Convert the assembly line presented in the chapter 7 application problem to a CONWIP production strategy. The assembly line was described as follows:
A new serial system consists of three workstations in the following sequence: mill, deburr, and wash. There are buffers between the mill and the deburr stations and between the deburr and the wash stations. It is assumed that sufficient storage exists preceding the mill station. In addition, the wash station jams frequently and must be fixed. The line will serve two part types. The production requirements change from week to week. The data below reflect a typical week with all times in minutes.
Arrivals represent demands for completed products. Demands are satisfied from finished goods inventory. Each demand creates a new order for the production of a product of the same type after it is satisfied. The completed product is place in the finished goods inventory.
Three quantities must be determined through simulation experimentation:
1. The CONWIP level, that is the maximum number of parts allowed on the line concurrently.
2. The target FGI level for part type 1.
3. The target FGI level for part type 2.
Two approaches to setting these values could be taken. Choose either one you wish.
1. Approach one.
a. Set the FGI inventory level for each product as described in this chapter. Set the CONWIP level to infinite (a very high number). Use an infinite (again a very high number) FGI inventory level to determine the minimum number of units needed for a 100% service level.
b. Determine the inventory level needed for a 99% service level during the average replacement time analytically. The average replacement time is the same for each part type. Determine the average lead time using the VUT equation for each station. Sum the results. Remember that ca at a following station is equal to cd at the preceding station. Hints: 1) The VUT equation assumes that there is only one part type processed at a station. Thus, the processing time to use a the mill station is the weighted average processing time for the two part types. The weight is the percent of the total parts processed that each part type is of the total: 60% part type 1 and 40% part type 2. The formulas for the average and the variance for this situation are given in the discussion of discrete distributions in chapter 3. 2) The formula for the variance of a uniform distribution is given in chapter 3. 3) Ignore the downtime at the was station for this analysis.
c. Assess the service level for the inventory level midway between the lower and upper bound.
d. Pick the lowest level of inventory of three that you have tested that yields close to a 99% service level. Note average and maximum WIP on the serial line for this value.
e. Set the CONWIP level to the lowest value that doesn't negatively impact the service level. The minimum feasible CONWIP level is 3. Try values of 3, 4, 5, ... until one is found that does not impact the service level. Confirm your choice with a paired-t analysis.
f. Compare the maximum WIP before the CONWIP level was establish to the CONWIP level you selected.
2. Approach two:
a. Find the minimum CONWIP level that maximizes throughput. Set the two FGI levels to infinite (a very high number) so that the service level is 100%. The minimum feasible CONWIP level is 3. Try values of 3, 4, 5, ... until one is found such that the throughput is no longer increasing. Confirm your choice with a paired-t analysis.
b. Compare the maximum WIP in the serial line without the CONWIP control to the CONWIP level you select. The former could be determined by setting the CONWIP level to a large number.
c. Estimate the finished goods inventory level need to satisfy customer demands using the approach described in this chapter and after the CONWIP level has been established. Use an infinite (again a very high number) FGI inventory level to determine the minimum number of units needed for a 100% service level
d. Determine the inventory level needed for a 99% service level during the average replacement time analytically. The average replacement time is the same for each part type: the average lead time at station j is given by the following equation discussed in Chapter 9 where M = 3 stations and N is the CONWIP level you selected:
e. Assess the service level for the inventory level midway between the lower and upper bound and pick the lowest level that yields close to a 99% service level.
Terminating Experiment:
Use a simulation time interval of 184 hours.
Application Problem Issues
1. How should the CONWIP control be modeled?
2. What should the ratio of the two FGI levels be if prior information is used?
3. Should the mean or maximum WIP level on the serial line with no CONWIP control be compared to the CONWIP level?
4. Given the CONWIP control, is it necessary to model the finite buffer space between the stations on the serial line? Why or why not?
5. How will verification and validation evidence be obtained?