Standard WIP (SWIP)

The minimum amount of material or product that must be in the process at all times to ensure smooth operation.

Standard Work is a little underrated concept in Lean Manufacturing. It is not simply standardization or work standards.

Standard Work is composed of three elements: Takt time, Work sequence and Standard Work in Process (SWIP). Takt Time is a fundamental concept of Lean Manufacturing, and Work Sequence is relatively intuitive. SWIP, however, is a bit more complex.

SWIP refers to the minimum necessary in-process inventory (work in process or WIP) to maintain Standard Work. It is not more or less than what is needed. To calculate the appropriate quantity for SWIP, one must ask a number of questions.

While a rough estimate of SWIP can be obtained by using the equation SWIP = Sum of Cycle Times / Takt Time, it is still necessary to determine where exactly this SWIP should be applied. The following steps provide a guide for determining the appropriate quantity of SWIP:

what’S the team size?

Standard Work is the most efficient combination of manpower, material, and machine, and is based on takt, work sequence, and Standard Work in Process (SWIP). By definition, it should include manual work. If a process is fully automated, it is not considered Standard Work. Instead, it is likely an NC program.

To determine the appropriate team size, the sum of manual cycle time is divided by Takt Time. Therefore, one piece of SWIP per person is required. The equation for manual SWIP would than be:

SWIP(manual) = Team member x (1 piece = person)

When determining the amount of SWIP, there should be no rounding, unless there is less than a full person. In that case, round up to the nearest whole number.

process steps as automatic one-piece cycle machines

Standard Work assumes the use of multiple processes or machines, and separates human and machine tasks as much as possible.

When using an automatic cycle, the worker will only be responsible for loading and unloading, and will not be present during the actual cycle. The automatic cycle time must also be shorter than the Takt Time, ensuring that there is always at least one piece in the machine during each cycle.

This is known as SWIP (single piece auto), and is calculated as the number of single-piece automatic cycle machines multiplied by one piece per machine. There is no rounding necessary as it is not possible to have less than a full machine. However, this only applies to single-piece automatic cycles, and calculations for batch processes or cycles with longer lead times may differ.

process steps as a single-piece non-machine automatic cycle

The term "non-machine automatic cycle" refers to process steps such as the drying time for paint, curing time for epoxy, and cooling time for hot parts.

These process steps may not involve machines, but they do require a certain amount of time for completion. The ratio of this time to the Takt Time is known as the Single-Piece Non-Machine Automatic (SWIP) cycle.

It is important to note that this value should always be rounded up to the nearest whole number. In some cases, equipment like turn tables or FIFO racks may be used to manage the curing process, ensuring that a finished product is available for each takt, and a new one is added for curing.

Process steps with a batch automatic cycle

Batch processes refer to situations in which equipment is designed to unload and load multiple pieces at a time, rather than one piece at a time.

A common example is heat treatment processes where a vacuum must be maintained and the door cannot be opened for hours. In such cases, a batch of parts is removed and then another batch is loaded. The cycle time per piece may be less than the Takt Time, but the overall automatic Cycle Time is greater than the Takt Time.

The Single-Piece Non-Machine Automatic (SWIP) cycle in this case is calculated as (Automatic time / Takt Time) x 2. The reason for this is that in batch processes, which do not allow for the addition or removal of individual pieces during the Takt, an extra quantity of complete parts is required. This concept can be compared to the idea of a pulley and bucket system used to retrieve water from a well, where one bucket is at the bottom of the well, full of water and another bucket is at the top, full of water, and during Takt, you empty out the bucket one by one and fill it back up one by one.

It's worth noting that in formulas 2, 3 and 4, manual cycle time is not included in the calculation because rule #1 takes care of that. This is because every manual Cycle Time must be within Takt by definition of Standard Work and since the unload/load time will involve one piece, there is no need to add manual time back into the calculation (in most of the cases).

Supermarket

When speaking of a supermarket in lean context we are talking about an independent instrument that is used to control production. In a supermarket raw material and pre-commissioned components can be found in defined areas. The amount is well organized according the replenishment times of each component, in general the inventory is limited and components are refilled as soon as they are used. Following the Pull Principle with the help of the Kanban-System.

Concerning the supermarket we see it as one of the pull strategies that can be implemented as link between two process steps when developing the future value stream. The supermarket is the third option when firstly One-Piece-Flow and secondly FIFO are no options at all.
The supermarket is a great methodology to help your organization to manage a variety of inventory where you don’t need to know in what order the components will be consumed. Through the so called Kanban-Pull-System “internal customers” will take components of the supermarket, which are replenished by the internal logistics following the Kanban-Pull-Principle or in a pre-defined interval. With this integrated pull system of the supermarket as link between logistics and production you can also speak of a general replenishment pull system. But let’s have a look at the supermarket, what it can be used for and how it could look like in your organization.

The supermarket itself is a mix of FIFO lanes for different components stored in Mobile Racks or a typical shelf where components are stored in bins or on pallets. Let’s assume on workstation A 3 different components are assembled, in the supermarket 1 lane would be dedicated to only this component following the FIFO principle. With this explanation you can see why FIFO itself is preferred the supermarket. The supermarket is either steered with two bin principle or kanban cards. According the replenishment time, the replenishment is triggered with the extraction of components and the stock dropping below the minimum quantity.
You see, the key question is when to use a supermarket instead of installing plane FIFO lanes or even follow the One-PIece-Flow.

