Value
Network Mapping (VNM): Visualization and Analysis of Multiple Flows in Value
Stream Maps
ZahirAbbas N. Khaswala and Shahrukh A. Irani[1]
Department of
Industrial, Welding and Systems Engineering
The Ohio State University
Columbus Ohio 43210
Abstract
A ‘Value Stream’ (VS) is “all the
actions (both value-added and non-value-added) 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
work-in-process (WIP) inventories.
However, the developers of VSM acknowledge that many value streams have multiple flows that merge. This would typically be the case in Make-To-Order 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 Multi-Product Process Chart (M-PPC) and From-To 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.
Introduction
Lean Thinking, a concept that is based on the Toyota Production System, extends continuous improvement efforts to reduce the costs of serving customer/s beyond the physical boundaries of a manufacturing facility, by including the suppliers, distributors and production system that support the manufacturing function (Figure 1). These improvements and cost reductions are achieved by eliminating the muda (wastes) associated with all activities performed to deliver an order to a customer. Wastes are defined as “all activities that consume resources (add costs to the product) but contribute zero value to the customer.” According to Jim Womack and Dan Jones, there are five steps for implementing Lean Thinking in an enterprise: 1) Define Value from the perspective of the Customer, 2) Identify the Value Streams, 3) Achieve Flow in the facility, 4) Schedule production using Pull, and 5) Seek Perfection through Continuous Improvement. Womack and Jones define the value stream as “the set of all the specific actions required to bring a specific product through the three critical management tasks of any business: …problem solving, …information management, …physical transformation” (Moore & Scheinkopf, p.17).
Basic Concepts
of VSM
Unlike traditional process mapping tools, VSM is a mapping tool that maps not only material flows but also information flows that signal and control the material flows (Figure 1). This visual representation facilitates the process of lean implementation by helping to identify the value-adding steps in a value stream and eliminating the non-value adding steps, or wastes (muda).
Using a VSM process requires development of maps: a Current State Map and a Future State Map. In the Current State Map, one would normally start by mapping a large-quantity and high-revenue product family. The material flow will then be mapped using appropriate icons in the VSM template. The (material) flow path of the product will be traced back from the final operation in its routing to the storage location for raw material. Relevant data for each operation, such as the current schedule (push, pull, and order dispatching rules in effect at any process ex. FIFO) and the amount of inventory in various queues, will be recorded. The information flow is also incorporated to provide demand information, which is an essential parameter for determining the “pacemaker” process in the production system. After both material and information flows have been mapped, a time-line is displayed at the bottom of the map showing the processing time for each operation and the transfer delays between operations. The time-line is used to identify the value-adding steps, as well as wastes, in the current system. The comparison between the processing times and the takt time (calculated as Available Capacity/Customer Demand) is a preliminary measure of the value and wastes in a stream. This takt time is mostly used as an ideal production rate for each operation to achieve. Ideally, the cycle time for each operation should be less than or equal to the takt time.
Based on the analysis of the Current State Map, one then develops a Future State Map by improving the value-adding steps and eliminating the non-value adding steps (waste). According to Rother & Shook, there are seven guidelines, adapted and modified based on the concepts of Lean Thinking, that can be followed when generating the Future State Map for a lean value stream (Rother & Shook, 1999, p. 44-54):
1) Produce to takt time
2) Develop continuous flow
3) Use supermarkets to control production where continuous flow does not extend upstream
4) Schedule based on the pacemaker operation
5) Produce different products at a uniform rate (Level the production mix)
6) Level the production load on the pacemaker process (Level the production volume)
7) Develop the capability to make “every part every (EPE) <time period>”
Advantages of
VSM
Ø Relates the manufacturing process to supply chains, distribution channels and information flows.
Ø Integrates material and information flows.
Ø Links Production Control and Scheduling (PCS) functions such as Production Planning and Demand Forecasting to Production Scheduling and Shopfloor Control using operating parameters for the manufacturing system ex. takt time which determines the production rate at which each processing stage in the manufacturing system should operate.
Ø Helps to unify several IE techniques for material flow analysis, such as Production Flow Analysis (PFA), Business Process Reengineering (BPR), and Process Analysis and Improvement (PA&I) that, to date, have been taught and implemented in isolation.
Ø Provides important descriptive information for the Operation and Storage icons that, to date, has not been captured in standard Flow Process Charts used by IE’s.
Ø Forms the basis for implementation of Lean Manufacturing by designing the production system based on the complete dock-to-dock flow time for a product family.
Ø Provides a company with a “blueprint” for strategic planning to deploy the principles of Lean Thinking for their transformation into a Lean Enterprise.
Disadvantages
of VSM
Ø Fails to map multiple products that do not have identical material flow maps.
