Heijunka is the Japanese term for production leveling. The goal of heijunka is to reduce operations batch size as much as possible while still meeting production volume targets. Heijunka facilitates just-in-time (JIT) production and significantly reduces all levels of inventory in a system. This contrasts with traditional production that tended to increase batch sizes to reduce downtime due to setups.
In short, the longer it takes to change production from one model or product to the next, the larger the batch size needs to be. That’s why reducing setup times is a prime concern of lean organizations.
Here are some benefits of reducing the batch size:
Easily identified problems: Removing the clutter created by large batch sizes allows you to quickly identify problems such as machine breakdowns because subsequent operations will soon run out of inventory if the machine feeding them breaks down and there’s no inventory for it to work on.
Improved quality and reduced waste: Small lot sizes allow you to discover quality problems quickly and prevent an operation from producing a large number of defective parts.
Less space requirements: With all the inventory eliminated, you need less production space. You can place operations closer together, which can enhance communication and process awareness.
Reduced inventory: Small lot sizes reduce not only the amount of in-process inventory (WIP) but also the requirement for raw materials (RMI) and the amount of finished goods inventory (FGI).
Reduced process flow time: Speed is a critical factor in a lean organization. Minimizing the batch size significantly reduces the wait time in the process. For a complete analysis on the effect of batch size on the process.
As Shigeo Shingo, a Japanese industrial engineer who was considered a leading expert on manufacturing practices during the lean revolution, discovered at Toyota, a major impediment to reducing batch sizes is the changeover time required to prepare the machinery for a different part.
Long setup times reduce active processing time and significantly reduce the capacity of the equipment because no parts are produced during the setup. Looking for a way to reduce the downtime, Shingo devised the single-minute exchange of die (SMED) method for changeovers. (Single minute doesn’t mean one minute but that the setup can be accomplished in less than ten minutes.)
In the strictest sense, the setup time is the time spent actually converting the equipment, while the changeover time includes the removal of all the old materials and products from the line and the time spent adjusting the equipment after it has been changed, before actual production begins.
SMED was first applied to change the dies on the stamping machines that shaped the metal of automobile body parts. These dies are very heavy, and the tolerances on the parts are less than a millimeter, making installation time-consuming because the machines need to be perfectly aligned. Although originally designed for die changes, the SMED method has applications for all types of machine setups. Here are the steps to SMED:
Observe the current methodology.
Document all the steps currently taken to accomplish the changeover. This can be in the form of a process flow diagram.
Separate the internal and external activities.
External activities are those activities that can be completed before the machine is shut down for the setup, and internal activities are those activities that can only be done when the machine is stopped.
Eliminate all non-value-added steps.
Are any steps in the process unnecessary?
Turn internal activities into external ones.
By taking activities off-line, you can reduce the time the equipment isn’t operating.
Simplify the internal activities.
For example, eliminate nuts, bolts, and screws, and replace them with clamps if a fastener is really needed. Use pins and jigs to simplify adjustments.
Simplify the external activities and make sure that they don’t delay any internal activities.
Document the new process and measure improvement.
Repeat the process.
Continuously reduce the setup time. You’ll likely need several cycles to get to a single digit setup time.
SMED reduced the time for die changes at Toyota to less than ten minutes per die, an improvement of over 40 times in most cases.