Page 142 - How To Implement Lean Manufacturing

How to Do Lean—The Four Strategies to Becoming Lean 121

How Do We Handle Variations in Supply and Demand?
Having this cycle time to takt time relationship will take care of our production prob-
lems in the long run, but what about the short-term problems we might have? Those
short-term problems are usually of two types: underproduction and short-term increases
in demand. Say, for example, that the customer calls on Monday and informs you that
for their Wednesday pickup they need twice their normal volume. Normally, there is no
way to handle this with cycle-time changes, so we hold buffer inventory for exactly this
situation: demand variation. Take a second example in which your customer picks up
daily and you have a problem with one of your parts and can’t make production. How
do you make the shipment? We will hold a volume of safety stock to cover this varia-
tion. Hopefully these do not happen often, but in both cases the answer is that we pro-
tect the supply to the customer with finished goods inventory.

Appendix B—The Basic Time Study

Background
The most fundamental tool to synchronize flow and analyze the work is the basic time
study. The Zeta Cell Time Study, shown in Fig. 7-1, is included for your reference. The
study can be done in many ways, but several cycles should be observed, and then a
rational decision made on the times to use in the study. Notice that, although the aver-
age value is employed, several times where the average was overstated due to prob-
lems found during the study, a lesser number was used. In addition, those problems
encountered were taken care of with kaizen activities.

The Time Study
The form used has a place for the:
• Process step number
• Flow chart identification
• The work element
• Eight time studies
• Summary statistics
• Final time selected
It is important that the times measured are for the work only and do not include the
wait times. If you wish to catalogue wait time, simply modify the form to have a work
column and a wait column for each cycle.
It is important to evaluate several cycles. In this case, we did eight. Ten is very com-
mon, and five is too few.

The Time Study Analysis
In this case, we put the data into a tool I call a Time Stack of Work Elements, see Fig. 7-2,
which shows the cumulative cell time on the left, with the work steps defined and notation
of which operator performs the work. Here, there was one operator per work station.
These data are then used in the balancing study. See Appendix C.