Sean M. Gahagan, Northrop Grumman Corp. / University of Maryland, College Park
Value stream mapping, or VSM, is an important technique used in the practice of lean manufacturing. It is usually carried out as a hand drawing, done as one walks the value stream on the shop floor or as a series of sticky notes on a wall chart. These hand drawn, or constructed maps, are not very effective communication tools, especially to decision makers unfamiliar with VSM and accustomed to slick visual presentations. As a result, a cruel irony of many VSM efforts is that the maps must be converted to PowerPoint or Visio for presentation and approval – a process most practitioners would call non-value-added. In order to add value to this process, I developed a VSM template in Arena, a simulation software package. This template allows the user to create maps in the Arena workspace that can be cut-and-pasted into other presentation applications, but are also fully-functional simulation models of the value stream. These models can be used to validate the findings of the mapping effort and to provide an animated laboratory in which stakeholders and decision makers can view and experiment with the value stream. I applied this template to a recent, highly successful VSM effort.
Value stream mapping, or VSM, is a potent tool for the practice of lean manufacturing, the discipline of eliminating waste from manufacturing systems. However, value stream maps can be misunderstood by non-practitioners, making it sometimes necessary to transcribe them. Transcribing maps is inherently non-value-added and therefore aggravating to committed lean practitioners.
In order to make transcription more valuable, I developed a simulation modeling template for VSM which allows the user to develop presentation quality graphics while simultaneously building a working simulation model of the value stream. The simulation model animation can then be used to further illustrate the behavior of the stream. This paper reviews value stream mapping, explains the simulation model template I developed and illustrates its utility by citing a real world example of its use.
The rest of this paper is organized as follows. Section 2 provides a review of lean manufacturing and the role of VSM. Section 3 includes a description of VSM technique and iconography. Section 4 describes the motivation for this work – the transcription of value stream maps. Section 5 introduces the VSM simulation model template. It describes the elements of the template, their attributes and how they work together to make a working simulation model from a value stream map. In Section 6, a simple transcription activity is described, illustrating the use of the template objects. Section 7 features a real world application of the template to a value stream in an electronics manufacturing facility. The paper concludes with Section 8, a summary of the material above and a brief discussion of future work based on the VSM template.
Lean is a management philosophy whose goal is the elimination of waste. Waste is broadly defined as any activity that does not create value for the customer. The first step in many lean initiatives is to take an inventory of all of a business’ activities with the goal of identifying those that produce value and those that do not. An important tool for this inventory is the value stream map. A value stream map is a simple iconic diagram that describes the sequence of activities a business undertakes to produce a product or family of similar products. Like many lean tools, its power is in its simplicity. However, also like many lean tools, its simple elegance is often corrupted by a western corporate culture that requires flashy PowerPoint slideshows.
One of the most well known tools for lean is VSM. In one of the most widely used texts on the subject, Learning to See, Rother and Shook (1999) describe the practice, nomenclature and iconography of VSM.
Value stream mapping describes the activities required to produce a family of products from door-to-door. The process begins by “walking the value stream;” literally following the product around the shop floor, from the shipping dock, all the way back to receiving. As the observer follows the product, they record the processes they encounter, using a very specific symbology, mapping the flow of material and information through the system. The result is a current state map. The symbology helps to quickly identify processes that add value and those that do not. Using the current state map as a baseline, the lean practitioner can then develop future state maps -- ideal system designs that reduce or eliminate the non-value-added steps. The key to this technique is the iconography of VSM.
Rother and Shook define three types of VSM icons: material flow, information flow, and general icons. In this work, we focus on the material icons. Figure 1 shows the principal material icons used to create value stream maps.
The manufacturing process icon is used to describe an activity or sub-set of activities in the value stream. Though largely arbitrary, the dividing lines between activities is usually drawn wherever the product is handed off from one resource to another or where a product must wait in queue for the next activity.
Each manufacturing process icon is usually accompanied by a data box icon. The data box records important attributes of the process. These may include, but are not limited to, process cycle time, changeover time, process yield, and resource quantity and availability. The truck shipment icon shows the arrival of raw materials to the value stream. Although it is rarely accompanied by a data box, it is usually denoted with the frequency and quantity of arrivals.
The inventory icon is used to show places in the value stream where products sit and wait for the next process. These icons are labeled with both the quantity of product found at the location and also by how much time the products there wait before moving on.
Using these material icons, together with the other icon types, complete value stream maps can be created.
Value Stream Maps
Putting these icons together, one can describe an entire value stream in one compact diagram. Figure 2 shows a map of a simple value stream. This example is a material-flow-only value stream map. A normal value stream map would show a flow of information as well.
In this map we see a three stage manufacturing process with regular deliveries and shipments. At left, a truck shipment introduces raw materials on a daily schedule. Arrows indicate the flow of material through three manufacturing processes. Each process has a data box describing its behavior. The material flows into an inventory, where it waits for a weekly truck shipment.
