Lean Manufacturing Tools

Fixed-time, unfixed-quantity
 conveyance In this type of handling system, a material handler performs a standard route through a facility at precisely determined time intervals such as every 20 minutes. The amount of material moved each time may vary, but the time interval is exact. During this interval the material handler follows a predetermined, standard route, picking up kanban cards signaling what materials to deliver next, and delivering the materials to production locations. This system often is coupled with a heijunka box in which the withdrawal intervals in the columns of the box correspond to the time required for the standard material handling route. This type of system often is employed in assembly operations where a large number of components need to be delivered to many points along a line. It also is called mizusumashi or waterspider conveyance.

Fixed-quantity, unfixed-time conveyance This type of handling system acts on signals from downstream locations to deliver exactly the materials needed when they are needed and in the right amounts. The material handler is signaled to collect materials from a preceding process when a trigger point or predetermined stock level is reached. Because the material handler collects a standard quantity of materials from the upstream process (such as one tray or one pallet or one skid), the quantity of material is fixed but the timing of conveyance varies with need. This type of system often is employed in facilities with storage areas for materials that are produced in batches due to long changeover times. As the cell or machine depletes the materials in the storage area, a signal is triggered for the material handler to replenish from upstream processes the amounts consumed. This type of system often is termed a call system or a call-parts system. It is less common but possible to use waterspider conveyance in conjunction with fixed-quantity, unfixed-time conveyance. In this arrangement, the material handler will dart around from one production process to another picking up fixed quantities of materials from varying processes on a route that changes with time.