You need to run your machine shop like a clean room. Laminar airflow, high grade air filters, glove boxes, etc. Anyone who has ever even walked through a machine shop will know how challenging that would be. You probably aren’t going to do it in your garage.

For uranium there’s an actual public US government report.

https://www.osti.gov/servlets/purl/4284913

And here is an interesting write up on how hard it was to manufacture the plutonium spheres for The Gadget at Los Alamos:

https://www.tandfonline.com/doi/full/10.1080/00295450.2021.1...