When a component is required it must be produced - there is no work in progress or inventory to rely upon. This demands high machine reliability and a fast repair must be possible in the event of failure.
This often dictates that simple low functional machines are chosen rather than complex multi-function machines.
The operator must share the responsibility for routine maintenance of the machines in his care and must be able to carry out first line maintenance. This will reduce machine breakdown time and also increase operator interest.
A planned maintenance programme is necessary to remove unscheduled stoppages and in this case machine utilisation should not be planned to exceed more than 80%. This is in contrast to our normal concern with optimising machine utilisation. JIT says that you do not operate a machine if the output is not immediately required.
Any unscheduled stoppage causes problems in many ways:
- Bottleneck machines become more overloaded.
- Production planning is difficult.
- Lead times increase.
- Scrap and re-work is increased.
The way to avoid these problems is to plan your maintenance strategy along the following lines:
Plan maintenance into the shop schedule.
Learn to predict machine breakdown via a written history and performance measurements. Most machines suffer the same breakdowns repeatedly, a history will let you diagnose the problem quickly and predict what is going to go wrong. Go for those faults that occur most often on the bottleneck machines.
- Design out maintenance problems.
- Design the system for fast repair.
- Carry stocks of trouble prone items or better still design around them.
- Decentralise maintenance tasks to operators (train, train, train).
- Provide tool kits close to machines.
A similar type of maintenance programme needs to be developed for tooling and other critical production items. What about a people maintenance programme, after all they are critical production items.
The traditional 'push' approach has been to buy complex machines and to try to improve utilisation times. A JIT and cell approach decreases this emphasis and leans towards the use of small machines that can be copied easily as they are needed.
The traditional view has been that technology can work us out of the problems. When the super machines fail (as they must do) the whole line grinds rapidly to a halt. My experience is that they always fail on a Friday afternoon, the service technician can't get there (from Germany) until Tuesday, it is fixed on Thursday and fails again just as the technician's flight leaves from Heathrow on Friday afternoon. Start the cycle again.
When a small machine fails, the line or cells can keep on running with flow switched to another machine or cell. Probably the biggest benefit of small machines is that they allow a closer linearity between capacity and sales. In business it is often said, 'costs are certain, but sales never are.'
The truth of this is self evident, which is easier to prepare: the cost budget or the sales budget? If sales have been increasing over a period then it is tempting to forecast that this will continue.
This 'forecast' says that you will have sufficient volume to buy a supermachine that will be fully utilised. You then borrow money from the bank to fund the purchase. But sales are never certain, they tail off and you are left with an overdraft, a machine that never gets into it's stride and possibly a failed business.
Small machines require less investment at any one time and this can be staggered as your sales increase and finances allow, no overdrafts and a healthier business. The relationship is shown in Figure 1 and Figure 2.
Figure 1: The Super machine approach
Figure 2: The small machine approach
Small cheap machines give a multitude of benefits:
- Easier operator maintenance.
- Copies can be made to give quick changeovers.
- Many cells can be developed to allow parallel flow to take place.
- Can be developed internally.
- Lower emphasis on utilisation.
- Requirements can be steadily increased rather than huge jump.
- Simpler control.
- Lower capital cost.
- Shorter cycle times.
The one drawback with small machines is that they are not very sexy and you can't boast about them at the pub but that is sometimes the price you must pay for survival.
Within the JIT environment there is a need to reduce the size of transfer batches within a cell. How many fabricators cut or process in batches and what does this do to the cost of work in progress? JIT encourages the use of small transfer batches.
The Kanban is a card or indicator that tells you:
- How many items in the batch
- The component name and number
- Where it has come from.
- Where it is going to.
The card can be fixed to a box but it is easier to design a box to act as a Kanban. The container is full when it has the required quantity. Scrap is shown immediately and control of production is highly visible. The number of containers present at a work station tells that operator the exact number of products to process and he cannot produce more than this on the basis of 'just-in-case'.
If you have lots of space for work in progress e.g. many toast racks, then you can be sure that they will fill up but you can also be sure that it will fill up with the wrong stock i.e. the stuff you can never sell. Reducing the number of toast racks can speed the flow of work through your factory provided you insist that no work is produced if the toast rack in front of the station is full. Use the toast racks as Kanbans for your factory and cut down on the number.
On a similar vein why is it that the scrap bin is always full no matter how big it is?
"The Manufacturing Strategy" series is designed to give production managers and their staff some insights into new manufacturing methods and to prompt the industry into considering the benefits of alternative approaches to manufacturing. The series is:
Part 1: Setting the strategy
Part 2: The systems and MRP II
Part 3: Just in time (1)
Part 4: Just in time (2) (This section)
Part 5: Just in time (3)
Part 6: Optimised Production Technology (OPT)
Part 7: A fundamental quality
Part 8: Quality management techniques & tools
Part 9: ‘There's no accounting for manufacturing strategy’
Part 10: Performance measurement
Part 11: Changing roles and things to do NOW!
Last edited: 11/03/10
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