SMED stands for Single Minute Exchange of Dies. It is one of the methodologies of Lean to reduce the changeover time. The name SMED comes from the automotive industry where dies are used to make car auto-body parts. These machines use large hydraulic forces to press a pair of dies onto a sheet metal in order to impart a specific shape to the sheet metal such as a front hood of a car. Due to the large forces involved, some of these machines can be pretty large and changing the dies from one shape to another requires significant effort and downtime. In the past, the changeover time could be several days.
If the changeover time is large, then making frequent changeovers is considered non value added time as the equipment is not productive during changeovers. Hence, the number of changeovers is usually minimized, which means these machines end up making hundreds of parts of one type before a changeover is initiated to another die. The numbers of parts that are made are larger than what is immediately required causing excessive inventory in the factory floor. One of the root causes of large inventories is the high changeover time required to change dies. Taiichi Ohno, the father of Lean along with Shigeo Shingo came up with a method to reduce the changeover time to less than 10 minutes. Hence, this methodology was named as the Single Minute Exchange of Dies (SMED).
The techniques developed is not only applicable for dies but can be used anywhere a changeover reduction is required. Don’t get confused with the name as well. A successful SMED program could reduce the changeover time from 1 day to less than 1 hour (it does not always have to be in minutes). In some cases, the changeover time could be reduced to less than 5-10 seconds (as in NASCAR pit crew). This concept does not apply to manufacturing alone. For example, when an aircraft lands at an airport and before it can take off for the next flight can be considered as a changeover time. During this changeover, the flight has to be refueled, cleaned, all the necessary checks have to be performed, luggage of the arriving passengers has to be removed and those of the departing passengers has to be loaded etc. The longer it takes to make this changeover the lower is the efficiency of the entire process as the plane is not productive and generating revenue during the changeover.
Why a large changeover time is bad?
First of all, changeover time is not value adding. The reason being that during changeover, no product is being produced. Though the changeover can help us make the right product that the customer requires, if we are able to reduce the changeover time to a smaller value, we can still accomplish what the customer wants without it impacting the production too much. Hence, from this perspective, a smaller changeover time is preferable. More importantly, a larger changeover time implies that the organization will not do frequent changeovers in order to maintain the productivity numbers. This means that even if a changeover is required, to maintain efficiency numbers, the changeover may be delayed. Running the process for longer than required means that the produce inventory is in excess of what is required; this is one of the wastes of Lean (over-production). In addition, this excess inventory needs to be stored in some location causing transportation waste; there is a chance of obsolescence of the parts and rework etc. Hence, it is clear that a large changeover time is not beneficial for any process. If the changeover time can be reduced, then inventories can be reduced and thus we reduce the waste in the process. Smaller inventory in the process also implies a smaller lead-time and hence faster reaction to that the customer expects.
Changeover reduction can facilitate the following benefits:
- Lower cost (since there is less NVA activities, lower inventory levels)
- Smaller lot sizes (since we can have more frequent changeover)
- Faster response to customer demand
We can use the SMED methodology to reduce the changeover time. A typical SMED methodology consists of the following steps:
Step 1: Identify all the activities that are currently being performed during a changeover including the time for each activity. You may want to video tape the changeover activity with an on-screen timer to ensure that all activities are captured. In addition, you may want to observe multiple changeovers to ensure you capture the variation as well as document all possible activities. One thing to watch out for during the video tape process is that the process is not altered due to the videotaping process – you should try to capture the as-is situation as close to reality as possible.
Step 2: Classify each activity that is performed as either an internal activity or external activity. All the activities that are performed when the equipment is stopped are called internal activities. All activities that are performed when the machine is running are called external activities. For example, if you need to change the wheels of a car, hopefully the car has to be stopped when you perform the changeover – that would hence be classified as an internal activity. If you need to check the engine temperature or other parameters it could possibly be done by the driver when the car is still in motion and hence could be an external activity. The most significant way in which changeover time can be reduced is to convert all internal activities to external activities. During a changeover, a number of activities are performed such as:
- Allowing the machine tool to cool down (if required)
- Getting the required tooling & instruments to do the changeover
- Removing the old die (which may be attached using screws, fixtures etc.)
- Clean-up of the area as required
- Putting the new die (including fixtures, screws etc.)
- Calibration of the machine tool as appropriate
- Running the machine tool for the first few parts to ensure good quality output
- Removing the tools & instruments from the work area
By moving those activities that are performed when the machine is stopped to when the machine is still in production the overall changeover time can be reduced. For example, you could get all the required tooling & instruments to do the changeover while the machine tool is still in production. Before a flight lands at an airport, the flight attendants could clean up the aircraft as much as possible while the plan is still in the air – this would be an example of an internal activity that is converted to an external activity. Of course, not all internal activities can moved to external activities but we should try to move as many activities as possible.
Step 3: Review all internal activities that are left over to see if they can either be eliminated, made simpler or combined with other steps in order to reduce the total changeover time. Techniques that can be used here are:
- Eliminate bolts & screws using quick release mechanisms
- Eliminate adjustments and use standardized settings
- Eliminate motion by reorganizing the work area
- Modularize equipment to reduce changeover time.
When performing SMED, don’t ignore the people side. In addition to the above technical elements on reducing the changeover time, significant improvements can also be made by paying attention to the people element. For example, by having clear roles and responsibilities, coaching team members on the concepts of kaizen and continuous improvement to always look out for improvement opportunities and constantly improving the process, standardizing the work instructions so that work gets done in a consistent and repeatable fashion we can make significant improvements to our process as well.
A typical SMED event may last 1-3 days. This should not be seen as a one-time activity. Once a SMED activity is completed and the changeover time has been reduced, we should ensure that new process is followed for a period of time to standardize the new way of working. In the meantime, new ideas and suggestions may come from team members to further refine and improve the process. Once the process has been stabilized, this would be a good time to initiate a subsequent SMED activity to further reduce the changeover time. Remember that the ideal changeover time is zero.
It is also important to stress that once the changeover time has been reduced, the related processes should also be streamlined – for example you may need to recalculate the appropriate inventory levels, stock replenishment strategies etc. For example, there is no benefit of changeover time reduction for a flight landing if the aircraft sits at the terminal idle and we don’t capitalize on the reduced changeover time. The shorter the changeover time, the more the number of changeovers you need to plan in your process so that the benefits of reduced changeover times can really be leveraged by the organization.
You can use software such as Sigma Magic to facilitate your changeover analysis. The screenshot below shows the list of activities and classifies them as internal & external. You can perform a time study to capture the changeover time before (with the existing process) and after (after making the improvements to the process using the 3 steps described above).
The output of the analysis software is shown below. It contains the summary of the data you have collected and the improvements you have made to the process along with a control chart which highlights the reduction in changeover time along with information on whether the new process is in control.
 Sigma Magic Software: http://www.sigmamagic.com.