Value Stream Mapping is an tool for mapping information flows and physical flows throughout the process.
Value Stream Mapping (also called MIFA – Material and Information Flow Analysis ) is a tool for mapping the flow of data. information and physical flows of the entire process. She allows to :
- View the entire value stream.
- Share a common and pragmatic vision of a situation.
- Identify sources of waste.
- Provide a plan for improvements.
CIs a methodology that Toyota developed in the 1980s to enable them to be more effective in their waste.
1 – VSM “Current State”
1.1 Identify the perimeter
First step, the choice of the scope of analysis. As for most projects, choose too small, we will have few results, choose too large, we risk having an unreadable VSM.
For a first practice, we will choose a scope related only to the factory. We will avoid choosing a process that includes several different sites or companies.
Then we will choose the product or the product family. We will draw up a product / process chart such as the one below.
If a product is identical to 2 customers, then we will distinguish the 2 products.
Products with a minimum of 70% of the common part will be considered as identical.
We will choose the product or product family that goes through the most process. This will allow us to obtain results that will positively affect other products. 2 cases presents itself to us:
- Case 1 : We have 2 products that go through 5 processes out of 6, the others only go through 4 at most. But these are not the same products. Our choice will be on the most strategic product, either because we have the most problems, or because vis-à-vis the future of society, it is this one that is strategic.
- Case 2 : we have 1 product that goes through 5 processes out of 6, the others only go through 4 at most. We will then choose this product for the study, the latter being the most representative of our production.
1.2 Represent the customer
We will immediately calculate the Takt Time of the product we chose. It is noted that the Takt Time, in the case of the study of a family of products, is calculated on the need of all the products of the family.
Then we will start our mapping by representing the client. This is defined as follows :
- Name of the plant, the distribution center … of the customer : specify the name of the plant or the place, especially in the case where the customer has several.
- Specify versions or types of products.
- Quantity of product per day and version.
- Indicate the standard packing unit.
- Indicate the delivery time and the frequency of deliveries.
- Other information may be indicated as the type of transport…
1.3 Represent the process
In a first version, we will not yet collect accurate data. We will go to the field, go through the process starting at the end, and then go back through noting all the steps of the process.
Then, we will redo the course by raising the QCDSM data of each step of the process. The list below is generic, it is to adapt according to your situation :
- Cycle time. Li>
- The number of operators. li>
- The rate of non-quality line and customer. li>
- The serial change time. li>
- The OEE. li>
- The batch size. li>
- The level of upstream and downstream work in progress. li>
- Number of incidents. li>
- Service rate. li>
- Opening time, team number. li>
- Useful surface…
1.4 Represent the physical flow
We will represent the type of material flow between the value added items. In the case where the flow between stations is Kanban, we will not put anything at this stage because this is considered a flow of informations.
Finally, during this step, we indicate the pushed flows and the FIFOs, as well as the modes of transport of the parts between station or in entry / exit of the company..
Recall on the FIFO
The FIFO, First In First Out, indicates that the first element that enters a production sequence is the first that comes out. Most often, this type of flow is conceived by putting the products on roller rails with a slight slope. The upstream station places the object at the top of the rail, following the line. The downstream operator takes the first product he has in front of him.
This is opposed to LIFO (Last In, First Out).
1.5 Représenter les flux d’informations
A ce stade, nous avons une vue précise du flux des matières et des différentes tâches. Notre objectif est désormais de représenter le flux d’informations. Le flux d’informations étant l’information qui permet au personnel de savoir ce qu’il a à produire (Quoi, Quand, Combien). Ce flux d’informations se compose des systèmes du Juste A Temps (Kanban, Heijunka Box…), et de tous les moyens mis en œuvre pour faire passer l’information : téléphone, fax, mail, commande papier…
Pour chacun des éléments, on indique :
- Taille de lot.
- Fréquence d’arrivée ou de mise à jour des informations.
1.6 Calculate the different indicators
Last step of the development of the current state VSM, we will calculate various indicators that will allow us to build the future state of VSM and decide which Lean 6 Sigma projects to implement. We find :
- Value Added Time: it is equal to adding the cycle times of value added processes.
- The lead time of production: it goes from the arrival in the store of entry at the beginning of the truck with the finished products.
- Inventory levels of inflows, outflows and outstandings.
- Useful surfaces for production stations and stocks.
With these data, we calculate 3 indicators to qualify stock levels :
Outstanding stock ratio = number of pieces (good or not considered as discarded) in the various stocks of stocks / daily customer demand
The raw material stock ratio = nb of good parts in the initial stock / daily customer demand
The finished product stock ratio = nb of good parts in the final stock / daily customer demand
At most these ratios are low, at least we have stock.
