SMED (Single-Minute Exchange of Dies) is a system for dramatically reducing the time it takes to complete equipment changeovers. The essence of the SMED system is to convert as many changeover steps as possible to “external” (performed while the equipment is running), and to simplify and streamline the remaining steps. The name Single-Minute Exchange of Dies comes from the goal of reducing changeover times to the “single” digits (i.e. less than 10 minutes).
A successful SMED program will have the following benefits:
- Lower manufacturing cost (faster changeovers mean less equipment down time)
- Smaller lot sizes (faster changeovers enable more frequent product changes)
- Improved responsiveness to customer demand (smaller lot sizes enable more flexible scheduling)
- Lower inventory levels (smaller lot sizes result in lower inventory levels)
- Smoother startups (standardized changeover processes improve consistency and quality)
SMED was developed by Shigeo Shingo, a Japanese industrial engineer who was extraordinarily successful in helping companies dramatically reduce their changeover times. His pioneering work led to documented reductions in changeover times averaging 94% (e.g. from 90 minutes to less than 5 minutes) across a wide range of companies.
Changeover times that improve by a factor of 20 may be hard to imagine, but consider the simple example of changing a tire:
- For many people, changing a single tire can easily take 15 minutes.
- For a NASCAR pit crew, changing four tires takes less than 15 seconds.
Many techniques used by NASCAR pit crews (performing as many steps as possible before the pit stop begins; using a coordinated team to perform multiple steps in parallel; creating a standardized and highly optimized process) are also used in SMED. In fact the journey from a 15 minute tire changeover to a 15 second tire changeover can be considered a SMED journey.
In SMED, changeovers are made up of steps that are termed “elements”. There are two types of elements:
- Internal Elements (elements that must be completed while the equipment is stopped)
- External Elements (elements that can be completed while the equipment is running)
The SMED process focuses on making as many elements as possible external, and simplifying and streamlining all elements.
An excellent way to learn more about SMED is to walk through an implementation example. This section provides a step-by-step roadmap for a simple and practical SMED implementation.
Virtually every manufacturing company that performs changeovers can benefit from SMED. That does not mean, however, that SMED should be the first priority. In the real world, companies have finite resources, and those resources should be directed to where they will generate the best return.
So what should be the first priority? For most companies, the first priority should be ensuring that there is a clear understanding of where productive time is being lost, and that decisions on improvement initiatives are made based on hard data. That means putting a system in place to collect and analyze manufacturing performance data.
The de facto “gold” standard for manufacturing performance data is measuring OEE (Overall Equipment Effectiveness) with an additional breakdown of OEE loss categories into the Six Big Losses and a detailed breakdown of OEE Availability losses into Down Time Reason Codes (including codes for tracking changeover time).
Once a system for measuring manufacturing performance is in place collect data for at least two weeks to gain a clear picture of where productive time is being lost.
|SMED||If changeovers represent a significant percentage of lost productive time (e.g. at least 20%) consider proceeding with a SMED program.|
|TPM||Otherwise, consider first focusing on a TPM (Total Productive Maintenance) program.|
Step One – Identify Pilot Area
In this step, the target area for the pilot SMED program is selected. The ideal equipment will have the following characteristics:
|Duration||The changeover is long enough to have significant room for improvement, but not too long as to be overwhelming in scope (e.g. a one hour changeover presents a good balance).|
|Variation||There is large variation in changeover times (e.g. changeover times range from one to three hours).|
|Oportunities||There are multiple opportunities to perform the changeover each week (so proposed improvements can be quickly tested).|
|Familiarity||Employees familiar with the equipment (operators, maintenance personnel, quality assurance, and supervisors) are engaged and motivated.|
|Constraint||The equipment is a constraint/bottleneck – thus improvements will bring immediate benefits. If constraint equipment is selected, minimize the potential risk by building temporary stock and otherwise ensuring that unanticipated down time can be tolerated.|
In order to create a wide base of support for the SMED project, include the full spectrum of associated employees in the selection process, and work hard to create a consensus within the team as to the target equipment choice.
Once the target equipment has been selected, record a baseline time for the changeover. Changeover time should be measured as the time between production of the last good part (at full speed) and production of the first good part (at full speed). Be cognizant of the “Hawthorne Effect”; changeover times may temporarily improve as a simple result of observing the process. When possible, use prior data to baseline the changeover time.
