From the description of the end goal, it appears there is a faulty table design here.
It would not be proper design to have to store the same values from three different fields in a second table. I think I can guess what the tables actually look like based on this description, although this is subject to verification.
The proper design is to designate the relationship between two tables using a Primary Key in the first table and a Foreign Key in the second table.
So, your inventory table should have 4 fields (at a minimum).
InventoryItemID -- Long Integer. Primary Key. The unique value which identifies each inventory item. Most commonly, we use the AutoNumber for this.
PartNumber -- Short Text. Your company's designation as numbers and letters.
UPC -- Long Integer. The digits making up the Universal Product Code of the part.
Description -- Short Text (up to 255 characters) or Long Text (if your descriptions are more than 255 characters long).
---Corrected--
The PartsUsed table needs 3 fields (at a minimum).
PartsUsedID -- Long Integer. PrimaryKey. Identifies each use of an inventory item in a part. Most commonly, we use the AutoNumber for this.
InventoryItemID -- Long Integer. Foreign Key. Identities the InventoryItem used for this part.
PartBuiltDate -- Date Time. The date on which that inventory item was used for that part.
To use this in a data entry form, you would either use a subform, if more than one inventory item is included in each part, or you could use a combo box for the inventory item.
Tell us more about the actual usage, for more specific suggestions.