Finally, we calculate the value-added ratio :
Sum of cycle time of each operation / Lead Time of production
Expressed as a percentage, this ratio allows us to qualify our time as global added value. It is pretty easy to interpret: If we have 1hr of added value for 2 weeks in the factory, we “store” our room for almost all the time. We will therefore seek to have a ratio closest to 100%.
In our example, we have a daily demand of 920 pieces on average ((17,600 + 10,000) / 30), we get :
- Outstanding stock ratio = 24,670 / 920 = 27 days of work in progress.
- Ratio of raw material stocks = 11 days of stocks of raw materials.
- Finished product stock ratio = 9,600 / 920 = 10.4 days of good finished product stocks.
- Value Added Ratio = 167 / 2 246 400 = 0,0075 %.
In view of these values, we have a lot of stock, since we have more than 10 days of finished product in stock for a value-added time of less than 3 minutes.
2 – VSM “future state”
At this point, we have a good knowledge of our overall flow, inventory level and productivity. We will build our “future state“, which will represent the vision we have of our processes.
2.1 View the feed based on customer demand
We begin by comparing the cycle times of our different processes with respect to Takt Time. On construit le balancing chart which in our case is the following (Takt Time of 93 sec: 920 pcs days, for an opening time of 24 hr).
We can see that, overall, our cycle times are well below the Takt Time (we note that we have2 Takt Time, positions 3 and 4 are open to 3 teams while other positions are open on 2 teams). Which is normal, since we took into account that the need related to the 2 products studied while we used the overall opening time of each process. In the context of the example, this is not important since we have more a problem of stock than of capacity and that the cycle times are identical regardless of the product.
2.2 Where is the bottleneck ?
The first question to ask is what is the Goulot process ? (voir la Theory Of Constraint et la Little law pour plus de précision). In other words, the longest process in our value chain. 2 cases will present themselves :
- Cycle Time of the Bottleneck > Takt Time : we will focus on this one to eliminate theMuda and make sure it does not take longer than the Takt Time.
- Cycle Time of the Bottleneck< Takt Time : priority will be given to the fastest process that generates us overproduction.
In our example, we clearly have a problem with positions 3 and 4 since they require additional teams to be able to absorb the load. Our priority is to increase the productivity of these 2 positions.
2.3 The line is balanced ?
We will seek to have a line more balanced possible in order to come closer to the principle of One Piece Flow.
Taking our example, we observe that the positions 1 and 5 have already been much improved causing an imbalance in the chain. We will do the same with the 3 and 4 which pose us problems.
3. What is the pacemaker process ?
Before redrawing the value chain, we will choose the pacemaker of the line. The Pacemaker will allow us to adjust the rate of production of the line and it is on him that we adjust the line. Once this one is selected, we will put the whole line in FIFO. Unless otherwise noted, the pacemaker is always the closest to the customer, the last one before shipping. This includes the fact that the focus is on delivery on time, in the right quantities and the quality requested by the customer.
This is where we find the fundamental difference between Lean and Constraint Theory. Both methods agree on identifying and “breaking” the bottlenecks. On the other hand, the TOC gives as Pacemaker the bottleneck where the Lean gives the customer as Pacemaker.
4. What kind of flow can we put in place ?
At this point, we know what process we need to adjust the line and we have an idea of the final productivity and the projects we will have to implement. Ultimate step of the analysis, choose the type of fired flow that we will implement. We will choose between :
However, it is sometimes necessary to have an operation in stocks, where one alternates “pulled flow and pushed flow. These are the typical cases where :
- There is a lot of series change generating high variability.
- You can not balance jobs effectively (too much job …).
We will then use the DDMRP.
5. Draw the new stream
We can then recalculate the new ratios, redefine the Takt Time and identify the times of operations. This allows us to set the objectives of future projects and to compare with the current situation.
Below, examples of VSM Future State for each of pull flow strategies (simplified examples to make it easier to read).
Make To Stock
The customer will have to wait for the assembly flow time to receive his product.
Assembly To Order
The customer will have to wait for the assembly flow time to receive his product.
Make To Order
The customer will have to wait the time of flow of the production to receive his product
Engineering To Order
In this case, adapted to the custom-made, it will be enough simply to set up FIFO.
3 – The action plan
From this future state, we will identify and plan the Lean projects to implement to arrive at the desired situation. We will start with the processes closest to the customer and prioritize the others using the gain/cost matrix.
M. Rother, J. Shook (2008) – Leearning to see ; value stream mapping to add value and eliminate muda
M. A. Nash, S. R. Poling (2008) – Mapping the total value stream
D. R. Locher (2008) – Value stream mapping for Lean Development