Understanding Time Buffers
Time Buffers are an important element of DBR Scheduling, but often
misunderstood.  Some view time buffers as additional blocks of time
added to the traditional product chain layout. In fact, DBR time buffers
are the result of rearranging the queuing, waiting, and moving time
already included in a traditional product chain.   
Traditional scheduling approaches use 3 components, queuing time, setup/run
time
, and wait-move time to determine the start and end point for each
workcenter operation.  It is not known exactly how much setup/run time will be need
at Operation #1 (See Figure 1).  Exactly how long a job will wait before being
moved to Operation #2 is also not known.  This is due to variability present in all
systems.  To set a feasible starting point for the Operation #2, time is added to the
Interoperation time to protect (buffer) against possible variability.  However, this
protection is limited to only Operation #1.  Any unneeded time is lost while the job
waits for its start date/time at Operation #2.  Protection (buffer) against variability
at an earlier operation does not offer any protection for a later operation (See
Figure 2).  DBR considers this as  buffer time and unnecessarily increasing
product lead-time.  Instead, DBR positions time buffers to provide protection for
multiple operations.  DBR also uses the term
time buffer to cover all
non-operational activities, including queuing, waiting, and moving.  Figure 2 can
be converted to DBR by rearranging the blocks. Work is grouped together
followed by queuing/wait-move time (See Figure 3).  Now the buffer time is
positioned to provide protection for all operations.  This rearrangement brings
another change - no firm start and end date/times for each operation.  Only  release
dates and ship dates are firm.  Material is queued upon arrival at a workcenter and
processed as soon as possible following a FIFO priority rule.  This facilitates faster
movement of material through the shop. Pooling buffer time also reduces the total
amount of buffer needed, decreasing the product lead-time.  Figure 4 shows the
basic DBR Planning Template
without a drum and is appropriate if the constraint for
the product chain is in the market.  When a drum is present, a time buffer is placed
in front of the drum and sized appropriately to handle all upstream variability.  The
Drum Buffer is critical for protecting the drum from any disruptions.  With the
introduction of a drum buffer, the downstream buffer only needs to protect the
portion of the system
after the drum.  The downstream buffer size can be reduced
and is commonly referred to as the Shipping Buffer.  Figure 5 shows the basic
DBR Planning Template for a process
with an internal drum.
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