"Waterspider," "Tugger," "Transporter," etc. Whatever the name of this functionality, the responsibility is to transport the materials from where it is stored to where it will be used. One common problem is material not arriving at the point of use in a stable way.
One typical approach to this problem is to add inventory—two hours, one shift, one day, one week etc. We add and add more stock, but unfortunately, this solution does not eliminate the problem. The quantity of inventory and stability are not related. Even if we had a large amount, we still have material shortages due to missing exact part numbers, lost in a pile of materials, etc. Even worse is that large inventory creates this false perception and culture that we can violate some material handling rules. That makes it even more unstable condition. Just because we keep stock, this does not guarantee stability.
Another approach is to calculate the periodic work of material handling. Suppose that it takes 15 minutes to go to the warehouse and pick up 100 parts. Nine hundred seconds / 100 pieces, so 9 seconds per part. Let add 9 seconds to each cycle time as periodic work. Is this going to make the process stable? No, because the fact that it takes fifteen minutes to perform the periodic work does not change. The fifteen minutes of material handling to the warehouse will become a downtime of cycle work. The only way to make this happen without downtime is that during the production of 100 parts, the process needs to move ahead to build enough inventory to cover that 15 minutes. Is that possible? What happens if there are multiple part numbers? What happens if there are various processes? Even if it is possible, all operations are asynchronous. It is a mathematically suitable but unmanageable condition.
For the above reasons, it is better to separate the periodic work and delegate the task as the material handler to stabilize the process. The material handler is like blood vessels and the nerve system of our body. It delivers what is necessary throughout our body system. It also carries some form of information throughout the system. Just like our body, the material handler can not have arrhythmia in the system. The system should run consistently.
Just because the system needs to be consistent, it can not be too rigid. As Taiichi Ohno said, this system requires "Autonomic nerves" to adjust to the minor shop floor and market changes. So we give flexibility in timing or quantity. If we fix both, the system is a push system. If we do not fix both, it is chaos. We can only design fixed timing flexible quantity system or fixed quantity flexible timing system. The choice between the two methods depends on the materials and mode of transportation.
Once the method of this system is selected, we need to standardized the points – lines – areas. The critical points for the material handlers are delivery points and pick points. The exact locations on where to pick up and drop off the materials need to be defined. In today's IT world, the place of materials can be flexible. The system will find the open space for upcoming material and place it randomly. Yet, this will create variations of the material handler's work. The location should have an "address" and "mailbox" to place material. Randomly thrown newspapers get damaged or lost, causing unstable delivery. Even if it is quick for the material handler, it is not the best method of delivery. Also, materials that do not fit the "mailbox" will overflow and goes somewhere. This temporary placement of the material is another cause of instability. The "mailbox" should be large enough, or the frequency of delivery should be frequent to make each delivery small to avoid overflow. Or before the surge happens, the material handler should stop and notify since there is very likely a problem.
Once all the delivery points and pick-up points are defined, these points are connected to form the route. The route should be in a single stroke (Ex. Seven Bridges of Königsberg). If this is not possible, we have to rethink the mode of transportations or locations of materials. The route is made from two parts, moving with material or empty-handed. The worst waste of transportation is the empty-haded segments. Minimizing this segment is essential. One key challenge to stabilize this route is to give the material handler the right of way. Very often, the material handler has to wait for someone else, causing fluctuations in its work. The designated route just for material handler helps accomplish such conditions. Similar to this right of way thinking, inside the workstation, the right to work belongs to the operator. To prevent material handlers from interfering with the operators, we need to design the racks and other material-providing devices to eliminate the need for the material handler to enter the workstation.
Once these points and lines are designed, we can define the standardized work of the material handler. But simply combining these points and lines is not good enough. Once I designed a material handler's standardized work. The cycle time was 27 minutes. But then, my boss started challenging me on what would happen if we kept repeating this work? If repeated, then each cycle will end at 27, 54, 81, 108… Then what will happen when they do break or end of the shift? The work will stop in the middle. Stopping the material handler's work in the middle will become instability. The location where the work ends becomes random. The first thing to do will be to find the material handler's work vehicle. Do the following shift know precisely where the last delivery or pick-up ended? The material handler's standardized work has to be the divisor of working hours between shifts.
Most factories do not care about how long should be the working hours between each break. But randomly selected length of work time makes the standardized work challenging to repeat. In case my 27 minutes material handler, what did I do? First, I thought about adding 3 minutes, but then I recognized that that is adding three minutes of waiting waste. So I decided to challenge 20 minutes as my target. To accomplish that, I just had to destroy some walls to create shortcuts on the route.
Is material handling simple work? It should be. Unfortunately, if the above points – lines - areas are not defined, the material handler's work becomes complex and unstable. Because I believe the material handler is equivalent to our body's blood vessels and nerve system, inconsistent material handling is unhealthy for the entire factory.