Single-Minute Exchange of Die
(SMED) is one of the many methods for reducing waste in a manufacturing
process. It provides a rapid and efficient way of converting a manufacturing
process from running the current product to running the next product. This
rapid changeover is key to reducing production lot sizes and thereby improving
flow.
The phrase "single
minute" does not mean that all changeovers and startups should take only one
minute, but that they should take less than 10 minutes (in other words,
"single-digit minute"). Closely associated is a yet more difficult
concept, One-Touch Exchange of Die, (OTED), which says
changeovers can and should take less than 100 seconds, is a tool used in
manufacturing. However SMED's utility of is not limited to manufacturing.
History
The
concept arose in the late 1950s and early 1960s, when Shigeo Shingo was
consulting to a variety of companies including Toyota, and was contemplating
their inability to eliminate bottlenecks at car body-molding presses. The
bottlenecks were caused by long tool changeover times which drove up production
lot sizes. The economic lot size is calculated from the ratio of actual
production time and the 'change-over' time; which is the time taken to stop
production of a product and start production of the same, or another, product.
If change-over takes a long time then the lost production due to change-over’s
drives up the cost of the actual production itself. This can be seen from the
table below where the change-over and processing time per unit are held
constant whilst the lot size is changed. The Operation time is the unit
processing time with the overhead of the change-over included. The Ratio is the
percentage increase in effective operating time caused by the change-over. SMED
is the key to manufacturing flexibility.
Changeover time Lot size Process time per
item Operation time
Ratio
8 hours
100 1 min
5.8 min 480%
8 hours
1,000 1 min
1.48 min 48%
8 hours
10,000 1 min 1.048 min 5%
Toyota's additional problem was
that land costs in Japan are very high and therefore it was very expensive to
store its vehicles. The result was that its costs were higher than other
producers because it had to produce vehicles in uneconomic lots.
The "economic lot
size" (or EOQ) is a well-known, and heavily debated, manufacturing
concept. Historically, the overhead costs of retooling a process were minimized
by maximizing the number of items that the process should construct before
changing to another model. This makes the change-over overhead per manufactured
unit low. According to some sources optimum lot size occurs when the interest
costs of storing the lot size of items equals the value lost when the
production line is shut down. The difference, for Toyota, was that the economic
lot size calculation included high overhead costs to pay for the land to store
the vehicles. Engineer Shingo could do nothing about the interest rate, but he
had total control of the factory processes. If the change-over costs could be
reduced, then the economic lot size could be reduced, directly reducing
expenses. Indeed the whole debate over EOQ becomes restructured if still
relevant. It should also be noted that large lot sizes require higher stock
levels to be kept in the rest of the process and these, more hidden costs, are
also reduced by the smaller lot sizes made possible by SMED.
Over a period of several years,
Toyota reworked factory fixtures and vehicle components to maximize their
common parts, minimize and standardize assembly tools and steps, and utilize
common tooling. These common parts or tooling reduced change-over time.
Wherever the tooling could not be common, steps were taken to make the tooling
quick to change.
Example
Toyota found that the most
difficult tools to change were the dies on the large transfer-stamping machines
that produce car vehicle bodies. The dies – which must be changed for each
model – weigh many tons, and must be assembled in the stamping machines with
tolerances of less than a millimeter; otherwise the stamped metal will wrinkle,
if not melt, under the intense heat and pressure.
When Toyota engineers examined
the change-over, they discovered that the established procedure was to stop the
line, let down the dies by an overhead crane, position the dies in the machine
by human eyesight, and then adjust their position with crowbars while making
individual test stampings. The existing process took from twelve hours to
almost three days to complete.
Toyota's first improvement was
to place precision measurement devices on the transfer stamping machines, and
record the necessary measurements for each model's die. Installing the die
against these measurements, rather than by human eyesight, immediately cut the
change-over to a mere hour and a half.
Further observations led to
further improvements – scheduling the die changes in a standard sequence (as
part of FRS) as a new model moved through the factory, dedicating tools to the
die-change process so that all needed tools were nearby, and scheduling use of
the overhead cranes so that the new die would be waiting as the old die was
removed. Using these processes, Toyota engineers cut the change-over time to
less than 10 minutes per die, and thereby reduced the economic lot size below
one vehicle.
The success of this program
contributed directly to just-in-time manufacturing which is part of the Toyota
Production System. SMED makes Load balancing much more achievable by reducing
economic lot size and thus stock levels.
Effects of implementation
Shigeo Shingo, who created the
SMED approach, claims that in his data from between 1975 and 1985 that average
setup times he has dealt with have reduced to 2.5% of the time originally
required; a 40 times improvement.