An unfortunate reality of business is the pervasiveness of PowerPoint slideshows and Excel spreadsheets as means of corporate communication. Rother and Shook specifically cite the “beauty of this bureaucracy-free, PowerPoint-free method.” (p.9) Value stream maps however, be they hand-drawn in pencil on the shop floor, or pieced together with Post-its on the wall of a conference room, do not make compelling presentations for some audiences. A reality of lean practice in large companies is the need to transcribe value stream maps into other applications for further presentation and analysis.
While inherently non-value-added, transcription is sometimes necessary to succeed. When non-value added processes are necessary, lean philosophy dictates that the time and cost required for such processes be minimized. One way to lower the cost of transcribing maps is to use pre-defined data modules that can be re-used and re-arranged. In fact, the iconography of VSM is particularly well-suited to this type of modular instantiation.
In this work, I developed VSM modules in Arena, a discrete event simulation application (Kelton, Sadowski and Sturrock. 2007). This is not the first application of Arena to VSM and lean. Lian and Van Landeghem (2002) built simulation models of value stream maps to provide decision makers more insight into the behavior of their system and a tool with which to test different future state maps. However, they did not develop any VSM-specific model objects. Treadwell and Herrmann (2005) built custom Arena objects to facilitate pull production control – an important concept in lean practice. However, they did not specifically address value stream mapping.
Arena was chosen because it incorporates elements of both visual presentation and data organization. The Arena interface is visually similar to value stream maps. In the Arena interface upper right frame, the simulation model is represented graphically, in an attractive format that can easily be cut and pasted into a document or slideshow for presentation. In the lower right frame, the model is represented as a spreadsheet. This format can be easily cut and pasted into a spreadsheet for further analysis. In the left frame, there are sets of re-usable data objects, templates, that the user can drag-and-drop into the model.
This seemingly superficial resemblance to value stream maps led to the development of a special Arena template for creating VSM maps.
To facilitate the creation of value stream maps in Arena, I created a template of data objects specific to the task. In so doing, I drastically reduced the time necessary to create and edit value stream maps.
Arena’s Professional Edition provides the tools to create a custom template of data objects with both graphic and data properties. I created a template called VSM. The template contains three objects: process, delivery, and shipment. Figures 3, 4, and 5 show these three objects as they appear in the Arena model window.
Process is the most important of the three VSM template objects (see Figure 3). It takes the place of three of the four material flow icon elements introduced earlier. Process does the jobs of the manufacturing process icon, the data box icon and the inventory icon, all in one. It provides a manufacturing process graphic that can be connected to other process objects. It stores and displays the data from the data box as object operands. It also provides a built-in upstream inventory location. In the figure, the inventory location is represented by the sideways T-shape above the manufacturing process block. In the VSM template there is no inventory object per se. Since the objects make up a functional simulation model, the inventory levels can be observed when the model runs and can be measured in order to validate the map. However, if the user wishes to prescribe a queue time, an operand is provided in Process to store and implement the observed inventory queue time.
Delivery is a straightforward object. As shown in Figure 4, the object is equipped with operands to store each of the important characteristics of an arrival process: interarrival time, time of first arrival, number of units per arrival and maximum number of arrivals. The operands are all displayed in the graphic view of the object.
Shipment is even more straightforward than Delivery. Its purpose is to get rid of the products in the simulation model. Its graphic simply features a counter to display the number of products that have been processed during a simulation run (see Figure 5).
These three modules together make a tool box with which to build a simulation-based value stream map in less time than it would take to create a non-functional PowerPoint image.
In order to better understand the usage of the VSM template, we can use it to transcribe the example value stream map shown in Figure 2.
To begin, we start Arena and open a new model. Next, the VSM template must be attached to the model by right clicking in the template window and selecting “attach.” This opens a browser window in which to locate the template file. The template file, VSM.tpo may be stored anywhere on the computer, but is best stored in the default template directory. Once installed, the three VSM objects will appear in the template window and we are ready to begin transcribing.
Following the Rother philosophy, start in shipping by dragging a Shipment object to from the template panel, to the right side of the model window. Edit this object by double-clicking on the object to access the dialog window or, as this author prefers, by editing the fields in the data window at the bottom of the screen. All shipment needs is a unique ID. Call it “Ship.”
Next, drag a Process object to just left of the Shipment and edit the operands in the data window. Use the following values:
Batch Size: 1
Queue Time: 24
Batch Time: 0.5
Run Time: 1.0
Value: Non-Value Added
Batch Size: 1
Queue Time: 24
Batch Time: 0.5
Run Time: 1.0
Value: Non-Value Added
Note that all times are expressed in hours and that we have assumed that one day equals eight hours. Batch size is used if the products traveling through the system are actually batches of products. For a batch size greater than one, the processing time is equal to batch time plus run time multiplied by batch size. Queue time is the time spent in the upstream inventory location. This operand takes the place of the inventory icon. It is not used during simulation runs. In a simulation model, queue time is an output, not an input. It is included in the object for completeness of the VSM transcription and to facilitate analysis when exported to a spreadsheet model that requires it. Batch time and run time correspond the VSM terms changeover time and run time, respectively. Capacity is the number of processing resources (workers, machines, etc.) available to perform the activity.