Ø Fails to relate Transportation and Queuing delays, and changes in transfer batch sizes due to poor plant layout and/or material handling, to operating parameters (ex. machine cycle times) and measures of performance (ex. takt time)[2] of the manufacturing system.
Ø
Lacks any worthwhile economic measure for “value” (ex.
profit, throughput, operating costs, inventory expenses) that makes it similar
to the Flow Process Charting technique used by IE’s.
Ø Lacks the spatial structure of the facility layout, and how that impacts inter-operation material handling delays, the sequence in which batches enter the queue formed at each processing step in a stream, container sizes, trip frequencies between operations, etc.
Ø Tends to bias a factory designer to consider only continuous flow, assembly line layouts, kanban-based Pull scheduling, etc. that are suitable mainly for high volume and low variety (HVLV) manufacturing systems[3].
Ø Fails to consider the allocations and utilization of an important resource – factory floor space – for WIP storage, production support, material handling aisles, etc.
Ø Fails to show the impact on WIP, order throughput and operating expenses of in-efficient material flows in the facility ex. backtracking, criss-cross flows, non-sequential flows, large inter-operation travel distances, etc.
Ø Fails to handle complex product BOM’s branched and multi-level Operation Process Charts and Flow Diagrams that result in complex value streams.
Ø Fails to factor queuing delays, sequencing rules for multiple orders, capacity constraints, etc. in any map[4].
Ø Lacks the capability, due to the manual mapping method, for rapid development and evaluation of multiple “what if” analyses required to prioritize different alternatives for improving a Current State Map when time and/or budget constraints exist.
Industrial
Application of VSM in a Fabrication Jobshop
C.O.W.
Industries, Inc. (http://www.C.O.W.ind.com)
is a fabrication jobshop specializing in the manufacture of precision metal
products. The 75,000 sq. ft. facility contains fabrication, machining and
welding equipment. The company produces a variety of products, ranging from
large equipment cabinets to small turned parts. Process capabilities include punching, grinding, turning,
milling, forming, and painting. A
typical finished product consists of multiple unique components produced in the
Press shop that are welded into a single unit.
Hence, the material flow network for any welded product provided the
opportunity to study value streams with multiple flow paths that merged into a
single path after the welding step. The
traditional VSM method was found inadequate for mapping such a flow
network. hence, the proposed approach
of Value Network Mapping (VNM) was developed, applied and tested for general
use in similar manufacturing facilities.
Limitations of Value Stream Mapping
The
product used for this study was an equipment cabinet, ED1M009-32, that was
recommended by the client company. This
particular product belonged to a family of similar products and accounted for a
significant proportion of the annual production volume and sales of the company[5].
The cabinet consists of twenty-one components.
Each component has a unique sequence of operations that require a large
variety of processes.
The
basis for development of a Current State Map for a value stream is the
manufacturing routing that specifies the sequence of workcenters that must be
visited in order for that product to be produced. However, when the authors began to draw the Current State Map for
the above-mentioned multi-component fabricated product using the standard VSM
method, the following difficulties were encountered:
·
Given the large number
of manufactured components, it was difficult to map each of their unique flow
paths on a single 11 x 17 sheet of paper. To address this problem, Rother and Shook suggest that “(when) many value
streams have multiple flows that merge ….. do not try to draw every branch if
there are too many. Choose the key
components first, and get the others later if we need to” (Rother & Shook, 1999, p. 19). However, no
decision-making process is suggested to select a subset of key components to
map. Also, if the components and
sub-assemblies in the end-product are not completed and made available in
appropriate “kits” as necessary, then the welding and subsequent assembly steps
could not be executed.
·
Given the large number
of manufactured components, it was difficult to map each of their unique flow
paths on a single 11 x 17 sheet of paper. Rother and
Shook suggest that “(when) many value streams have multiple flows that
merge, draw such flows over one another” (Rother &
Shook, 1999, p. 19). However, in order “to draw one flow over another”,
one needs to identify which flow paths are identical, similar or
non-identical. This task is non-trivial
and cannot be done manually for any but the simplest of fabricated products. An
additional drawback of this “aggregation” will arise when generating the
timeline for compiling the production lead time for a fabricated product. The scheduling-related delays that occur
when multiple activities in a complex product must access one or more common
resources cannot be accounted for using a pencil-and-paper technique[6].
·
In many multi-product manufacturing
facilities, there is significant backtracking observed in the flow paths of
several products. This occurs when the
same process/workcenter is required for multiple non-consecutive operations in
a manufacturing routing. In such
situations, should the process box be duplicated in the map or should the
material flow travel back to the previous machine? The current VSM methodology
does not explain how to represent this case in the Current State Map.