However, the power of SMED is
that it has a lot of other effects which come from systematically looking at
operations; these include:
Stockless
production which drives inventory turnover rates,
Reduction in
footprint of processes with reduced inventory freeing floor space
Productivity
increases or reduced production time
Increased
machine work rates from reduced setup times even if number of changeovers
increases
Elimination
of setup errors and elimination of trial runs reduces defect rates
Improved
quality from fully regulated operating conditions in advance
Increased
safety from simpler setups
Simplified
housekeeping from fewer tools and better organization
Lower expense
of setups
Operator
preferred since easier to achieve
Lower skill
requirements since changes are now designed into the process rather than a
matter of skilled judgment
Elimination
of unusable stock from model changeovers and demand estimate errors
Goods are not
lost through deterioration
Ability to
mix production gives flexibility and further inventory reductions as well as
opening the door to revolutionized production methods (large orders ≠ large
production lot sizes)
New attitudes
on controllability of work process amongst staff
Implementation
Shigeo Shingo recognizes eight
techniques that should be considered in implementing SMED.
1. Separate internal from external setup
operations
2. Convert internal to external setup
3. Standardize function, not shape
4. Use functional clamps or eliminate
fasteners altogether
5. Use intermediate jigs
6. Adopt parallel operations
7. Eliminate adjustments
8. Mechanization
NB: External setup can be done
without the line being stopped whereas internal setup requires that the line be
stopped.
He suggests that SMED
improvement should pass through four conceptual stages:
A) Ensure that external setup
actions are performed while the machine is still running,
B) Separate external and
internal setup actions, ensure that the parts all function and implement
efficient ways of transporting the die and other parts,
C) Convert internal setup
actions to external,
D) Improve all setup actions.
Formal method
There are seven basic steps to
reducing changeover using the SMED system:
1. OBSERVE the current methodology (A)
2. Separate the INTERNAL and EXTERNAL
activities (B). Internal activities are those that can only be performed when
the process is stopped, while External activities can be done while the last
batch is being produced, or once the next batch has started. For example, go
and get the required tools for the job BEFORE the machine stops.
3. Convert (where possible) Internal
activities into External ones (C) (pre-heating of tools is a good example of
this).
4. Streamline the remaining internal
activities, by simplifying them (D). Focus on fixings - Shigeo Shingo observed
that it's only the last turn of a bolt that tightens it - the rest is just
movement.
5. Streamline the External activities, so
that they are of a similar scale to the Internal ones (D).
6. Document the new procedure, and actions
that are yet to be completed.
7. Do it all again: For each iteration of
the above process, a 45% improvement in set-up times should be expected, so it
may take several iterations to cross the ten-minute line.
This diagram shows four
successive runs with learning from each run and improvements applied before the
next.
Run 1
illustrates the original situation.
Run 2 shows
what would happen if more changeovers were included.
Run 3 shows
the impact of the improvements in changeover times that come from doing more of
them and building learning into their execution.
Run 4 shows
how these improvements can get you back to the same production time but now
with more flexibility in production capacity.
Run N (not
illustrated) would have changeovers that take 1.5 minutes (97% reduction) and
whole shift time reduced from 420 minutes to 368 minutes a productivity
improvement of 12%.
The SMED concept is credited to
Shigeo Shingo, one of the main contributors to the consolidation of the Toyota
Production System, along with Taiichi Ohno.
Key elements to observe
Operation Proportion of time
Preparation, after-process
adjustment, and checking
raw materials, blades, die, jigs, gauges, etc. 30%
Mounting and
removing blades, etc. 5%
Centering,
dimensioning and setting of conditions 15%
Trial runs and
adjustments 50%
Look for:
1. Shortages, mistakes, inadequate
verification of equipment causing delays and can be avoided by check tables,
especially visual ones, and setup on an intermediary jig
2. Inadequate or incomplete repairs to
equipment causing rework and delays
3. Optimization for least work as opposed to
least delay
4. Unheated molds which require several
wasted 'tests' before they will be at the temperature to work
5. Using slow precise adjustment equipment
for the large coarse part of adjustment
6. Lack of visual lines or benchmarks for
part placement on the equipment
7. Forcing a changeover between different
raw materials when a continuous feed, or near equivalent, is possible
8. Lack of functional standardization that
is standardization of only the parts necessary for setup e.g. all bolts use
same size spanner, die grip points are in the same place on all dies
9. Much operator movement around the
equipment during setup
10. More attachment points than actually required for the forces to be
constrained
11. Attachment points that take more than one turn to fasten
12. Any adjustments after initial setup
13. Any use of experts during setup
14. Any adjustments of assisting tools such as guides or switches
