Manufacturing Strategy for Window Fabricators 3 - Just in Time




  Download Adobe Acrobat file of complete series to date

Sell daily - make daily

JIT began life in Japan during the 1960s at the Toyota company and for many years it was referred to simply as the 'Toyota Manufacturing System'. The rapid rise in quality and low costs of the system began to be noticed by other companies in Japan and by companies in the West in the 1970s and the group of techniques developed by Toyota was renamed 'Just-In-Time'.

One of the major attractions of JIT is the simplicity of the basic initial concept. The goal of JIT is the production and delivery of the required items at the required time in the required quantity to specific orders and not to a theoretical forecast or schedule. JIT is therefore well suited for 'bespoke' industries, such as window fabrication, where manufacture is always to order. JIT is known as 'pull' or demand driven because nothing is produced until just before the customer or the next work station needs it. The demand for parts or finished goods thus pulls product through the system with essentially no production for stock or inventory. The phrase 'sell daily - make daily' encapsulates JIT but in terms of moving the product along the production line there is an essential difference between JIT and the conventional line.

In a conventional production line the individual work stations produce goods at a speed dictated by the line speed irrespective of whether the subsequent operator is ready or not. One of the major problems with this is line-balancing to make each operation take exactly the same amount of time. Where line-balancing fails (as is almost inevitable with a mix of products) there is a build-up of Work in Progress (WIP) in front of the bottlenecks or slower operations. This ties up cash, in the form of WIP in the production line. The result of this is often seen in fabrication units - there is a build-up of stock in front of the slowest operation (which moves depending on the product mix).

Unfortunately this means that just when the orders are greatest and the factory is busiest then the WIP builds up and makes production the most difficult! It is easy to see the difference - if the factory looks any different when it is busiest then JIT is not running. 

In the proper JIT system an item is only passed to the next station when they are ready for it and want it. This prevents cash being tied up in WIP and decreases the product throughput time. In the words of an American JIT practitioner 'Don't make nothing, no how for nobody, make them come and get it!' JIT factories look exactly the same when there are lots of orders as they do when there are few orders - the only product is that being worked on and, of course, the finished goods awaiting despatch.

In the 1990s, many companies misunderstood the basics and saw JIT as a supplier problem; they simply reduced their raw materials stock levels and required their suppliers to deliver more frequently. This was JIT, or so they claimed, but this 'VAT approach' of simply forcing the inventory reduction back up the supply chain did nothing for the internal inventories of WIP and finished goods, forced no radical internal production improvements and was generally judged to be relatively ineffective. Any good idea can fail if implementation is misunderstood and only takes the soft option.

Low tech - high results

While regarded as a production management system, JIT is much more involved with methods than with technology. It is a total philosophy for production improvement and a radical change that drives all production areas with the single concept. The real strengths are the simplicity and the ease with which it can be explained to all levels of staff. 'Don't make anything until the next station needs it' sounds simple but preventing people from working is often difficult when we have been programmed to seeing all the labour busy (even if they aren't doing useful work). 

Computerisation of JIT is at an early stage but even when computerized it only involves limited computing power to replace the manual systems. This makes it accessible and can serve as a method for uniting employees in the common goal of improved performance.

The process of production under JIT can be compared to the progress of a boat on a river with large rocks under the water. The moving river is the movement of materials and the depth of water represents the WIP. As the water level is lowered (by the JIT process of driving down inventories) the rocks are gradually revealed. These rocks must be removed to allow the river to have less total water in it but to continue to flow smoothly and swiftly. Typical rocks encountered are quality, changeover times, vendor conformance, and product design. Failure to 'remove the rocks' will lead to system failure. Too many companies lowered the water, failed to do anything about the rocks and then blamed JIT for the problems.

The river and rocks model of JIT
The depth of the river is the inventory and work in progress in the factory. Under a conventional manufacturing system the inventory allows all of the other concerns to be hidden - at the cost of time and money. JIT removes the inventory and this exposes some of the rocks. Action must be taken to lower or remove the rocks or serious problems will result. Simply lowering the inventory is easy; it is dealing with the problems that this exposes that is difficult.

Despite the many good things in the concept, JIT can also be a risky manufacturing system. Minimal inventories make planning essentially short term and if the rocks are not removed swiftly once they are found then there is no inventory to rely on and the whole business can founder.

Push versus pull

Conventional production planning and developments such as MRP and MRPII (discussed last month) are of the 'push systems'. Materials and jobs are issued to production so as to hopefully achieve the required completion dates while at the same time minimising labour and machine downtime. An important measure in push systems is machine usage and the attempt to balance production lines is given a lot of effort. The build up of WIP in the production line is made worse by floating bottlenecks in the operation. Push systems are often unresponsive to changes in customer requirements, i.e. they tend to be inflexible and cannot cope with the need for increased variety and short runs. The concept of Economic Batch Quantities (EBQ) is generally very important in such systems and the need to schedule and run 'push' systems generally results in complex computer databases and programmes.

By contrast, in 'pull' systems the products are only produced when there is an order or requirement for them. This generally results in simpler control systems where inventory levels and batch sizes are greatly reduced. In a JIT environment, machine and labour utilisation are not relevant because machines are only used when they are needed. An operator passes work along only when a signal (KANBAN) is given that the next operator is ready for the work. Such signals are generally visual and easily understood.

Despite this easy sounding explanation, JIT is not simple to implement and many 'rocks' need to be worked on simultaneously. JIT is not a quick fix and it needs quick reactions when things go wrong!
Successful implementation demands careful planning, a firm strategy, the right environment (awareness, industrial relations etc.) and determination. The rewards of successful implementation are dramatic, improved cycle times, improved customer response time, improved quality and reduced costs.

For and against JIT


  1. Radically reduces product throughput times.
  2. Reduces or eliminates conventional inventory.
  3. Concentrates the mind on quality and set-up times.
  4. Provides a conceptual framework for strategic developments in many areas.


  1. Real cultural changes are necessary and these can be difficult to implement and hold.
  2. Close involvement with suppliers who may not share your goals is necessary.
  3. Lack of contingency planning can give problems (Life is both in the fast lane and on the edge of a razor).
  4. No long-term planning method.


After these pages on JIT you probably find that it involves more than you first thought it did. JIT is well suited to the window industry because that it makes simple control possible and can have a dramatic effect on the throughput of the factory. It is easy to experiment with JIT by letting your work in progress go down and then only making things as they are necessary. Fabricators who have carried out this simple experiment often find the results amazing.

'Manufacturing Strategy' Series.

The 'Manufacturing Strategy' series is designed to give window fabricators a set of ideas for managing production. The series is being published in Fenestra on a monthly basis and published here after the Fenestra publication. The series is:

Part 1: The Essential Part 
Part 2: The Systems
Part 3: Just-in-Time

Part 4: Optimised Production Technology

Part 5: Work Cells

Part 6: Machines
Part 7: Machines (2)

Part 8: Scheduling

Part 9: Waste (Methods)

Part 10: Waste (Materials)

Part 11: Supply Chain

Part 12: Measurement
Part 13: Things to do NOW!
Part 14: The Cost of Quality

Part 15: The Hidden costs of inventory

Part 16: Environmental management

Part 17: Continuous Improvement


Last edited: 11/03/10

© Tangram Technology Ltd. 2003

Our standard disclaimer regarding Internet data applies.