`Just-in-time’ is a management philosophy and not a technique.
It originally referred to the production of goods to meet customer demand exactly, in time, quality and quantity, whether the `customer’ is the final purchaser of the product or another process further along the production line.
It has now come to mean producing with minimum waste. “Waste” is taken in its most general sense and includes time and resources as well as materials. Elements of JIT include:
- Continuous improvement
- Attacking fundamental problems – anything that does not add value to the product.
- Devising systems to identify problems.
- Striving for simplicity – simpler systems may be easier to understand, easier to manage and less likely to go wrong.
- A product oriented layout – produces less time spent moving of materials and parts.
- Quality control at source – each worker is responsible for the quality of their own output.
- Poka-yoke – `foolproof’ tools, methods, jigs etc. prevent mistakes
- Preventative maintenance, Total productive maintenance – ensuring machinery and equipment functions perfectly when it is required, and continually improving it.
- Eliminating waste. There are seven types of waste:
- Waste from overproduction.
- Waste of waiting time.
- Transportation waste.
- Processing waste.
- Inventory waste.
- Waste of motion.
- Waste from product defects.
- Good housekeeping – workplace cleanliness and organisation.
- Set-up time reduction – increases flexibility and allows smaller batches. Ideal batch size is 1item. Multi-process handling – a multi-skilled workforce has greater productivity, flexibility and job satisfaction.
- Levelled / mixed production – to smooth the flow of products through the factory.
- Kanbans- simple tools to `pull’ products and components through the process.
- Jidoka (Autonomation) – providing machines with the autonomous capability to use judgement, so workers can do more useful things than standing watching them work.
- Andon (trouble lights) – to signal problems to initiate corrective action.
VED stands for vital, essential and desirable. This analysis relates to the classification of maintenance spare parts and denotes the essentiality of stocking spares.
The spares are split into three categories in order of importance. From the view-points of functional utility, the effects of non-availability at the time of requirement or the operation, process, production, plant or equipment and the urgency of replacement in case of breakdown.
Some spares are so important that their non-availability renders the equipment or a number of equipment in a process line completely inoperative, or even causes extreme damage to plant, equipment or human life.
On the other hand some spares are non-functional, serving relatively unimportant purposes and their replacement can be postponed or alternative methods of repair found. All these factors will have direct effects on the stocks of spares to be maintained.
Therefore, it is necessary to classify the spares in the following categories:
Vital items which render the equipment or the whole line operation in a process totally and immediately inoperative or unsafe; and if these items go out of stock or are not readily available, there is loss of production for the whole period.
Essential items which reduce the equipment’s performance but do not render it inoperative or unsafe; non-availability of these items may result in temporary loss of production or dislocation of production work; replacement can be delayed without affecting the equipment’s performance seriously; temporary repairs are sometimes possible.
Desirable items which are mostly non-functional and do not affect the performance of the equipment.
As the common saying goes “Vital Few — trivial many”, the number of vital spares in a plant or a particular equipment will only be a few while most of the spares will fall in ‘the desirable and essential’ category.
However, the decision regarding the stock of spares to be maintained will depend not only on how critical the spares are from the functional point of view (VED analysis) but also on the annual consumption (user) cost of spares (ABC — analysis) and, therefore, for control of spare parts both VED and ABC analyses are to be combined.