The default system settings currently allow for the spontaneous generation of free metal even without intentional exploitation. I don't THINK this is a mod interaction, but do let me know if it's a known one (since it would render the following analysis moot):
EDIT: Yep, it's a mod... 'interaction', alright. Buried deep within the feature list of the Useful Stuff mod, underneath a dropdown at the very bottom, is that all metal tool grant 50 metal units' worth of bits when broken. I didn't expect Useful Stuff to include a feature that so dramatically affected the 'baseline' of the game, especially with so little fanfare.
A breaking tool already generates 10 bits of metal, which is 50 metal-units, or half the ingot. However, it also generates a "broken metal fragments" item which is chiselable for a randon amount, which is usually more than 50 metal-units. Given that this was all produced from a tool that only cost 100 metal units to make, we've already made a 'profit'. This is before taking into account metal bits that are generated from the cut-function during forging, meaning you can easily end up with a 50+ profit simply by forging and using metal tools regularly
Simply removing either the stack of 10 bits of metal or the chiselable pile of fragments should be enough to pull the number back down into the range of merely reducing "overall" metal usage; i would suggest the former.
That being said, some kind of system that "knows" how much metal a tool requires and thus setting an upper bound for how much the broken metal fragments item should be 'worth' might also be useful. A knife should produce less metal scraps than a shovel, since you get more bits during the forging process with the knife. It should also mean 'forward compatibility' for any tools made out of more or less metal.