Product Wheel scheduling is a
concept which allows for standardized, noise-reduced production of fast and
slow moving products made to stock and made to order. It came about
first as a tool to introduce ‘lean’ principle – which were thought out
primarily in the automotive industry – to process manufacturers. Product Wheels
find now widespread acceptance in the chemical, pharmaceutical and food
processing industry as it allows for the scheduling of large batches and
considers the difficulties with switching over from one product batch to
another.
There are some specifics to be considered when using the
product wheel in the process industries and, with this writing, I’d like to
provide you with some ideas on how a product wheel could be configured into
SAP.
What
is Product Wheel Scheduling?
If your company is a process manufacturer, you most likely
mix, blend, cure or otherwise process your products on a production line. One
of the characteristics of processed products is that you can't disassemble
them. An automobile you can usually 'unscrew' and put the components back in
inventory (even though that is not true 100%, it is an approximation to
generalize the difference between process and discrete manufacturing).
Also, in process manufacturing you may have by- and
co-products; unfinished yield that may be re-introduced into the process. And
you often can't predict what exactly comes out of the process. So you have to
work with ranges (of specifications) and chemical formulas. All of that is
provided with recipes and process orders in PP-PI. As "lean
manufacturing" came primarily from the automotive industry, process
manufacturers always asked the question if they can reduce waste as well. Why
not? You cannot introduce 'one piece flow' but that's not the only lean
principle. Why not heijunka level a production program or make every product
every interval (EPEI)?
Peter L. King has written a book, “Lean for the Process
Industries. Dealing with Complexity”, which beautifully translates all the
'automotive lean principles' to process manufacturing. One of the most
interesting ideas is the 'product wheel' is that it represents heijunka for
processed products. Products wheels allow you to schedule, capacity level and
sequence your production program all at the same time. It is a mixed model scheduling
concept which allows you to automatically fill a processing line to it's
capacity, in a setup-optimized sequence, ensuring that the smallest possible
lot size is processed as many times as possible within a planning cycle.
Within this concept the circle represents the lengths of the
planning cycle, each spoke is a batch size (the lengths in time to produce it)
of a specific product and the gap in between represents the time it takes to
setup, clean or prep the line for the next product. Note that there are spokes
for MTO and spokes for MTS. The MTS spokes are planned based on a forecast,
whereas the MTO spokes are reserved time / capacity which can be filled by
customer requests which are made to order.
A planner will first identify how much time is available
during a planning cycle to get around the wheel. If that time span is one week,
we simply sequence the total forecasted quantity for all products on that line
and for the week around the wheel. If, with that, we get 2/3rds around the
wheel, then there is 1/3 available for MTO capacity and setup time. Peter L. King
calls that open time PIT – Process Improvement Time.
The Product Wheel is a production scheduling method with its
design based on average demand but it is executed to actual demand. The phases
to use a Product Wheels are:
1.
Identify
the location (line or line segment) on your production floor where the product
wheel is to be used for scheduling
2.
Design a
standard sequence using all the products which may be produced on the line
3.
Determine
the lengths and periodicity of the cycle of the wheel
4.
Schedule
or load the product wheel for the next cycle so that it meets planned demand
5.
Execute
the schedule for the cycle according to the plan and fulfill incoming orders
from inventory
The last point is of special importance as this provides
adherence to the core philosophy of product wheel scheduling: produce
to inventory according to a set plan in a frozen zone and fulfill actual demand
from inventory which was replenished from the previous production cycle
Product Wheel scheduling brings with it transparency and
insights that help to continuously optimize the way we produce. A uniform,
level production schedule will maximize equipment and labor utilization, and
smooth out requirements for raw materials. One of lean manufacturing’s major
change in thinking is that we must take variable demand and find a way to
distribute it evenly. Product Wheel scheduling does exactly that. It goes away
from scheduling customer demand on an instantaneous basis, but rather integrates
the variable demand into some longer time frame
Implementing
the Product Wheel with SAP
First perform a segmentation to classify our
products (the ones we manufacture) into four categories:
u
MTS high volume – every cycle - these are your
front running items. You are are maintaining an inventory level which provides
high availability (high service levels) to your customers
u
MTS low volume - every other cycle - these
products are demanded less frequently and find their ray onto the product wheel
only every other cycle or even on c, every third or fourth cycle
u
Only when inventory requirement is breached -
here we are dealing with products which are demanded only from time to time.
