JIT (Just-in-Time): the first pillar of TPS

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JIT (Just-in-Time): the first pillar of TPS 9

Just-in-Time: the first pillar of TPS

Just-In-Time (JIT) is a logistics and production method aimed at eliminating stock and material inventories in the factory. It is based on the concept of producing only when needed, i.e. when there is demand from the customer immediately downstream following the flow of the process.
This method of organising the launch of production, together with the adoption of increasingly smaller batches made possible by the introduction of rapid set-up techniques (SMED), eliminates or drastically reduces the storage of raw material awaiting processing, thus reducing the total transit time from days to hours.
The JIT consists of three elements:

  • Pull system
  • One-piece flow system
  • Takt Time
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JIT (Just-in-Time): the first pillar of TPS 10
SMED

The Pull System

Pull production contrasts with traditional (push) systems, which are based on production schedules set in advance and therefore inevitably fail to reflect actual demand. With the pull system, on the other hand, the production flow is driven by customers: upstream, goods are not produced until the downstream customer, whether internal or external, requests them.

The Kanban

The tool that governs this system is KANBAN (which means card), a visual system – a card, a signal, an area, etc. – that transmits a series of instructions communicating information on the materials to be procured or the components to be produced. A typical example of Kanban is a card accompanying a container or pallet on which the following information is displayed:

  • an identification number
  • a component number
  • a description of the component
  • where it comes from (supplier)
  • where should he go (customer)
KANBAN 3
JIT (Just-in-Time): the first pillar of TPS 11

There are two types of Kanban signs:

Handling/picking Kanban: are used to move components or materials towards a production process

Production Kanban: represent actual production orders authorising the upstream process to produce a certain component for a downstream process.

The handling Kanban authorises the movement of a component between two specified production centres. It indicates the types and quantities of components that the next process must collect from the previous process and is used to trace consumption between the various processing stages – operations.
The card circulates between the external storage point or the supply centre exit (where the part is produced) and the user production centre's incoming storage point. The card is always associated with a standard parts container when it is moved to the production centre that uses them.
The information on the handling label includes:

  • the component code number
  • the capacity of the container
  • the badge number
  • the supplier's production centre number
  • the number of the outgoing storage point for that production centre
  • the number of the user's production centre
  • the number of the incoming storage point for that production centre
  • the type and quantity of product that the downstream phase must collect from the upstream phase

When a parts container is selected for use from the incoming storage point, the tag
The handling tag is detached and placed in a collection box. It will be collected and returned to the supply centre as authorisation to collect another container of parts. Handling tags therefore only circulate between production centres and each one relates to a specific component.

The production Kanban authorises the production of a standard container of parts to replace one that has just been removed from an outgoing storage point. It indicates the exact quantity and type of product to be produced by the previous process (upstream phase). These cards are only used at the production centre and its outgoing storage point.
The information on the production sheet is:

  • the code number of the part to be produced
  • the capacity of the container
  • the supplier centre number

What are the rules of Kanban?

  1. Containers (trolleys, pallets, etc.) must always contain the same number of items (predefined).
  2. The parking area for containers (both full and empty) is fixed and predefined (horizontal signage, shelving labels).
  3. A Kanban card always contains the same (predefined) number of pieces (base quantity).
  4. The labels must be affixed to the containers.
  5. A full container can only be moved if it has a label.
  6. A Kanban card shows: the description of the part – its code – the number of parts in the container – the total number of parts to be produced (if greater than the base quantity).
pull production
JIT (Just-in-Time): the first pillar of TPS 12

Each production centre is equipped with a storage point at the entrance for the necessary materials and at the exit for the finished product of the centre itself. Components are stored in standard containers, which will always have a single label of one of the following types attached:

  • Transfer Kanban card (KB-T): authorises the removal of a container from a production centre and its transport and delivery to another centre.
  • Production Kanban card (KB-P): authorises a production centre to produce components to fill a container.

The full containers at the storage point at the entrance to the production centre have a KB-T label, while the full containers at the storage point at the exit have a KB-P label.
When a component runs out, the operator removes the full container from the storage point at the entrance, detaches the corresponding KB-T, attaches it to the empty container and sends it to the upstream department, thus authorising the transfer of a container. At the exit point of the upstream department, a KB-P is removed from a full container, replaced with the KB-T that has arrived and sent downstream. The KB-P remains in the department and authorises the production of a batch of parts (note that the batch sizes are equal to the contents of the containers).
Each tag must contain all the information necessary to identify and produce the component and the number of pieces in the container. It is used for a single part and circulates between a well-defined pair of production centres. The flow of tags between departments may also include an external supplier of parts.

The system logically begins at the storage point at the entrance to the final assembly line. Production starts with a request for a finished product, and each centre acts as a supplier to the centre downstream and as a customer to the centre upstream.
Production progress is “pulled” in that Kanban, starting from the planning of the finished product, pulls the necessary quantities of sub-products along the transformation cycle, precisely measuring the demand according to the final order. Compared to normal planning systems that ’push“ materials through the various stages to the finished product, the Kanban system is more streamlined and scalable and avoids excesses in the line.

Single Card, Dual Card and Batch Card Kanban

Single Card Kanban

Use only the movement Kanban. This is generally used in cases where the operational departments are particularly close to each other. It is the most commonly used type, providing a number of containers with a predefined quantity of parts for a given component, and each container is associated with a Kanban for replenishment. When a container is emptied, the Kanban associated with it acts as a replenishment order for the supplier.

Dual Card Kanban

It uses a handling Kanban and a production Kanban. It is generally used in cases where the operational departments are located far apart from each other.

Kanban Batch Card

Batch Kanban is implemented in production systems when the supplier's production batch is large compared to the customer's consumption. Batch Kanban is structured like normal Kanban, with the only difference being that the supplier waits until a certain number of cards for that code have accumulated before starting production. This type of Kanban involves the use of special boards for accumulating Kanban cards in columns divided by code.

Often on these boards, the columns are divided into three areas that are filled in succession:

Until I have filled the green zone with Kanbans and entered the yellow zone, I cannot produce that component.

Once the Kanbans begin to fill the area, the supplier can put that code into production.

As soon as a Kanban is placed in the red zone, the supplier must immediately put that component into production.

The One-Piece-Flow System

flowing production
JIT (Just-in-Time): the first pillar of TPS 13

The One-Piece-Flow system is a way of organising the progress of materials “one at a time”, with the possibility of changing product models at each stage, in a continuous flow.
In this way, individual pieces pass from one production phase to another without accumulating between machines, contributing to a reduction in the time line (the material passes through the departments as quickly as possible), achieving maximum flexibility, significantly reducing intermediate stocks, and recovering physical space within the line, thanks to the use of smaller machines, which are placed closer together due to the presence of small batches.
Unfortunately, the One-Piece-Flow system is not always possible. This happens, for example, when:

  • the upstream processes use machines with cycle times that are too slow for the production levels of the final assembly, which usually operates on 1 or 2 shifts;
  • In the process, there is a stage that inevitably requires longer set-up times than the other stages (e.g. semi-finished products are manufactured using large automatic machines and assembled manually in the final stage).
    In these cases, it is necessary to fall back on solutions that are closer to the One-Piece-Flow system and are therefore characterised by minimum batches, frequent set-ups and shipments, synchronised, reliable and physically close machines.

Takt Time

Takt time is the parameter that links production to the market. Takt time is a number that expresses a time: in this time, one unit of product must be obtained. It is essentially the pace of production.

takt time
JIT (Just-in-Time): the first pillar of TPS 14
cycle time

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