@MelissaMuz
If it is literally a one off project and there is nothing in the projects table that is required other than the name then another alternative is to have a separate field to record the project number - you can include a project record with a name like 'one off project' and if selected, the other field becomes available.
Access says to use it on pretty much every table and the Primary key needs to be "meaningless"
I would interpret that as 'should not be assigned any meaning' other than as a unique identifier for the record. So should not be something like ProjectNumber which has given it a meaning.
@micks55
Personally, I would not store an ID anyway. I would expect part numbers to be unique so adding a unique ID is overkill. By storing the part number in full you will know what was fitted at the time of the repair.
If your part number is text and say typically 8 characters long, your app will suffer from poor performance once you have a large number of records. An autonumber (a long) takes 4 bytes, strings require 2 bytes per character plus 2 bytes so an 8 character code will take 18 bytes - 4.5 times bigger. Indexing reads 4000 bytes at a time so will capture 1000 longs, or 220 strings - so simplistically strings will be nearly 5 times slower. This won't be apparent until you have 220 records although even then, with modern computers the time difference will not be that noticeable, but get into the 10's of thousands and you will start to see a degradation in performance.
And don't forget the child records will also need indexing - so if you have a parent record with 10 child records, for autonumbers, the parent record takes up 8 bytes (4 for the record and 4 for the index) and the child records 80 bytes - a total of 88 bytes, whilst your string equivalent will require 36 bytes for the parent record and 360 bytes for the child records - which takes up more space on the disk
So by all means stick with your string PK if you don't expect to hold many records (I do use them for things like listing abbreviations as PK and fullname for US states, counties in the UK and the like) . But don't dismiss autonumbers as overkill.