·
VSM does not incorporate
the material handling information between any and every pair of consecutive
process boxes, such as transfer batch size, frequency of product batch
transfers between the two process locations, type/s of equipment used for
material handling, travel distance and
travel time. In practice, the material
handling delays between consecutive process steps contribute a significant
portion of the Non-Value Added time in the production lead time for a
product. And, if the cycle time for
material handling between any two process steps is not matched with the process
cycle times, then it would be difficult
to complete orders at a rate specified by the takt time. This mismatch between material handling
rates and process cycle times results in inventory buffers being observed at
each process box in a Current State Map.
Based
on the above limitations of the standard VSM methodology described in Rother
& Shook, the authors have developed an alternative method – Value Network
Mapping (VNM) – that extends the current VSM methodology to handle fabricated
products with complex BOMs.
Specifically, the new approach (a) helps to identify and merge multiple
flow paths in a value stream that are either identical or similar and (b)
considers all in-house and outsourced parts that constitute the BOM and
assembly structure of the product instead of focusing on “ the key
components first”.
Value Network Mapping (VNM): An Enhancement of Value
Stream Mapping for Jobshops
Value Network Mapping (VNM) was developed to eliminate the
limitations imposed on the traditional methodology when “many value streams
have multiple flows that merge”. A
Current State Map for a single component (or assembled product) is built upon
the manufacturing routing (or Assembly Operations Process Chart) for the
component (or product). Hence, VNM
utilizes algorithms for clustering of similar manufacturing routings and design
of facility layouts to identify families of similar routings for which a single
composite Current State Map could be developed. In addition, these algorithms utilize special data structures
that capture the complete assembly structure of the product instead of
extracting the key components only. Figure 2 presents a flowchart that gives a
step-by-step explanation of the proposed VNM approach and compares it with the
VSM approach. Results obtained from an
industrial case study to evaluate this approach are also presented. The steps in the VNM approach are explained
below
1.
Form
a Product Family: VSM
defines a product family as “a group of products that pass through similar
processing steps and over common equipment in your downstream processes ” (Rother & Shook, 1999, p. 6).
Since VSM is a
manual diagramming method, the products that have been studied to date have few
components in their BOMs and little or no differences in the manufacturing
routings of the components contained in the BOMs. Products manufactured by a typical fabrication jobshop will
exhibit the properties such as “multiple flows that merge”, “flows that are
identical or differ by at most one or two process steps” and “mulitple branches
in the product BOMs. This is because,
even within the family of welded cabinets produced by C.O.W. Industries, Inc.,
they were found to differ in (a) the components contained in their product BOMs
and (b) the manufacturing routings of the components contained in their product
BOMs. To form product families, VNM
utilizes a combination of the following methods – Product - Process Matrix
Clustering, Product - Component Matrix Clustering and PQRS Analysis – that have
been computerized using the PFAST (Production Flow Analysis and Simplification
Toolkit) package (Irani et al, 2000). Note that this step was not executed in this
particular study since the client company had already determined the product to
be mapped.
2.
Visualize
the Flow: Using a
product BOM and the manufacturing routings of the components in the BOM, the
Operations Process Chart for the product can be generated and transformed into
a Multi-Product Process Chart (MPPC). When these charts are mapped onto the
physical layout of the facility, the Flow Diagram for development of the
detailed Value Network Map is generated.
For our case study, Figure 3 shows the
MPPC for all components and subassemblies in the cabinet. Figure 4 shows
the Flow Diagram for this product. Note that the chaotic and congested
material flows in the facility, due to the backtracking and crossing of
different flow routes, would not have been identified using the simple VSM
methodology.
3.
Collect
data for the process boxes: The Flow Process Chart (FPC) is a classic data collection tool used by
Industrial Engineers to record all operation, storage, transport, delay and
inspection steps in the flow path of a product. The VSM methodology has the unique feature that it records the
information flows associated with the material flows in the same map. Hence,
VNM utilizes the Enhanced FPC to attach material handling and
scheduling-related information to the material flows mapped in the Flow Diagram
(Figure 4). For our case study, Figure 5 shows the Enhanced FPC for one component
contained in the BOM for the ED1M009-32 product.
4.
Merge
similar routings:
This step in the VNM approach facilitates the placement of the process boxes on
the 11x17 sheet of paper when developing the Current State Map without
sacrificing the exact assembly structure of the complete product. The merging
of similar routings helps to “draw similar flows over one another” but reduces
the number of process boxes to be drawn on the paper. However, it is important not to lose the overall material flows
contained in the Operations Process Chart for the product. This is achieved using the Modified
Multi-Product Process Chart (MMPPC) derived
from the MPPC. For our case study, Figure 6 shows
the complete map of the product generated from Figure
3.
5.
Group
Similar Routings into Component Families: This step in the VNM approach helps to group
components with similar manufacturing routings into families. Thereby, one could design multiple component
manufacturing cells that would supply parts to the Welding department. This is done using the Machine-Part Matrix
Clustering algorithms in PFAST (Irani et al, 2000). For our case study, Figure
7 shows the cluster analysis dendogram generated by PFAST that guided
the grouping of components into different families.
