The rope is actually a mechanism that controls two different functions. First, it is the mechanism that determines how much and when to release inventory into the system. The most common practice is to tie an artificial “rope” from the constraint operation back to the front of the line. When the constraint produces and completes one unit of work and passes it to the next operation, then the rope is pulled to signal the front end of the line to release one more unit of work into the system. Rope signaling systems can vary. The rope signal is equal to the output of the constraint operation, no more and no less. This release mechanism, tied to the drumbeat of the constraint, will allow for a synchronized work flow and a smooth transition of work through the system.
The second
function of the rope is to initiate and maintain subordination for all other
processes in the line. By default,
following the cadence of the rope release signal causes subordination to the
remaining non-constraint processes to be executed. The non-constraints processes can only work on
what has been released to work on. By
releasing work only to the drumbeat, all other operations are held in check to
the rule of subordination. Even if the
non-constraints can do more work, they are restricted by subordination and only
allowed to work on parts or products required by the constraint.
The
systems inventory not only includes the work located at the buffer, but also
the cumulative total of inventory (work) at other process locations as
well. It is possible, and recommended,
that you establish an additional buffer at the shipping location. The shipping buffer can be used to help
control any system variation that occurs after the constraint. Bad things can and do happen after processing
at the constraint. The constraint buffer
provides the necessary protection in front of the constraint, and the shipping
buffer provides protection after the constraint. The shipping buffer is just a mechanism to
absorb and manage the inevitable variation that will occur. Buffer sizing at these two locations is a
variable, but you do need to start with something.
Consider, as a starting point for the buffer
at the drum (the constraint) location to be about one and a half for whatever
units of time you are measuring. For
example, if your constraints can produce ten units in one day, then the buffer
should be set at fifteen units (or 10 x 1.5 = 15). You may decide in time that the buffer is too
large or too small, so you can adjust it either up or down depending on the
need. The shipping buffer could be three
or four days or less depending on the speed of product through the system. It doesn’t need to be necessarily large in
quantity or long in time. It just needs
to be sufficient to protect against variation after the constraint. It’s also important to consider your shipping
buffer time in your scheduling calculation to determine the correct release
date into the system for on-time delivery.
If you watch your buffer locations carefully, you can make good decisions
to increase or decrease them based on some supportive historical data. If the buffer is constantly on the high end,
then reduce it. If it is constantly on
the low end, then increase it. Apply the
rule of common sense to determine the correct buffer.
When you know and understand the constraint location, and you buffer the work activity, and you send the correct release signal to the front of the line to release more work, then you have in essence implemented a system of synchronized flow.
In my next post we will continue our discussion of Drum Buffer Rope and will get into what a DBR system might look like.
Bob Sproull
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