Now, drag two more process objects into the model window and configure them in the same way to match the model in Figure 2. Note that process 1, inspection, has a yield of only 80 percent. In that process object, the yield operand would be set to 0.8. For any yield value less than 1.0, an additional exit point appears on the right side of the process block, allowing you to route the failed units into a separate process flow. Provide a destination for these products by dragging another shipment object into the model window called “junk.”
Complete the map by dragging a delivery object into the model. Configure it as follows:
Interarrival Time: 8
First Arrival: 0
Max Arrivals: Infinite
Interarrival Time: 8
First Arrival: 0
Max Arrivals: Infinite
Here it is again assumed that a day is 8 hours and that simulation time zero is the beginning of the first day.
Finally, connect the modules together using connections. Figure 6 shows the finished example model. Refer to Figure 2 to compare the Arena VSM model with a traditional value stream map. This simple technique was used to successfully transcribe the value stream map of an electronics shop.
The VSM Arena template was recently applied to a lean initiative in an electronics shop. It was used to transcribe hand-drawn value stream maps collected on the shop floor into electronic format for presentation and analysis. It was during this effort that the template was created, so the map was transcribed using Visio, Excel, and Arena, presenting a side-by-side comparison of the applications.
To limit the comparison, we will look at a sub-set of the complete map: the wedge-bond process. In this process, tiny ribbons of gold are used to electrically connect MMIC chips to the packages in which they are mounted. The original, hand-drawn map was first transcribed into Visio, as shown by Figure 7. The wedge-bond process has 10 steps. These products are cleaned and pre-heated. Then they are wedge-bonded, a process with a 90 percent yield, requiring a separate repair activity. After wedge-bonding, the products are inspected then transported to the next process in the shop.
The map was then transcribed using the Arena VSM template. The Arena model is shown in Figure 8. Visually, the Arena model is virtually identical to the Visio version. The time needed to create the Arena version was comparable with the Visio version. However, an Excel spreadsheet of the mapped process was also needed. From the Arena version, the values were easily cut and pasted from the data window. If a new spreadsheet had to be created, it could easily have taken an hour to complete. This time savings may seem trivial, but the Arena model offers the added power of a fully functional simulation model. In the time it took to create a PowerPoint slide, we created a simulation model that the stakeholders could watch run as a visual double-check of the map validity, measure as a statistical triple-check and re-configure easily to create and test new future state maps.
At this time, possible future state maps are under consideration and the VSM template model is a key part of that effort. In this application the Arena VSM template was an unqualified success due to its power, flexibility, and ease of use.
Value stream mapping is an invaluable tool of the lean practitioner. When a map is simply not sufficient to communicate the message, the VSM template reduces the effort required to translate the map into electronic media. As a bonus, it brings the additional persuasiveness of animation to this previously static tool. Use of simulation models permits users to work in a virtual laboratory where they can develop and test future state maps with near-real-time performance feedback.
Although this initial deployment of VSM was very successful, much work remains to make it a complete VSM solution. As discussed earlier, the current VSM template addresses only the material flow icons from the traditional value stream mapping. In order to make a truly useful tool, the information flow icons must be modeled. In other parts of the case study electronics shop, standard simulation modeling objects were required to fulfill the information flow needs. The work of Treadwell and Herrmann (2005) provides a compelling starting point for the development of information flow objects.
Overall, the VSM model template provides one more useful tool for the lean practitioner by adding real value to value stream maps.
The author would like to thank Northrop Grumman Corporation and particularly the AMEC Continuous Improvement Team for their willingness to try something new, and Dr. Jeffrey Herrmann at the University of Maryland for his minor outside editorial assistance.
Kelton, W.D., Sadowski, R.P. and Sturrock, D.T. Simulation with Arena, 4th edition. McGraw-Hill, Boston, Mass. 2007.
Lian, Y-H and Van Landeghem, H. “An Application of Simulation and Value Stream Mapping in Lean Manufacturing,” Proceedings of the 14th European Simulation Symposium, A. Verbraeck and W. Krug eds., SCS Europe BVBA. 2002.
Rother, M and Shook, J. Learning to See; Value Stream Mapping to Create Value and Eliminate Muda, Version 1.2. Lean Enterprise Institute, Brookline, Mass. 1999.
Treadwell, M. and Herrmann, J. “A Kanban Module for Simulating Pull Production in Arena,” Proceedings of the Winter Simulation Conference, Orlando, Fla. Dec. 4-7, 2005.
Sean Gahagan is an industrial engineer at Northrop Grumman Corp. in Baltimore, MD. He is also a doctoral candidate at the University of Maryland. He received a B.S. from the University of North Carolina, Charlotte in 1995 and an M.S. from the University of Maryland in 2002, both in mechanical engineering.