Penultimate Support Systems makes fairly good speaker and equipment support stands for music groups. The assembly process involves two operations: (1) fabrication, or cutting aluminum tubing to the correct lengths, and (2) assembly, with purchased fasteners and injection-molded plastic parts. Set-up time for assembly is negligible. Fabrication set-up time and run-time per unit, assembly run-time per unit, and the production schedule for next week follow. Organize the work to minimize makespan (the time it takes to complete the product) and idle time, and create a Gantt chart. Can this work be accomplished in two 40-hour shifts? At Penultimate, it is normal practice to complete lots in one process before sending them to a following process.
Fabrication Assembly
Setup Run Time Run Time
Model Quantity (hrs.) (hrs/unit) (hrs/unit)
A 200 2 0.050 0.040
B 300 3 0.070 0.100
C 100 1 0.050 0.120
D 250 2 0.064 0.060
Use "Johnson's Rule" which is an approach for multiple jobs (the different models in our example) on two distinct processes or tools, as in this case - Fabrication and Assembly:
Johnson's Rule consists of the following steps:
1. List the operation time (combined setup and run times) for each job on both processes.
2. Select the shortest operation time.
3. If the shortest time is for the first process, do the job first; if it is for the second process, do the job last.
4. Repeat Steps 2 & 3 for each remaining job until the schedule is complete.