AUTOMATIC PART RESUPPLY
By Dr. David M. Anderson, P.E., CMC
The illustration below shows two rows of part bins which are set up for resupply by the two-bin kanban system.1 Initial assembly starts with all bins full of parts. When the part bin nearest the worker is depleted, the full bin behind moves forward, as shown by the empty space in the illustration. The empty part bin then is returned to its "source," which could be the machine that made the part, a sub-assembly workstation that assembled the part, or a supplier. The source fills the bin and returns it to this assembly workstation behind its counterpart which is now dispensing parts.
The beauty of Kanban part resupply is that the system ensures an uninterrupted supply of parts without forecasts or complicated ordering procedures. The number of parts in a bin is based on the highest expected usage rate and the longest resupply time. The size of the bins are determined by the bin quantity and size of the parts. For large parts, some companies use two-truck kanbans, in which parts are drawn from one truck trailer while the other trailer goes back to the supplier for more parts. Alternate systems include kanban squares for larger parts and a two-card system where the cards travel (or are faxed) back to the source instead of the bins. Electronic equivalents are also possible.
In order for Kanban systems to work, there must be enough room to dispense all parts at the points of use. This, again, emphasis the importance of part standardization.
The "lowest hanging fruit" in material logistics is the breadtruck delivery system for small, inexpensive parts. Instead of counting on sales forecasts to trigger an MRP system to generate purchase orders, all the small, inexpensive parts can be made available in bins at all the points of use. A local supplier is contracted to simply keep the bins full and bill the company monthly for what has been used, much like the way bread is resupplied by the breadtruck in a small market.
All the MRP/purchasing expense is eliminated and this type of delivery can assure a constant supply of parts, thus avoiding work stoppages. Being off the forecast/MRP system, the supply of these parts can be assured for "forecast-less" operations such as Build-to-Order. Typical parts suitable for breadtruck deliveries are fasteners, resistors, capacitors, and almost any small, inexpensive part.
As companies become more agile, they may include slightly more expensive and slightly larger parts into the breadtruck system. The more expensive parts may incur some inventory carrying cost, but that should be outweighed by savings in purchasing, materials overhead, expediting, and avoiding work stoppages.
Criteria for Breadtruck Deliveries:
1. David M. Anderson, "Build-to-Order & Mass Customization, the Ultimate Supply Chain and Lean Manufacturing Strategy for Low-Cost On-Demand Production without Forecasts or Inventory," (2004, 520 pages, CIM Press,1-805-924-0200; www.build-to-order-consulting.com/books.htm); Chapter 7, "Spontaneous Supply Chain."
Dr. Anderson is a California-based consultant specializing in training and consulting on build-to-order, mass customization, lean/flow production, design for manufacturability, and cost reduction. He is the author of "Build-to-Order & Mass Customization, The Ultimate Supply Chain Management and Lean Manufacturing Strategy for Low-Cost On-Demand Production without Forecasts or Inventory" (2008, 512 pages; CIM Press, 1-805-924-0200, www.build-to-order-consulting.com/books.htm) and "Design for Manufacturability & Concurrent Engineering; How to Design for Low Cost, Design in High Quality, Design for Lean Manufacture, and Design Quickly for Fast Production" (2008, 448 pages; CIM Press, 1-805-924-0200; www.design4manufacturability.com/books.htm). He can be reached at (805) 924-0100 or email@example.com; web-site: www.build-to-order-consulting.com.