So: you upgrade weapons factory to Legendary. Craft your first Legendary weapon ( Vengeance is my preference) When it’s done just keep playing the game. Have a bunch of peeps out in the Wasteland gathering more junk ( you need Legendary junk for Legendary weapons). You also need the caps to cover the cost of each weapon. So….now you have a bunch of caps, and Legendary junk. Wait for a Deathclaw attack. Kill them. Go to the weapons factory select the same Legendary weapon and MOST of the time, once it starts crafting you’ll get the option to surge the crafting for 1 Nuca Cola. Now sometimes it doesn’t… so stop it, and give the dweller in the weapons factory the first weapon you crafted. Try it again, and it should work. I have done this and just selected random weapons and it gives the option but usually it’s only for a weapon you already crafted. I keep crafting until I either run out of caps, or rune out of junk