I've mostly worked in computer related sheet metal fabrication, it seems that the effective life span of a major customer like Cabletron or Lucent is about 5 years before they need to either purge their product line canceling all the old orders and redesigning, or they sell out, or the market takes a dump.
In the 8 years I made parts for Cabletron I was laid off 3 times, each time it was long enough to take a Temp job till I was called back.