Crafting Mundane Items
We know from the description of the Crafts skill that making things is an overcome roll, not a create an advantage roll:
Overcome: Crafts allows you to build, break, or fix machinery,
presuming you have the time and tools you need.
However, it's usually not need to roll to see if you can actually create the item. The basics chapter tells us that
some of the things you’ll do in a Fate game require you to roll dice
to see if your character succeeds or not. You will always roll the
dice when you’re opposing another character with your efforts, or when
there’s a significant obstacle in the way of your effort.
If there's no significant obstacle in your way, there is no need to roll the dice, just let it happen.
As for how to go about crafting things like a Weapon:1, that's really up to your GM. If your game is using weapon and armour rating (remember that in Fate Core it's an option rather than a standard assumption) then the player and GM need to have a conversation about what can and cannot be crafted.
Going off of your specific questions, I'm assuming it's a fantasy game. With that in mind, any decent PC smith should be able to craft a tempered sword—no need to have it be an aspect, that way leads to aspect spam. Depending on what each weapon/armour level represents, the GM should know what ratings are allowed to be crafted during downtime and which needs quests to get the special ingredients needed to make.
Using Create an Advantage
Let's say the characters are taking some downtime and the smith
decides to craft some armor. The GM calls it a "Create an Advantage"
action and assigns a difficulty to it. The smith succeeds, creating a
Plate Mail aspect on a character with a free invoke. Does that sound
right? Is that the kind of aspect that stays around for a while? Can
that free invoke be saved indefinitely until the right encounter calls
If one is using weapon ratings, it's almost guaranteed that they will be using armour ratings, as well. So I see no need to create an advantage if you're crafting something with a weapon or armour rating.
Create an advantage, instead, should be used "to create aspects representing features of a piece of machinery, pointing out useful features or strengths you can use to your advantage or a vulnerability for you to exploit" (Crafts skill description).
This could be used after the item had been created (with the overcome action) to hone the blade sharper, or shore up weaknesses in the armour, and thus give it an aspect. It's easy enough to rationalize how and why weapons and armour don't keep their CaA-given aspects:
- Weapon are used and get dull
- Armour is used and gets dents, dings, and starts to fall apart
Use the create an advantage action when you have downtime to sharpen your blade with a whetstone, or use an anvil and hammer to beat the plate mail back into shape.