6.
Draw
the Current State Map: When drawing the Current State Map, VSM suggests to “choose key
components first, and get the others later if needed” (Rother
& Shook, 1999, p. 19). However, this would not be recommended when
mapping the flows for a welded structure that requires timely delivery of
multiple kits, each consisting of several different parts. Using the VNM approach, this mapping of a
large number of different flows could be done at two levels: At Level 1, we would map the flows of a complete product (or a family
of products) using the MMPPCs and Enhanced FPCs generated from their BOMs. At Level
2, we would map the
flows of components in any family using the MMPPCs, Enhanced FPCs and Cluster
Analysis dendograms. Both levels of
mapping essentially seek to combine/merge several flow paths in order to
generate more compact Flow Diagrams without eliminating any components in a
product’s BOM. For our case study, Figure 8 shows
a VNM at Level 1 for the ED1M009-32 product.
Figure 9 shows the VNM at Level 2 for
Component Family #1 in Figure 7. A unique
feature of the VNMs shown in both figures is the material handling information
– distance of travel and equipment used to move parts over that distance –
associated with every flow between any pair of machines. Figure 10 presents an alternative representation for
the VNM at Level 1 in Figure 8 – the Assembly Operations Process Chart – that
shows the optimal flows of components, subassemblies and the final product
without losing the assembly structure of the product.
Future Work
The current version of VNM lacks detailed analysis of the material handling systems and processes connecting different pairs of process boxes. Also, unlike the simpler maps produced using traditional VSM, the VNM needs to include information on lot sizing, job sequencing at each process and WIP buildup at each process due to queuing delays. A critical element of future VNMs needs to be the overall system throughput when multiple components and subassemblies require to use capacity-constrained resources at one or more process boxes.
Conclusion
This paper introduced a Value Network Mapping (VNM)
approach that, unlike Value Stream Mapping (VSM), is able to map value streams
that have multiple flows that merge. VNM utilizes a variety of material flow
analysis and product grouping tools that can be executed using a software
package called PFAST. Product grouping helps to merge flows whereby it becomes
easier to visualize multiple flows in the value stream for a product that has a
complex BOM, components with dissimilar routings and components whose routings
share several process resources.
In addition, VNM utilizes classical IE methods, such as Flow Process
Charting and Systematic Handling Analysis, to show how facility layout and
material handling make possible the design of “lean” value streams. Future work
will focus on enhancing the VNMs to include WIP, cycle time, lot sizing and throughput
information required to design the Future State Map.
Acknowledgment
We wish to sincerely thank Yuri Wibowo and Sadono Djumin who made prior contributions to this ongoing project. Yoseph Setiadi, IE at C.O.W. Industries, contributed his valuable time during the data collection phase and critiquing the results of this pilot project. The management and shopfloor personnel at C.O.W. Industries ensured the smooth execution of this project at all times. Lastly, we wish to thank the National Science Foundation for funding Yuri Wibowo through an REU supplement for Grant No. DMI-9796034.
Reference
Irani, S.A., Zhang, H., Zhou, J., Huang, H., Udai, T.K. & Subramanian, S. (2000). Production Flow Analysis and Simplification Toolkit (PFAST). International Journal of Production Research, 38(8), 1855-1874.
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Rother, M. & Shook, J. (1999). Learning to See: Value Stream Mapping to Add Value and Eliminate Muda. Brookline, MA: Lean Enterprise Institute (www.lean.org).
Womack, J. P. & Jones, D. T. (1996). Lean Thinking: Banish Waste and Create Wealth in your Corporation. New York, NY: Simon & Schuster.
“What is the Theory of Constraints, and How does it compare to Lean Thinking?” http://www.lean.org/Lean/Community/Resources/thinkers2.cfm.
Figure 1 Material and Information Flows in a Supply Chain
Figure 2 Comparison of Value Stream Mapping (VSM) and Value
Network Mapping (VNM)
Figure 3 Multi-Product
Process Chart (MPPC) for ED1M009-32
Figure 4 Flow Diagram for ED1M009-32
Figure 5 Enhanced Flow Process
Chart (FPC) for ED1M009-32
Figure 5 Enhanced Flow Process Chart (FPC) for
ED1M009-32 (contd.)
Figure 6 Modified
Multi-Product Process Chart (MMPPC) for ED1M009-32
Figure 7 Thresholds for Component Family Formation to generate Level 2 VNMs
Figure 8 VNM at Level 1 for ED1M009-32
Figure 9 VNM at Level 2 for Component Family #1 for ED1M009-32
Figure 10 Assembly Operation Process Chart for VNM at Level 1 for ED1M009-32