Wright Williams & Kelly
Factory Explorer® v2.4 Technical Alert #20
Raw Process Time Calculations for Models with
Scrap or Rework on Active/Inactive Process
Steps
Summary: Factory Explorer®'s capacity analysis engine incorrectly calculates
raw process times for models that specify scrap or rework on active/inactive
process steps. To avoid this problem, use a goto with a reverse image of the
offending step's active/inactive product list. This problem is fixed in
Factory Explorer® v2.5 beta 2.
Details: When calculating raw process times, Factory Explorer® sets up and
solves a system of linear equations to compute the expected number of visits
to each process step assuming no scrap or rework. Within these calculations,
there was an error that caused the system of equations to be biased if a
model included scrap or rework on a process step that specified
active/inactive products. The error was generally small, but it was related
in size to the probability of scrap or rework - the larger the probability,
the larger the error in the raw process time calculation. The error would
only occur for products that were inactive for the steps specifying scrap or
rework.
Workaround: Generally, active/inactive steps occur in blocks within a
process flow. For example, a block could be a set of steps that are active
only for product XYZ. In front of each block that contains scrap or rework
steps, insert a 100% goto that branches around the block. On this goto step,
specify a reverse image of the offending block's active/inactive product
list. In our example, this would mean specifying that the goto is inactive
only for product XYZ. This will cause Factory Explorer® to completely skip
over the offending block during raw process time calculations for products
other than XYZ, and for product XYZ the calculations will be performed
correctly. These additional goto's will be unnecessary with the next release
of Factory Explorer®, so you may then delete them.
Techalert tafx20-9808
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