Value Network Mapping (VNM): Visualization and Analysis of Multiple Flows in Value Stream Maps

Abstract

A ‘Value Stream’ (VS) is “all the actions (both value­added and non­value­added) currently required to bring a product through the main flows essential to every product” (Rother & Shook, 1999, p. 3). The process of mapping the material and information flows of all components and subassemblies in a value stream that includes manufacturing, suppliers, and distribution to the customer is known as Value Stream Mapping (VSM). VSM has proved effective in identifying and eliminating waste in a facility with similar or identical product routings, such as in assembly facilities. Using VSM, many companies have changed their existing facility layouts, material handling, inventory control, purchasing and scheduling systems to reduce the total throughput times of parts and current levels of work­in­process (WIP) inventories. However, the developers of VSM acknowledge that many value streams have multiple flows that merge. This would typically be the case in Make­To­Order jobshops that make products with complex BOM’s, such as welded fabrications, furniture, stamping dies, etc. In order to map multiple flows in a value stream, Rother & Shook suggest to “draw such flows over one another. But do not try to draw every branch if there are too many. Choose the key components first, and get the others later if you need to” (Rother & Shook, 1999, p. 19). Instead of this “sampling” step in VSM, this paper introduces an alternative approach – Value Network Mapping (VNM) – that is able to map the complete network of flows in the value stream corresponding to a complex product BOM (Bill Of Material). Our approach integrates basic Industrial Engineering (IE) tools for material flow mapping, such as the Multi­Product Process Chart (M­PPC) and From­To Chart, with a software package for material flow analysis, PFAST (Production Flow Analysis and Simplification Toolkit). In particular, the software is effective for visualization and analysis of multiple flows in a value stream that has products with dissimilar routings that share common resources. Also, unlike standard VSM, the proposed approach helps to view a value stream at any and all levels of assembly in a product BOM. Lastly, this approach supports facility improvements to merge/streamline multiple flows in the facility, such as the creation of manufacturing cells and improvements in the current material handling methods. The development and benefits of this approach are demonstrated using results from a pilot study done in a local welding fabrication jobshop. Outline of this Paper First, the concept of Lean Thinking is introduced and reviewed. This is followed by an explanation of the basic concepts of Value Stream Mapping (VSM), with a listing of the advantages and disadvantages of VSM. Specifically, it is shown that the original VSM methodology breaks down in the case of “multiple flows in a value stream that merge” in the case of complex product BOMs. Next, the development of the proposed approach, Value Network Mapping (VNM), is explained in detail. Finally, the results from an industry project are analyzed and the potential benefits of the proposed approach are presented.