KNOWLEDGE PROVIDERS
JN FACTORY
 
  3. THE BUFFER
HOME BUFFERS INTRODUCTION
 
   
   
   
   
   
 

When does an operator needs the material from the previous operation in front of his resource? Must he go and find the raw material when he finished a container? Must he do it some time before the container is finished ? The answer is for sure the last option (do you agree ?). But how long before hand must he have the material in place? Should it be 1 hour? Or 1/2 day? Or may be 1 day? Perhaps it should be 1 week?
We need to issue the raw materials at such a time, that they will arrive in time at the constraint. In time meaning to protect the constraint against starvation (=losing money) by putting enough INVENTORY in front of it.

We also have to guarantee the due date performance to our customers. This means that the inventory we put in front of the constraint, should allow us to produce in the right sequence.

Here is the role of the non constraint resources: Guarantee that the materials get on  time, in the right sequence, in front of the constraint.

Question: In order to do so we tie a rope from the DRUM to the material issue operation. How tight should be the rope. Or how long before hand must Raw Material be issued?

A very tight rope means a continuous irritation between the first soldier and the slowest soldier. It would be better if the rope has a small slope. If, for whatever reason the slowest soldier slows down, then he will not at once be pulled over by the first soldier. The rope should be able to handle most of the incident between the gate operation and the constraint. And incidents will happen between the first and the slowest operation! Those incidents are caused by:

  • A breakdown of a resource (often called Murphy)
  • A resource can be occupied in another job (= non availability of non constraint resources)
  • Statistical fluctuations:
    • Average operation times at the constraint and non constraint operations
    • Average set up times at the constraint and non constraint operations

A tight rope might cause problems by not having enough work in process in case of incidents in front of the constraint operation. A too wide rope causes too much work in process on the floor. And too much work in process can cause major problems on the floor (space, stealing for another job etc.). Again, how tight should the rope be ? Or how long beforehand must Raw Material be issued?

Can I calculate the risk of Murphy ? I know Murphy will come and then I can reduce its impact ...but I will never know when Murphy will pay me a visit. Can we calculate the non availability always? It seems to me, that we have a fair chance here. But always accurate? Operating times are average times, how do I handle the statistical fluctuations?

The rope is an estimation of a risk factor between two elements:

  • The risk someone wants to run to loose sales by not being ready in time, because of a starvation of the constraint.
  • The risk in having too much protection by much work in process, which is difficult to handle and leads indirectly to a starvation of the constraint.

There are in fact no written rules for it, except the rule: 'Use your knowledge of your particular situation, follow your intuition and be paranoid, but not hysteric'. The rules are based on intuition of the total quality of the process and as such a function of the reliability of the resources in front and behind the constraint.