I worked for a while in an outfit that made generator skids for offshore and onshore oil production, all custom. They had been doing it for years.
One day they got a hair across their butt about cleanliness, and implemented '5S' or something like that. Basically, they cleaned house, and then some. They gathered up all the little offcuts and other pieces left over from decades of previous jobs, and sold them for scrap. Then, since they didn't need them anymore, they sold their stock racks and now empty storage shelving, for scrap. The money from the scrap dealer paid for a modest pizza party.
Then they got another project. As usual, the specs changed after construction started, multiple times. They had to buy a full length, 20 feet, of a given size of angle, in order to get the couple of feet that they needed. ... and again, and again. Worse, every time they needed a new piece of stock, they had to call around and get bids, wait a couple of days, pay for the small delivery, and worst, find something else to do while they waited.
It was a friggin' disaster.
They delivered late, and lost their shirt.
They are now pretty much out of business.
You may be able to extract a lesson from that.
What I extract is that a company that builds the same thing again and again can predict its material needs, minimize onsite inventory, expedite tool changes that can't be avoided, and stuff like that.
Whereas, a custom shop really needs to have a lot of odds and ends lying around, or better, stored in an organized way, so they have the agility to implement small changes with essentially zero delay, and certainly don't want to be in the position of waiting for any external vendor to deliver a small quantity of anything.
Mike Halloran
Pembroke Pines, FL, USA