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Buffer Management-M'c Interference
When we looked at “Flowing the Operation” we saw how the “key operation” i.e. the slowest operation, influenced the output of the assembly line.  However “Machine Up-Time” can also significantly alter the output.  Machine Up-Time is the amount of time the machine is running during the shift.  In the following example, let’s assume that all the machines run at the same speed and the same up time.

  • If all the machines stop at the same time the best Up-Time performance of the line would be 90%.
  • If all the machines stop at different times the Up-Time performance of the line could be –

90% x 90% x 90% x 90% x 90% = 59%

  • If one machine forces others to stop and then they in turn breakdown (i.e. they run for 90% of the time that they are able to run) it could be even worse.

  Lets look at machine number 3 and see how it interfaces with the rest of the line –

  • Machine 3 can stop because it is broken down.
  • Machine 3 can stop when either, preceding machine 1 or machine 2 has stopped and therefore not supplying parts to it.
  • Machine 3 will have to stop when either, following machine 4 or 5 has stopped and therefore machine 3 cannot pass its finished work onward.

  Let’s now introduce a couple of buffers into the system and see what happens -

Machine 3 will still stop due to its own breakdown but it now has a degree of protection from the rest of the line in the form of buffers.  Normally these buffers will have a minimum and maximum bank.  In this case machine 3 will stop only if the preceding buffer reaches its minimum level or the following buffer is full.

The size of each buffer is dependent upon a number of factors –

  • Percentage up-times of the preceding and following machines.

  • Speed of the preceding and following machines.
  • Average duration of stoppage time of the preceding and following machines.  (i.e. machines can stop frequently for short durations or less frequently but for longer durations) therefore you must look beyond the up-time %.

Particular care must be taken of the line constraint (i.e. the slowest machine either, through its cycle time or up-time or combination of both). 

Effective buffer management can dramatically improve line performance.

One down side to this approach is longer lead times through the system. This is particularly noticeable when a line is started after it has been flushed during vacation periods.  Poor line performance on the start up shift is normally blamed on machines troubles and parts shortages but a major factor can simply be line fill time.    

OK that is straightforward, so now let’s take look quick look back at one-piece flow, which is where we started.

Lean manufacturing does not like inventory so it tries to remove buffers where possible.  You will find that should you decide to go into lean and design a “one piece flow line” with existing equipment and practices you will probably experience problems and wish to turn back to buffer management.  To continue you will be forced to pay particular attention to up-time and improve it through the introduction of planned maintenance programs, root cause analysis (5 whys) and simplification of machines etc.