8.4: The Case Study with Additional Machines

The need to add one or more machines causes a re-start of the simulation process at the third step, Identify Root Causes and Assess Initial Alternatives.

8.4.1 Identify Root Causes and Assess Initial Alternatives

The simulation experiment uses the same design as in Table 8-1. However, the number of machines at the shaper station is increased by one. Table 8-4 shows the results. The average service level of 56.4% is less than the required 95%. The maximum waiting time at the polisher is much higher than at any of the other stations. Thus, an additional polisher will be added.

Table 8-4: Simulation Results –Additional Shaper Case

Replicate	Service Level	Maximum Waiting Time at the Lathe (Hours)	Maximum Waiting Time at the Planer (Hours)	Maximum Waiting Time at the Polisher (Hours)	Maximum Waiting Time at the Shaper (Hours)
1	17.6	1.9	4.4	10.2	1.5
2	65.6	1.7	3.3	7.3	1.4
3	22.1	1.9	8.6	20.3	1.2
4	56.7	1.1	5.1	7.5	1.3
5	98.0	1.2	2.6	3.8	1.0
6	81.1	1.2	5.0	5.1	1.1
7	72.1	1.7	2.9	6.7	1.9
8	76.7	1.9	3.9	7.2	1.2
9	1.3	1.3	4.1	13.6	1.4
10	39.1	1.2	4.8	8.6	1.8
11	78.6	1.7	3.6	5.9	1.1
12	47.5	1.1	8.8	7.7	1.1
13	61.7	1.3	7.0	7.0	1.9
14	78.4	1.3	3.1	5.9	1.8
15	43.8	1.5	4.1	12.3	2.0
16	83.2	1.6	3.1	6.7	1.6
17	81.7	1.7	3.0	7.0	1.2
18	67.6	2.0	5.3	7.8	1.4
19	43.5	1.4	3.8	10.5	1.6
20	12.1	1.6	5.3	13.8	1.8
Average	56.4	1.5	4.6	8.7	1.5
Std. Dev.	27.1	0.3	1.8	3.8	0.3
99% CI Lower Bound	39.1	1.3	3.5	6.3	1.3
99% CI Upper Bound	73.8	1.7	5.7	11.2	1.7

The simulation results for the case of an additional polisher and shaper are shown in Table 8-5. The average service level, 95.1%, now exceeds 95%. The approximate 99% confidence interval for the service level is 91.1% to 99.2%. Thus with approximately 99% confidence, it can be concluded that the true service level is in a range that includes 95%.

In two of the replicates the service level was less than 80%. In these replicates, the maximum waiting time at the planer exceeded 8 hours. In addition, the average maximum waiting time at the planer was 4.6 hours.

Table 8-5: Simulation Results –Additional Shaper and Polisher Case
Replicate	Service Level	Maximum Waiting Time at the Lathe (Hours)	Maximum Waiting Time at the Planer (Hours)	Maximum Waiting Time at the Polisher (Hours)	Maximum Waiting Time at the Shaper (Hours)
1	95.6	1.9	4.4	2.0	1.5
2	98.9	1.7	3.3	1.5	1.4
3	79.4	1.9	8.6	1.4	1.2
4	96.0	1.1	5.1	1.3	1.3
5	99.0	1.2	2.6	1.7	1.0
6	96.6	1.2	5.0	1.5	1.1
7	99.2	1.7	2.9	1.5	1.9
8	97.5	1.9	3.9	1.4	1.2
9	96.3	1.3	4.1	1.8	1.4
10	94.5	1.2	4.8	1.6	1.8
11	98.7	1.7	3.6	1.3	1.1
12	76.5	1.1	8.8	1.6	1.1
13	92.0	1.3	7.0	1.6	1.9
14	99.2	1.3	3.1	2.0	1.8
15	98.4	1.5	4.1	1.6	2.0
16	99.2	1.6	3.1	1.7	1.6
17	99.4	1.7	3.0	1.4	1.2
18	93.5	2.0	5.3	1.9	1.4
19	99.2	1.4	3.8	1.4	1.6
20	93.9	1.6	5.3	1.3	1.8
Average	95.1	1.5	4.6	1.6	1.5
Std. Dev.	6.3	0.3	1.8	0.2	0.3
99% CI Lower Bound	91.1	1.3	3.5	1.4	1.3
99% CI Upper Bound	99.2	1.7	5.7	1.7	1.7

8.4.2 Review and Extend Previous Work

Management was pleased that the addition of two machines was sufficient to meet the service level requirement. It was decided that the job shop would have the following configuration of machines:

Lathes	-- 2
Planers	-- 3
Polishers	-- 3
Shapers	-- 3

In addition, the congestion, in terms of work in process, at the planer station would be monitored. Action would be taken to help avoid excessive waiting time at this station, which now appears to be the bottleneck.

8.4.3 Implement the Selected Solution and Evaluate

The job shop was implemented with the number of machines decided upon at the management review meeting. A monitoring system for work in process at the planer station was put in place.