ||
| Labor | 18 | ||
| Equipment Depreciation | 12.92 | Equipment Depreciation | 5.2 |
| Total | 239.74 | Total | 115.5 |
From the above table, it can be concluded that the total cost for one hour of continuous operation is 239.74 CNY for the plasma process, and 115.5 CNY for the CNC flame process.
Appendix: A comparison of time and cost between plasma and flame cutting for a framework.
| Item | Time (min) | Amount (CNY) | |
| Plasma Cutting | 76 | 297.8 | |
| CNC Flame Cutting | Single torch | 316 | 598.3 |
| Double torch | 158 | 299.2 | |
Chassis and Frame Production Capacity Estimation Table
| Chassis | Flame Cutting | 6.5 hours/set | 20 sets/month | 8×26=208h | 62.5% |
| Plasma Cutting | 1.2 hours/set | 11.5% | |||
| Framework | Dual Torch Flame Cutting | 2.63 hours/set | 100 sets/month | 126% | |
| Plasma Cutting | 1.57 hours/set | 75.5% |
Appendix
Comparison of Typical Sheet Thickness Between Plasma and Flame Cutting
| Plate Thickness (mm) | Speed (mm/min) | Cost (CNY/Meter) | Cutting Quality | ||
| 16 | Plasma | 3200 | 1.25 | Good（0.84） | |
| Flame | Single Torch | 400 | 4.81 | Good | |
| Double Torch | 800 | 4.80 | Good | ||
| 25 | Plasma | 2800 | 1.43 | Average（1.3） | |
| Flame | Single Torch | 320 | 6.0 | Good | |
| Double Torch | 640 | 6.0 | Good | ||
| 30 | Plasma | 2650 | 1.51 | Average（1.57） | |
| Flame | Single Torch | 300 | 6.40 | Good | |
| Double Torch | 600 | 6.39 | Good | ||
| 50 | Plasma | 700 | 5.71 | Poor（2.62） | |
| Flame | Single Torch | 200 | 9.60 | Good | |
| Double Torch | 400 | 9.56 | Good | ||
