We are now almost ready to calculate the resources – both people and machinery – required for pTool’s Mixed Model Flow line. Two preliminary tasks remain: determining the effect, if any, on rework and scrap; and calculating standard labor and machine times for each process. We’ll address the volume adjustments potentially cause by rework and scrap here in Exercise 4.
When you study a set of resources (a process) for a Mixed Model Flow line design you must always understand the process / volume relationships:
INPUT -> THROUGHPUT -> OUTPUT
Takt time must be calculated based on the process throughput volume. Required throughput volume can be affected by the amount of rework or scrap to be expected.
Adjustments for Known Rework. The units lost to rework can be recovered. However, the processes in the loop between the defective unit’s departure and return points need to have the capacity to re-process the units. If you were to assume that the reworked units are repaired in one pass, the calculation for the throughput volume is:
VolTH = VolOUT x (1.00 + Rework %)
105 = 100 x (1.00 + 5%) Example
VolTH = VolOUT x (2.00 – Yield %)
That is, if we require output of 100 units in a given time, and are incurring a 5% rework rate, then our affected process(s) must be capable of a throughput of 105 units in that time period.
In some cases, the “one pass” assumption may not be valid. For those cases, we assume that it will take infinite passes to repair the units. Then, the calculation is the same we used for scrap:
VolTH = VolOUT / (1-Rework %)
VolTH = VolOUT / (Yield %)
You will now calculate the increased throughput required for two processes, WIRING and ELECTRICAL TEST, which have been troubled by rework issues. These adjustments will then be entered in the Resource Calculation Spreadsheet you are building. (We will not be calculating for scrap in these Exercises, but the process is very similar.)