What you're describing here sounds like the classic one-to-many scenario, with the 'one' side being the PartNumber and the 'many' side being the Detail Fields. This is usually handled with a Main Form/Subform set up.
In general terms the Main Form would be a Single View Form and based on a PartNumber Table. The Subform would be, as you’re currently doing, displayed as a Continuous View Form (to better display the 'Detail Fields') based on a Detail Fields Table.
The Fields would typically be something like this:
PartNumberTable
PartNumberID 'Primary Key
PartNumberName
...and so forth
Detail Fields Table
PartNumberID 'Foreign Key
Detail FieldID 'Primary Key
DetailField1
DetailField2
..and so forth
Once you've designed your Main Form and Detail Fields Form, based on PartNumberTable and Detail FieldsTable, respectively, open the Main Form in Form Design View and add a Subform Control to it. The Subform Wizard will walk you through the process. When asked, base your Subform on the DetailFields Form. Seeing that both Tables the Forms are based on have a Field named PartNumberID, the Wizard should ask if you want to link the Main Form /Subform using these Fields. Answer OK and Bob's your uncle!
As you move through the PartNumber Records in the Main Form, Access will automatically update the Detail Fields Records in the Subform and only show those Detail Fields related to that Area.
And when you add a New Detail Record, the PartNumberID from the Main Form will automatically be entered in the related Detail Record.
None of this requires VBA coding, as all the 'heavy lifting' is done by the Access Gnomes by way of the Subform Wizard.
Good luck with your project!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007