However, we cannot afford to wait with production until we have the customer
order in place because our customers would no of accept to wait out the
replenishment time but rather buyfrom some place else. This is why, for these
products, we will keep some inventory and trigger more production based an A
breach of a reorder point.
u
MTO - for products with infrequent demand which
happen to be valuable, perishable and/or relatively short to replenish, we
trigger production only when a customer order is present.
While performing the segmentation you’ll have to consider
some determining factors that may be described as follows:
u
Cost of inventory – requires short cycles
u
Cost of change over – requires long cycles
u
Shelf life – requires short cycles
u
Short term product demand variability
u
Minimum practical lot size
Then, before you can use the product wheel for production
scheduling you must define some standards. If you hold an annual strategy
meeting, this is the best time to set the wheel’s cycle duration, performance
boundaries and identify the places where the wheel is used for scheduling.
Once these decisions were made, one can implement the
standard steps and sequence by which a planner may design and subsequently run
or schedule the product wheel. Some of the steps to implement product wheel
scheduling are given below.
1.
Value
Stream Map – create an SAP value stream map with all master data, decoupling
points and pacemaker / wheel locations
2.
Where on
the floor? – decide the locations where you want to run product wheel
scheduling
3.
Demand
volume and segmentation – perform the segmentation as described above
4.
Sequencing – establish a changeover
matrix
5.
Wheel time (cycle) – fastest, most
economical, shelf life, demand variability, min lot size
6.
Wheel frequency for each product
7.
Distribute
products across cycles - balance cycle to cycle
8.
Visualize wheel cycles – diagrams
9.
Calculate inventory requirements
To define the standard sequence (as suggested under point 9)
proceed as follows:
A standard sequence provides a template for the actual
sequence. In it we identify all products which could ever run on the line and
provide a mechanism by which every actual sequence (with only those products on
the schedule which are actually demanded in that cycle) will go by.
To set the standard sequence in SAP we are using the setup
matrix with its fields SETUP GROUP CATEGORY
and SETUP GROUP KEY. Configuring the settings
to the fields in SAP’s customizing will enable us to define each product’s
place in the sequence. This is done in the product’s standard routing or
recipe. Go to the sequence of operations and from there drill into the details
of the operation with your production line. In there you will find the
fields SETUP
GROUP CATEGORY and SETUP GROUP KEY.
Pick from the list of options those values which place the product you are
maintaining into the right place of the sequence as shown below
For the routing displayed above we pick group “C” which
places the product on top of the sequence. Next we pick the setup group key.
Value 2 is being picked here which places the product in
second place within the third group “C” (which was picked as setup group
category) of the sequence
If you keep on assigning setup group category (the group)
and setup group key (the sequence within the group) to the routings of the
materials you manufacture, you are, in fact, building a standard sequence by
which these products fall into place should they be demanded and a planned
order is present.
Next I’ll demonstrate how orders can be scheduled using this
sequence by way of the Dispatch Sequence in SAP’s
scheduling transaction CM25.
Product
Wheel Scheduling with CM25
After all settings (changeover matrix, sequence schedule,
routing data, material master policy) have been setup, we can now schedule the
infinite supply plan, generated by the MRP Run, into a finite supply plan using
transaction CM25.
As you can see below, all generated, unscheduled
planned orders are visible in the order pool in the bottom window.
What we need to do is to pick the frozen zone period and
schedule relevant (within the time period) orders from the pool onto the
processing line. This must be done within the available capacity and in the
correct sequence.
To determine the correct sequence we must use the dispatch
key that uses the changeover matrix we configured in the system. This is done
by way of the strategy profile.
You can now select all the relevant planned orders from the
pool and push the dispatch button. This will distribute the orders in a given
sequence, within the available capacity on the processing line.
The result can be seen here
Product wheel scheduling can run very automated in SAP if
you put in some work upfront to set up all the relevant master data.
