Manufacturing Strategy for Window Fabricators 5 - Work Cells

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Work Cells

More new tools to improve the flow for window fabricators. This month we look at using small work cells to reduce inventory, speed production and provide simple but accurate control. A review of the basics by Robin Kent of Tangram Technology.

Fast, flexible and short

When fabricators start out they normally have around 3 people and produce around 60 windows per week. Generally, as they expand, they buy new machinery and set up their factory based on the traditional production line. On this basis: When they expand to 6 people what does their production output go up to?

Logic tells us that it should be 120 windows per week but if it was then it would be very surprising, the general result is around 100 windows per week. So, what happens to the missing output and who isn't pulling their weight?

The answer is that small groups of 2-3 people can be run by 'team leaders' who also do productive work. But by the time you get to 6 people you start to need a manager who does no productive work but simply manages the rest - he becomes a 'wealth dissipater' rather than a 'wealth creator'. For the conventional factory there is little linearity between the numbers of people and actual output, i.e. doubling the number of people rarely doubles the output. Work cells give us a way out of this common dilemma. 

A work cell is a small natural group of machines and/or people and skills. The various machines and skills will be such that a cell can manufacture a whole product or component, e.g. rather than have a welding area where all welding is carried out, the welders will be distributed around the factory in small cells where they are making products. This will result in reduced transport costs (a non-value activity or waste), reduced WIP and better control of quality. Work cells create an environment of ownership and autonomy; they also control quality and almost naturally seek to improve the process and product.

The work cell idea can best be explained by considering the work flow in the current factory. Work moves around the factory along a long production line, which is never truly balanced, so that some areas require more labour than others. An increase or decrease in sales requires staff changes but they are never whole numbers of people and you always end up with a few more or less people than you need.

Work flow in the conventional factory Work moves around the factory in a long linear flow. There is little attempt to minimise the distance travelled (a good indicator of the time taken to produce the product), handling activity is high, WIP is high and balancing of the line is a continual struggle as the product mix changes from day to day.

 

Work flow in a work cell based factory Work moves around the factory in short cell based flows, distance travelled is minimised, handling activity is low, WIP is low because there is nowhere to hide it, cells are self-balancing for production flow, operatives become multi-skilled. Work cells are most effective when the cell forms a U shape. Work flows into the cell, around the U and out the other end. This gives a small, compact and easily defined cell boundary that the operators can be responsible for.

The distance parts travel (a crude but simple measure of efficiency) is quite high, indicating low efficiency. In this model of the factory we have a 'serial' type of production flow, i.e. the work passes along one line via a series of individual stations. This is the conventional production line model that we all feel comfortable with. 

The production line gives a well-defined flow of materials but always gives problems with line-balancing and most importantly with poor operational reliability. If a work station in a 'serial' line fails then the whole line either stops or inventory rapidly builds up in front of the failed work station. Line balancing also means that most stations will be working at less than the full capacity (to match the bottleneck) and the total line will be working at greatly less than its full capacity.

Serial lines have some advantages in that it is easy to change the number of work stations and if the volumes are high enough for each product variant, then changeover between different types can be easy. The disadvantages are:

• Only one type of product can be produced at any one time.
• Full production cannot be started until the whole line is changed and ready to operate.
• The standard order of assembly cannot be easily changed.

The work cell approach is to divide the factory up into separate cells where similar operations are performed in various independent cells.

The cell approach is the equivalent of having many small fabrication shops running side-by-side and using cells it is possible to:

• Produce many different types of product at any one time to cope with the need for variety. You could have a woodgrain cell, a white cell, a door cell, a T/T cell, etc. Product variations are easily coped with by the cell concept.
• Change production, repair or service machines in one cell whilst all the others keep producing at normal rates.
• Get good linearity between the number of people and output. Doubling the number of fabricators really can double the output.
• Cope varying sales levels by opening up setting up or closing down individual cells.

Work cells offer a proven method for increasing output, increasing quality, improving workforce involvement and skill whilst at the same time providing greater flexibility for the fabrication process.

Failure proofing

Serial line layout

If a serial line machine stops working then the whole line stops producing (once any buffer stock has been used up). For example: if each machine has a reliability of 95% (it is capable of working 95% of the time when needed) and the line contains 8 work stations then the probability of the line delivering completed product is only 66%.

Parallel lines layout

If a parallel line machine stops, the other cells are free to keep working and production continues. Labour can be transferred to the operating cells and customers continue to get products.
Increasing the number of cells increases the probability of continued output from the line

Parallel lines based on work cells have a much higher operational efficiency and many different products can be produced at the same time.

For the same line conditions given in the example above, the probability of the parallel line delivering completed product is 100%, i.e. the system is highly reliable. 

Perhaps this explains why some companies seem to spend more time fixing machines than producing product and why every machine breakdown stops the output totally?

Multiple benefits

Work cells have many additional benefits such as:

• Standard working practices can be developed for each product.
• Visual control of scheduling is easier.
• Reduced waste.
• Easier labour balancing.
• Multi-skilled labour that can carry out all of the cell activities.
• Multiple machines:
  • Cheap to purchase.
  • Built in redundancy and capacity.
  • Easy to maintain (by operators).
  • Backup for machinery.
• Reduced and visible WIP.
• Improved quality from experience and more involvement in complete production.
• Short flow routes - when the distance travelled is short the throughput is increased and the inventory is decreased.
• Ownership of the process by the operators.

Summary

Work cells set the scene for significant cost reductions through additional techniques that affect many other production areas. Most significantly work cells lead on the 'one-piece' flow concept where the whole system is set up to produce one piece at a time - flexible, fast and cost effective.

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: 17/05/05

© Tangram Technology Ltd. 2003

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