Here are some examples when this is the case:

• Two main material flow streams come together before or are split after the supermarket

• Your organization follows the made to stock principle, then the supermarket is at the end of production and stores finished goods

• With the help of a supermarket different lead times of suppliers paired with a high variance can be leveled and production can be smoothly supplied

• Upstream processes are lacking quality, downstream process steps can easily replace defective parts/components (interim solution until problem is solved)

• Different change over times, when a downstream process needs a change over the upstream processes can fill the supermarket as a overflow stock that is drained after change over is done

All of this examples have in common that the final target is to eliminate the supermarket itself and improving the material flow in a way that simple FIFO lanes or a Two-Bin-Principle at workstations can be realized.
The size of a supermarket is always determined by the components and their space needed for storage and their replenishment lead times. So it is a good piece of advice to have a clear overview of your components, their recurring demand including their replenishment times and don’t forget about a little safety stock on top.

As already mentioned there are two principles that are already well-known for implementing and steering a supermarket. The first one we have a look at is the Kanban-Principle and the second the so called Two-Bin-Principle, in which the bins itself trigger the replenishment.

A supermarket running on Kanban Cards can be seen shematically in the picture below. Every component stored in the supermarket is represented by an individual card, on which all required information is listed in order to trigger the replenishment process. The Kanban-Card can be seen as order slip for suppliers. Usually Kanban-Cards are placed on the so called Kanban Board. This kind of a supermarket can be seen most of the time.

The Two-Bin-Principle is a kind of supermarket where the bins itself are utilized the same way the Kanban Cards are used. In this approach all components have e.g. two assigned bins, filled with the dedicated components for this exact bins. The full bins are placed at the workstation, components are used and the empty bins are placed on the empty conveyor, ready to be collected by the Mizusumashi. the Mizusumashi refills the bins with the defined components and brings the full set back to the workstation. This kind of “decentralized” supermarkets are typically used for small and C-parts, which are consumed by not only one but several workstations, e.g. screws, washers, etc. as the financial impact of c-parts are low and the space needed is small.

In the end the supermarket is the last possible way to implement a Pull System after failing on implementing a One-Piece-Flow or FIFO. The target is and always will be to reduce the size of the supermarket by changing it into a FIFO system or change the material flow in to One-Piece-Flow. Therefore the supermarket can be seen as needed but temporary. The size of a supermarket is always defined by the size of the components itself, the replenishment time incl. some safety surcharge and the consumption lead time of the production. By simply removing or adding Kanban Cards or Kanban Bins the level of WIP can be adjusted.

Signal Kanaban

Every container or bin in your production facility is marked with a so called Kanban card or signal. The time the last component is taken, the Kanban is send to the source it came in production and is added to the so called Kanban Board. This board actually shows you the amount of inventory you have in production. It is a great tool to minimize the risk of overproduction and can be used to steer your production facility. Besides this it helps you to have control of your inventories itself of course . Based on the design of Kanban Cards it is possible to identify directly where the container belongs to and in what interval you have to check on it.

The checking and supply of material or components lies in the hands of the mizusumashi.

Push vs Pull Principle

The pull principle is one of the main blocks in lean manufacturing > pull manufacturing. The customer demand determines which good will be produced and in which quantity. Max/Min inventory levels are agreed with suppliers and customers. This gives you the opportunity to limit overproduction.

In a pull process a workstation or process step is only being triggered when the downstream processes are free to take the output. This means that only when the customer initiates a need the value adding is started. By this a customer can be seen as internal as well as external one. This is lovely as you make sure that only what you will sell will be produced.

Pull is always to prefer against push. As it allows you to manage the WIP and inventories in between workstations which has a straight impact on your lead time. To successfully install a Pull System you have three choices, it is either the well known continuous flow installed, sequential pull or the replenishment pull.

All three types of connection will help you to achieve different levels of Pull in your environment.

No matter what it always comes down to three factors in your system:

1. Having max one piece of inventory between two process steps

2. Having a fix production sequence

3. and having a max number of parts waiting

Thinking about the three types of a Pull System, the continuous flow system has the highest level of Pull, as all three factors are highly involved. Only One Piece at the time and in between two process steps which means a fixed sequence as well as max one part waiting for the next process step.

If you have several workstation linked in one main manufacturing process working on one final product you have a work cell or simply a production line, depending on the shape you can call it e.g. U-cell.

With the sequential pull system you will have the second best choice of a pull manufacturing. It will have a fixed quantity, the sequence of production is defined but with a max number of WIP allowed between process steps. Basically you will allow buffers in your manufacturing. This can typically be seen in FiFo lines (First in First out).

Last but not least we have a look on the Replenishment Pull. This is in other words your supermarket in the production and your third and last option of implementing pull. The supermarket has a maximum number of components, parts or products waiting to be worked or processed on. But it is not known which one will be next. This type of inventory is usually steered or controlled using the Kanban systematic.

To put it in a nutshell - the following table should be fine to give you an overview: