A lot of people have been asking about making lv40 weapons (especially the cestus) so I thought I'd write a guide on the whole thing.
The whole process is somewhat lengthy so I'm going to split it into three parts.
There's a few things you need to know before crafting a lv40 weapon:
-Not all of the lv40 weapons can be crafted
-Gathering all the materials and the blueprint for a one handed weapon (dagger, rapier, sword, mace, cestus, shield) costs ~80 gold. Doing so for the other weapons costs twice as much.
-The craftable lv40 weapons each has a 3rd material which requires fame to buy. Getting the fame level required to unlock the material may cost up to 250 gold.
-Crafting the weapon itself does not have 100% success rate. You will lose the blueprint plus all of the materials if the fusion fails.
-Gathering aid can be used for a slight boost in success rate for a Job
Now let's get to the guide:
Preparing to craft
Before you can do anything you will need to decide which weapon to craft and find out what materials are required to craft the weapon. Weapon blueprints are sold by "Weapon Designer" NPCs in guild towns. Hit "G" -> "Find Town" -> "Towns" to bring up a list of player towns and which crafting NPCs they have. I'm just going to shamelessly advert the Atelier guild town because they're top on the list, they have all of the weapon NPCs and their tax is only 5%. Once you decide on the town hit the "Enter" button to enter it. If your are in a guild with a town and the NPCs for the weapon type you want, you should use that instead of buy tax-free.
Once you arrive at the town, find the Weapon Designer NPCs to list the available Blueprints. (they're all at the Bunker Market if you picked Atelier town) The lv40 weapons are only at the bottom of the list. The materials needed are listed in blue below "Required Materials:"
*Very Important - Pay attention the 3rd item on the required material list, Weapons with Foolish Thought or Rancid Blood as 3rd material CAN NOT be crafted*
Gathering the first Materials
Now depending on the weapon you picked, you will either need Hard Iron or Pine Tree as the first material. To get them you need to go a guild town with a Mine for iron or Farm for pine, (Atelier only has mine) buy the paper which gives the material from the merchant, and then use the paper to assign a job onto the miner/farmer.
Please note that the job doesn't always succeed, you need to buy more papers and assign more jobs if it fails. When the job is complete, you'll hear a sound and can click the orange scroll icon to bring up a list of active/finished jobs to claim the materials.
The Volcanic Diamond and sky grass Sky Grass is obtained in the same way. The mirage rune by doing pretty much the same at an Altar, but you need to kill the lv40 monster summoned instead of waiting for job to finish.
Unlocking the Third Material
edit: The 3rd material cannot be traded, but the lv40 weapon is not bound to character when made. If you have trustworthy friends, it might be worthwhile to pool together money to gain fame on only one character, use that character for crafting and then trade the weapon to the player who paid for the materials/needed weapon after it has been made.
The last material you need is either Ore Separator or White Flame. Those are bought from Arthur Hammer or PyroWind but you need 1000 fame with the association before you can buy the materials. Each association has 3 NPCs, one which you can buy stuff from and two which you can give stuff to in order to increase your fame to that association. If you click on them you will get a list of stuff with which fame range you can turn in the item to get more fame, how many items you need to turn in at a time and how much more fame you get for turning in that many items. Here's a picture of the 2 NPCs for Arthur Hammer
From What we see here we need to feed the bear 150 "Raw Coppers" to get above 100 fame, and then "Refined Coppers" or "Terrified Thought" untill the fame is above 400 and then "Hard Irons" untill the game is above 1000.
Terrified Thoughts are from killing monsters in Wetlands or Kings Valley. The minerals and Crystals can be mined in the same way as Hard Irons.
The 3rd material you needed becomes buyable after you get over 1000 fame.
After you have the Blueprints and the Materials needed, go to either the Molding Master or Famous Smith in a guild town to get your weapon made. (They're both at the foundry in Atelier). Bite down and hope the fusion doesn't fail. I'd post a screenshot of me making a weapon but I don't have the gold to do so.
hope this helped and good luck