No need to apologise, it is your database, not mine :)
Why is pipe and coupling separate tables? Aren't they both manufactured products and therefore should be the same table? Forget that couplings can be derived from pipe, that is irrelevant. What you are really saying is that product b (coupling) was manufactured from feed stock a (pipe), which may have been itself a product that was manufactured from feed stock c. Again, not knowing what the processes are but just working from general principles, the database I would envisage would start with feed stock and end up with products. It shouldn't matter if the product is a pipe, a coupling, or a gate post.
Now, this is just me, but I would have thought that thinking of it as pipe -> coupling -> DWA is blurring the issue. A more general approach would seem to be feedstock -> product -> DWA. This would cover any case, no matter how many processes or intermediate products existed in the manufacturing chain.
It is really hard to give specific answers to this without understanding the full requirements of your application - which is probably too big a topic for this forum.
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