I see a need for a date field that denotes when a tool was changed out due to end of life. That way, you can calculate (not in tables, remember?) and display the "end of life" date by adding the life expectancy value to the swap date. However, that would only be advisory. The life span would be [replace date] minus [install date] (2 fields for the one record). When those 2 dates are entered, that record is complete (other than perhaps to add comments or such) which means you'd need a table for tool swaps. A table for comments would make more sense than a field in the swaps table if comments would not usually be made.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.