My name is Steve Chapman. I am a retired faculty member from North Carolina State University.
After a long career in industry, consulting, and academics.
I am also co-author of the book Introduction to Materials Management.
Mapping the value stream provides a big picture view of a process using lean concepts and
symbols to visualize lean improvements.
Assessors first create a current state map to show how the system works.
Then a future state map is developed to show the desired improved state.
First, let’s discuss the current state map.
Here is a current state map for Beta Toys.
They are piloting lean in their plastic toys family to see if it can reduce costs and lead
times while increasing customer responsiveness.
Note the three major areas of a value stream map:
At the top are information flows, which includes planning.
In the middle are material flows.
At the bottom are lead and process times.
Seeing the big picture is a key benefit of value stream mapping as it allows you to visualize
the links between the flows.
Value stream mapping starts with the customer and works backwards to ensure the process
can achieve customer requirements.
The first step in creating a current state map is to add the customer.
and record annual unit demand
Then translate this into units of demand per day.
And finally calculate takt time.
Next, we add process boxes for this product family’s operations.
Working backwards, the assessor times how long it takes for one unit to be processed
at each operation not including any wait or setup time.
This is called process time.
The next step includes recording details for later use in the map, such as noting scrap,
calculating cycle time, process time, and setup time.
Once the data has been gathered, we enter key data per operation on the map.
The top line is for process time, which is the time it takes to complete each process.
Cycle time is listed if these are identical.
Cycle times are listed below process times if these numbers are different.
For example, if it takes 4.5 minutes to pack, but you have 3 operators, the cycle time is
cut to 1.5 minutes.
You should also include any changeovers or setups
As well as number of shifts
and amounts of scrap.
In the final part of this step, we add the number of process operators using this symbol
for each person.
Next, we add an inventory symbol and list units where inventory is stored or is temporarily
at rest.
In the next step, we add the longest lead time or most important supplier to the top
row.
Now we start adding information flows using symbols for various communications.
We start the information flows with the customer and move backwards.
Here the customer sends weekly purchase orders and annual forecasts electronically so we
use the electronic information flow symbol.
Next we add our internal planning and scheduling processes
such as the ERP system and planning bucket.
Here we add monthly production targets, master scheduling and MRP, weekly production schedules,
and shop orders.
Next we determine how communications with the supplier occur.
Resin requirements use electronic data interchange, or EDI,
so this uses an electronic communication symbol.
but we submit a paper purchase order so this uses a straight line.
Next we determine whether we use push or pull between each process.
These are the symbols for push and pull
and here is where you will place them in the map.
Since this is a conventional manufacturing planning and control system, all steps are
push steps.
Finally, we add a lead-time ladder at the bottom that steps up to show lead time per
unit in days and steps down to show process time per unit.
This is usually shown in seconds, but we will keep using minutes here to simplify discussion.
Lead time is the days of inventory at each location, and for the most part, the lead
time areas show non-value added time.
and process time areas show value added time.
These times are summed separately, which helps show the large difference between value added
versus non-value added time in most processes.
Now we have completed the current state map.
Can this process be run at takt time?
To answer this, we look at cycle times.
Since none of the cycle times are greater than 1.79 minutes per unit, this process can
already run at takt time.
Now let’s discuss the future state map
When making a future state map,
start with the current state map
and brainstorm process improvements
In our example, we could add supermarkets at both ends of the process and replenish
them daily by changing deliveries to occur daily in a milk run.
This keeps supermarket inventories low.
We could also combine the Assemble and Pack operations
to eliminate the inventory between operations
Since neither has setup time, this is feasible.
Next, because our fledgling lean operation can’t yet do one-piece flow, we propose
using a material flow technique of first-in, first-out.
The idea is to set a buffer at a certain number of days of inventory and keep our days of
inventory low but steady.
As one unit is added to the end, a unit is taken from the front.
The next change is to add kanban for Injection Molding.
A pull signal called the pacemaker will indicate when to start the process, which sets the
pace at takt time.
We also add heijunka leveling, which is a level schedule based on daily demand for each
type of unit.
The weekly schedule can be eliminated as subsequent operations instead use the pace set by the
pacemaker.
Next we calculate lead time savings and any change in process time.
We will keep 2 days of inventory in each location,
plus 20 days of inventory at the finished goods supermarket.
The combined process times of 3
plus 4.5 minutes
equal 7.5 minutes,
but dividing this by 5 operators is still a cycle time of 1.5 minutes, so there is no
change in cycle time.
The future state map is then finalized as shown.
Once approved, it becomes an action plan for process improvement events.
Once the new status is achieved, it is time to start over and find new improvements.
We hope you have enjoyed this overview of value stream mapping.