Thanks for the answer, you saved me a giant headache and a lot of work!

Adding an "empty" reordering rule (all quantities are zero) seems to do the trick (which seems quite counter-intuitive to me).

In case someone stumbles upon this at some point in time and you are having some troubles with this, here a short summary:

- A manufacturing order is only going to be created after a sales order, if the product is set as 'make to order' and 'manufacture' and if it can be sold.

- A purchase order for missing parts of a manufacturing order is only going to be created for the parts in the BOM if either:

a) there is a reordering rule for that part (all quantities can be zero) => Odoo will then check the stock (don't forget to run the inventory shedulers, otherwise the orders will not be created)

b) the part is marked as 'make to order' => Odoo will always order the part count from the BOM and not check the stock

In case purchase requests are not generated automatically, you might have to check if the parts are delivered to the correct location (usually your default warehouse location). I had some troubles when the reordering rules were pointed to different locations, then Odoo would not consider them for reordering upon creating a manufacturing order. But I guess you could fix that by adding routes (don't know how though).