probably not the machines table - the machine relevant fields (sales/serial number etc) can be included in the parts table and left blank for ordinary parts.
You would have a lot of nulls rather than repeat the serial/model info for every part record?? It seems to me that the major purpose of the db is to track everything about the machine from the time of purchase to the time of sale. Certainly warrants a machine table in my mind.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.