Step Two – Identify Elements
In this step, the team works together to identify all of the elements of the changeover. The most effective way of doing this is to videotape the entire changeover and then work from the videotape to create an ordered list of elements, each of which includes:
- Description (what work is performed)
- Cost in Time (how long the element takes to complete)
Some useful tips for this step:
|Elements||A typical changeover will result in 30 to 50 elements being documented.|
|Sticky Notes||A fast method of capturing elements is to create a series of post-it notes that are stuck to a wall in the order in which they are performed during the changeover.|
|Man and Machine||Be sure to capture both “human” elements (elements where the operator is doing something) and “equipment” elements (elements where the equipment is doing something). As discussed later, the human elements are usually easiest to optimize.|
|Other Notes||While videotaping the changeover have several observers taking notes. Sometimes the observers will notice things that are missed on the videotape.|
|Observe||Only observe – let the changeover take its normal course.|
The deliverable from this step should be a complete list of changeover elements, each with a description and time “cost”.
Step Three – Separate External Elements
In this step, elements of the changeover process that can be performed with little or no change while the equipment is running are identified and moved “external” to the changeover (i.e. performed before or after the changeover). It is not unusual for changeover times to be cut nearly in half with this step alone.
For each element the team should ask the following question: Can this element, as currently performed or with minimal change, be completed while the equipment is running?
If the answer is yes, categorize the element as external and move it before or after the changeover, as appropriate.
Examples of candidate elements for such treatment include:
|Retrieval||Retrieval of parts, tools, materials, and/or instructions.|
|Inspection||Inspection of parts, tools, and/or materials.|
|Cleaning||Cleaning tasks that can be performed while the process is running.|
|Quality||Quality checks for the last production run.|
The deliverable from this step should be an updated list of changeover elements, split into three parts: External Elements (Before Changeover), Internal Elements (During Changeover), and External Elements (After Changeover).
Step Four – Convert Internal Elements to External
In this step, the current changeover process is carefully examined, with the goal of converting as many internal elements to external as possible.
For each internal element, the team should ask the following questions: If there was a way to make this element external, what would it be? How could we do it?
This will result in a list of elements that are candidates for further action. This list should be prioritized so the most promising candidates are acted on first. Fundamentally, this comes down to performing a cost/benefit analysis for each candidate element:
- Cost as measured by the materials and labor needed to make the necessary changes.
- Benefit as measured by the time that will be eliminated from the changeover.
Once the list has been prioritized work can begin on making the necessary changes.
Examples of techniques that can be used to convert internal elements to external are:
|Advance Preparation||Prepare parts in advance (e.g. preheat dies in advance of the changeover)|
|Jigs||Use duplicate jigs (e.g. perform alignment and other adjustments in advance of the changeover)|
|Modularize||Modularize equipment (e.g. replace a printer instead of adjusting the print head so the printer can be configured for a new part number in advance of the changeover)|
|Modify||Modify equipment (e.g. add guarding to enable safe cleaning while the process is running)|
The deliverable from this step should be an updated list of changeover elements, with fewer internal elements, and additional external elements (performed before or after the changeover).
Step Five – Streamline Remaining Elements
In this step, the remaining elements are reviewed with an eye towards streamlining and simplifying so they can be completed in less time. First priority should be given to internal elements to support the primary goal of shortening the changeover time.
For each element, the team should ask the following questions: How can this element be completed in less time? How can we simplify this element?
As in the previous step a simple cost/benefit analysis should be used to prioritize action on elements.
Examples of techniques that can be used to streamline elements are:
|Release||Eliminate bolts (e.g. use quick release mechanisms or other types of functional clamps)|
|Adjustment||Eliminate adjustments (e.g. use standardized numerical settings; convert adjustments to multiple fixed settings; use visible centerlines; use shims to standardize die size)|
|Motion||Eliminate motion (e.g. reorganize the work space)|
|Waiting||Eliminate waiting (e.g. make first article inspection a high priority for QA)|
|Standardizing||Standardize hardware (e.g. so fewer tools are needed)|
|Operations||Create parallel operations (e.g. note that with multiple operators working on the same equipment close attention must be paid to potential safety issues)|
|Mechanize||Mechanize (normally this is considered a last resort)|
The deliverable from this step should be a set of updated work instructions for the changeover (i.e. creating Standardized Work) and a significantly faster changeover time!
When implementing SMED it is helpful to recognize that there are two broad categories of improvement:
- Human (achieved through preparation and organization)
- Technical (achieved through engineering)
Experience has taught that the human elements are typically much faster and less expensive to improve then the technical elements. In other words, the quick wins are usually with the human elements. Avoid the temptation, especially with technically proficient teams, to over-focus on technical elements. Instead, focus first on the human elements.
The following chart illustrates this principle, showing example areas of opportunity for SMED projects.