Question

Four Step Process with Rework and Scrap

Consider the following four step assembly operation with quality problems. All resources are staffed with one operator.

-The first resource has a processing time of 4 minutes per unit

-The second resource has a processing time of 3 minutes per unit. This process suffers from a high yield loss and 50% of all products have to be scrapped after this step.

-The third resource also suffers from quality problems. However, instead of scrapping the product, the third resource reworks it. The processing time at the third resource is 5 minutes per unit. In the 30% of the products in which the product needs to be reworked, this extends to a total (initial processing time plus rework) processing time of 10 minutes per unit. Rework always leads to a non-defective unit.

-No quality problems exist at the first and final resource. The processing time is 2 minutes per unit.

For every unit of demand, how many units have to flow through the third step in the process?

Where in the process is the bottleneck?

What is the process capacity?

Answer 1

Answer:

​Step 1:

• Cycle time = 4 min
• Units produced from 1 unit of input = 1

Step 2:

• Cycle time of 2 is generally 3 min but since the scrap rate is 50%
• Effective cycle time = 3/ 50% = 6 mins
• Units produced from 1 unit of step 1 = 0.5

Step 3:

• 70% of the units take 5 mins and 30% takes 10 mins . So the cycle time is the weighted average of the same.
• Cycle time = 0.7* 5 + 0.3 *10 = 6.5 mins
• Unit produced from 0.5 unit of step 2 unit = 0.5

Step 4:

• Cycle time = 2 min

Hence the bottle neck is step 3, cycle time of the process is 6.5 mins, process capacity = 9.23 unit / hour

2 units are required for 1 unit output