GHOSTS, PHANTOMS & DOA's.byBrian Willcox CFPIM of Action MRPII |
| What do the dead or supernatural have to do with materials management or production control? The cynical might say its a fitting description for the production control manager - he's dead but not fallen over yet. That is not my definition. As an ex-production control manager, I would say they are the busiest, liveliest guys around who make things happen - so that's not the answer. |
| If we turn to Collins English dictionary, it tells us
that a "Ghost" is the disembodied spirit of a dead person, supposed to
haunt the living as a pale or shadowy vision. A "Phantom" is an apparition
or spectre, and "DOA" is an abbreviation for dead on arrival.
If we turn to the APICS dictionary, the nearest it gets is to define a
"phantom bill of material" as a bill of material coding and structuring
technique used primarily for transient (non stocked) subassemblies. This
of course is what it is all about. |
| It is so easy to fall into the terminology trap. All three are in fact different names for the same thing, which is most commonly known as a phantom. I ran across the term "ghost" earlier this year, which is the term used by one of the South African developed packages, and yes it does conform to the general understanding of a phantom. A DOA, which in fact stands for "drawing office assembly", is used by one of the English developed packages. Now we know we are all talking about the same thing, what is a phantom and what do we use it for? |
| If we go back to basics, the principles behind our bill of material (BOM) structuring and MRP logic, is that the parent of a BOM is stocked. If you have a BOM, you create a picking list, issue the material, produce the item, and deliver it to stock. It is then issued to the next level of assembly in the same manner until we have our finished main assembly in the finished goods store. The logic is simple to follow and it is practical most of the time. |
| Unfortunately there are times when we don't want to
follow that approach. Perhaps the parts are too big to return to stock,
the assembly is moving down an assembly line, or its paint in a 20,000
litre tank that we will fill into several different can sizes, or its a
pastry case that we use for ten different pies. The truth is there are
many practical reasons why we may not want to return the manufactured
assembly to stock. If we don't, we had better have some way of telling the
system about it.
The answer is the phantom indicator, which is usually a field on the product structure record. It allows us to tell the system that this component on the BOM is one of the exceptions. We have now said that a phantom can be an assembly (parent) that is not to be booked back into stock but will be consumed in the manufacturing process. In other words, although we have a separate BOM for the part, it will be made and used as part of the manufacturing process of its parent. Therefore there will not be a works order to produce it, nor will it be scheduled in its own right. As far as the system is concerned it doesn't exist. When the parent is kitted for manufacture, instead of calling up the
phantom to be issued, it calls up the phantom's components to be issued.
From a practical point, when we decide to make an item a phantom we need
to do two things. One is to set the batching rule to Lot for Lot, so that
the correct number of component parts are issued with the parent.
Secondly, the lead time should be set to zero and any additional time
required added to the parent lead time. Often the lead time of the parent
does not need increasing as several activities are performed in parallel.
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| You may ask why don't we change the BOM and save all this performance of phantoms. There are several reasons why we need them. For example; we are building under license, with overseas design control, which means that we receive engineering change notes referenced to the subassembly BOM as used by the parent plant. We need to keep in our system the same bill structuring to be able to reference and implement the changes received. Due to the different way we operate with different volumes and equipment we need to alter the BOM structuring, and find that by using phantoms it allows us to maintain the overseas structure whilst combining levels for our own use by making some assemblies phantoms. |
| Another reason is that we have a product that we market in a wide range of colours. Most of the parts used are identical. We create a common parts bill which consists of the parts used on each of the colour variants, and then each final product calls up the common parts bill and the unique coloured differences. By making the common parts bill a phantom we have kept those parts separate for BOM control purposes but can have them at the level they should be for manufacturing. It must be realised that the common parts bill cannot be built as a subassembly in its own right as it requires the unique parts to go with the common ones. |
| Companies who start to apply the JIT principles find that one of the objectives is to reduce waste, and two of the wasteful activities we have is lead time, and the effort we use in putting things into stock only to issue them out a few days later. To remove these wasteful activities means combining BOM from one or more levels of the product structure. One easy way to do this is to make some of the assemblies phantoms. |
| In summary, we can say that phantoms are another tool in our tool box we can use to make our systems reflect the real world. They are easy to use and our systems support their use. |
| November 1997 |
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