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One
Piece Flow refers to the concept of moving one workpiece at a time
between operations within a workcell. At the opposite
extreme, we might process an entire batch or lot at each operation
before moving it to the next operation.
This
idea has many benefits. It keeps WIP at the lowest possible level.
It encourages work balance, better quality and a host of internal
improvements.
We
often hear One Piece Flow pronounced as an absolute must for any
workcell. Like the search for the Holy Grail it is taken as a moral
imperative. At Strategos,
we find it more helpful to think of this question as the
"Transfer Batch" or "Internal Lot size." The
issue is: "What Internal Lot Size will help the cell meet its
performance goals."
To
approach engineering design using slogans, edicts, or other grand
pronouncements is a dangerous business. Yet many teachers,
consultants and practitioners do it. Here are some of the more
common edicts:
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Inventory
is evil and must be eliminated regardless of all other
considerations.
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The
only relevant measure of work-cell performance is throughput
time.
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Machine
utilization is of no consequence and should be ignored.
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Work-cells
must have a straight-through flow for all products.
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Work-cells
must have one-piece flow.
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Low-tech,
manual machine tools are superior to high-tech NC equipment.
Performance
goals may vary from one cell or factory to the next. For some,
Quality is the foremost measure of performance. For others, delivery
speed and customer response dominates. For others, labor efficiency
is foremost. Where equipment is expensive and capital scarce,
equipment utilization may play a role, although it is often
overrated.. |
The
question of Internal Lot Size is just one of many decisions that
specify the workcell design. Every
engineering design is a series of decisions, made in a
logical sequence and often
interdependent. For more on this, see our page on Rationalized
Workcell Design.
One
Piece Flow is an ideal that engineers should strive for.
But, it simply does not work when the transfer time begins to
approach the work time. Nor does it work with certain processes such
as shot blasting.
When
a cell must address a wide product variety with varied routes, work
times, and setup times, One Piece Flow is also counterproductive.
This often occurs in jobbing-type machine shops or sheet metal
shops.
In
situations such as those mentioned above, small-batch flow can be a
good answer. For example, the lot size required by a customer might
be 100 units of product. This is the "External Lot Size."
The optimum "Internal Lot Size" or "Transfer
Batch" might be 10 units.
In
operation, the first operation in the cell makes 10 units of the 100
and passes the 10 units to the second operation. The first operation
then makes 10 more units and passes them along. This continues until
the External Lot of 100 is complete.
The
best workcells are engineered designs
which address both the technical
and human issues. Learn more...
Workcells-
The Heart of Lean Manufacturing
How
To Design Workcells
Workcell
Design for Lean Manufacturing - Seminar
For Examples
of Inappropriate Use of One Piece Flow, Click Below
Problems With One Piece Flow |
