Modelling & Simulation for Operations Management Assignment case study for Witness modelling and process improvement

Drive shaft manufacture

You have been employed as a member of a production improvement task force for a Drive Shaft Manufacturing Company (DSMC). DSMC has been experiencing a severe backlog in sales orders due to antiquated machinery and poor production planning. The Company has just received an unprecedented contract to supply 450 drive shafts by the process given below. However, finances at present are tight and only £250,000 can be invested. Some of the order can be sub-contracted. Your task is to determine the maximum capacity of the current production line after the investment of the £250,000 hence how many parts must be made by the sub-contractor. The cost list of potential capital improvements is given on next page.

The current output of the line averages 180 shafts on the two 38-hour shift system.

Process:

The drive shaft manufacture starts with a bar of 0.3 metres long that is cut into three parts (blocks). The operations are as follows (one machine at each operation unless stated otherwise):

1. The bar arrives based on vendor's supply schedule in lots of 3, every 30 minutes. The bar stock buffer can hold only 10 bars.

2. In the Saw machine, the time to cut the bar into 3 blocks is 6 minutes. Blade A is replaced every 40 operation and blade B every 30 operations. Set-up time is uniformly distributed between 10 and 15 minutes. Time between failures is NEGEXP with mean of 100, and the repair time follows triangular distribution with min 10, mode 25 and max 30. An operator is required to run this machine.

3. After sawing the blocks go to a Fixed conveyor with index time = 0.25 min, length 20 parts.

4. Once sawed the blocks must be coated for protection. The Coating machine takes 6 parts per cycle and has a cycle time of 60 minutes. Every 10 cycles, the machine is set-up for 5 minutes.

5. Once coated, the blocks are placed in a staging buffer of capacity 10 part, adjacent to the Inspector.

6. The Inspector draws from the staging area one block at a time and checks the quality of the block's coating. The cycle time = 20 min, set-up = 1min per part. If the coating is bad, the Inspector puts the rejected block into a Rework buffer of capacity 10. The reject rate is 20%, and rejects are returned to be recoated.

7. Parts are cured on an individual basis on a chain conveyor (Hardener) of capacity 10 parts through a furnace, a process on a first in first out basis. Heat-treat time is 60 min.

8. The hardened parts go for Gear cutting: cycle time 2 minutes + 1 minute load and unload that is manual, 2 machines, tool change every 100 parts for 10 minutes. Breakdowns mean time to failure 5000 minutes NegExp, repair time normal mean 80 minutes σ = 10. One operator runs two machines and changes cutter.

9. Four parts are Loaded into a special fixture in 4 minutes before entering for grinding; 10 fixtures are available in the system and are stored in a Fixture buffer if not used.

10. The 4 parts and the fixture are loaded to the Grinders (2 machines): cycle times M/c 1 = 35 min, M/c 2 = 40 min; Time between failures is normal distribution with mean = 75 min, SD = 15. The repair time is NEGEXP of mean 15 min and is repaired by the Saw operator.

11. Once ground, the fixture is Unloaded in 4 minutes. The fixture returns to the Fixture buffer and the parts go to two buffers of capacity 10 parts each.

12. The parts are then cleaned in the Wash machine, a multi stage machine with 5 stations, 1 part per station, cycle time = 15. Time between failures NEGEXP of mean 120 min, the repair follows Erlang distribution with mean = 20 min, K = 3. The machine is set-up every 100 operations, time 5 min. Set-up and repair are carried out by the Saw operator.

13. The Plating process follows and takes 36 parts per cycle, cycle time 36 minutes, buffers on machine 72 parts both before and after process. Due to chemical change on Monday morning, 4 hours are lost. An operator is required.

14. The parts are then washed with a Wash machine identical to 12.

15. The finished parts are Packed in Pack cycle time of 1.25 min. Packed into flat pack boxes one drive shaft per box. Boxes arrive in batches of 1000, frequency of deliveries to be determined by model. One operator required.

16. The boxes are finally shipped.

Attachment:- Assignment Case.rar