February 1, 2007

By Darren Dolcemascolo

Value stream mapping is a planning tool; thus, if you create a current state map only, you have missed the purpose of VSM. The future state map is a picture of the lean transformation process for a specific value stream. Before you create a future state map, you should have already created a current state value stream map.

A future state map identifies improvement to be made to the value stream that will shorten the overall lead time. (In most lean literature, the time from which raw materials enter a facility until the time they ship to the customer as finished goods is termed the lead time; most manufacturing folks call this the cycle time.) To create an effective future state map, the following must be considered:

Takt time versus current cycle times. Takt time is the rate of customer demand measured in time. That is the number of working minutes available per day divided by the number of units of product the customer requires per day on average. Generally speaking, takt times should be based on monthly demand (i.e., takt times should be calculated on a monthly basis although there are exceptions to this rule.). Cycle times is defined as how often a part is completed by a given process within the value stream. A bar chart is useful for comparing takt time with cycle times for all processes within a value stream.

Connecting Flow through Either One-Piece Flow or a Pull System. Each process box in a value stream needs to connect to the process boxes immediately upstream and downstream. One piece flow is the ideal state. A one-piece flow process can be created by combining processes so that a workpiece flows from process to process with no interruptions and no WIP; this can only work for very reliable/repeatable processes that can be scaled to run at takt time. For processes that do not lend themselves to one piece flow, a pull system must be used to connect the flow. The pull system must link processes by controlling the upstream production (to avoid overproduction or producing the wrong items). There are two basic types of pull systems: replenishment pull and sequential pull. In a replenishment pull system, the downstream process pulls from a supermarket of parts. When a trigger point is reached, the upstream process is given a signal to produce additional parts to replenish the supermarket. A sequential pull system allows a fixed quantity of non-specific parts to be held in a FIFO lane (instead of a supermarket) between the upstream and downstream process. The downstream process pulls from the parts in the FIFO lane in the same sequence in which they were produced. If the lane becomes full, it is a visual signal to stop the upstream process.

Scheduling only one process (the pacesetter process) with level loading. One of the most important concepts of lean and value stream mapping is to schedule one process in the value stream and to level the volume and mix at this process. Monitoring/Managing Performance to Takt at the Pacesetter. The pacesetter process must have a method for measuring performance to takt. There are several ways to accomplish this; the most simple and most common way is to use an hourly “plan versus actual” production board.

To implement these concepts effectively, it is necessary to implement process improvements such as improving process yield, reducing defects, improving uptime, reducing changeover time, and eliminating wasted time/motion within processes, etc. I have found through experienced that a structured brainstorming session will often identify key process improvements that should be done to achieve a lean value stream. You will likely need to use traditional lean tools such as TPM, Single Minute Exchange of Die (SMED), Visual Controls, and 5S. You will also likely use statistical methods to improve and control processes that need improvement.

After you’ve applied the above concepts to the current state, you can create a future state value stream map using the same methods discussed in our current state value stream map article. Finally, you should create a value stream implementation plan to achieve the future state. You are now armed with a plan for a lean value stream that you can use for tracking/measuring your improvements throughout the year. Ensure that you have a "value stream manager" or project manager driving the plan to completion.

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