Lean practitioners are familiar with the concept of shadow boards. We frequently use them to organize tools so that it’s immediately evident if one is missing. If one is, a visual “sweep” of the board prompts the owner to stop and search until the tool is found and returned to its place.
Like shadow boards, shadow boxes are used for visual organization.
One form of a shadow box we’re all familiar with is the egg cartons. From grade school we’re taught how many eggs to expect in one: an even dozen. If an egg is missing, it is visually apparent.
In Lean, the same concept holds true. We use shadow boxes to organize parts. They become a visual quality tool. Here are a few ways we can use them.
TOO FEW PARTS: Before commencing work on the next product, a worker makes a quick visual sweep of the shadow box. This “sweep” allows them to ascertain immediately if all the parts are present and int he correct quantity. If they are not, the worker signals for assistance; e.g. pulls the Andon cord. This prompts a material handler or water spider (discussed in an earlier post) into immediate action.
The mere existence of shadow boxes implies Just In Time (JIT) delivery of the exact parts required to build one product. If there are too few, the product cannot be completed, so early identification can allow the parts to be delivered and installed within Takt Time.
TOO MANY PARTS: While having too many parts doesn’t always stop the line, it is an equally unfavorable circumstance. The mere use of shadow boxes implies that the correct part count is critical. That means that the shadow box is used as a quality check: too few, or too many, both imply a defect in the supply process.
The presence of too many parts can lead to installing too many parts in the product and cause a product defect. That is serious problem.
Shadow boxes can also help in the early detection of wrong parts. If the process (standard work) calls for four bolts all 3/4” in length, and one is clearly longer, that’s a potential defect. In effect, the operator only has three bolts and needs to take the same action as if there were too few parts.
As with shadow boards, each part has its own spot in the shadow box. For instance, the nuts that went with the four bolts addressed above would be in their own slot. Likewise, washers, cotter keys, rivets, etc would each have their own slots. Once the shadow box is set up, those slots stay the same.
A well laid out shadow box allows the operator to grab the right parts without taking their eyes off the product. For example, if the operator knows that the 1/4” lock washers are in the fourth slot from the left, they run their fingers along the top of the slots and count 1 – 2 – 3 – 4. When they hit “four,” they grab the washer in that slot.
Not every circumstance calls for shadow boxes, but it is one of the arrows in the quiver of a good Lean practitioner. Don’t hesitate to apply the concept when conditions call for it.
