==Corpse processing facility==
With the help of a necromancer, corpses your dwarves refuse to butcher can be brought back to life and re-killed to yield bones and skulls for your bonecarvers if they are mushed up enough.
1. The simplest way to do this is with the help of height. A 1x1 pit with a minecart stop that dumps corpses down the chute, and several alternating floor hatches that close and open (linked to a repeater) with necromancers behind windows overlooking each layer of hatches to revive the bits of corpses. 2 windows with a mechanism controlled door in between, in front of each necromancer group can be used to control vision; but the system can only be stopped by unlinking the minecart dump to the refuse pile in your routes. Note: when i built this i had 3 hatches with 6 necromancers overlooking each (i had plenty of them since i embarked close to 4 towers). Revived corpses drop to their death and explode onto a tile with unright spikes linked (note, some of them will survive, so you need the spikes with a repeater or lever). The corpses that explode from the impact of height (or from other body parts/undead crashing into them) will hopefully yield bones. You may choose to re-haul up the body parts for another round, but only body parts still attached to a grasping part or the head will be revived, and this system isn't very efficient in the first place, so it may not be worth the trouble of hauling the arms and other bodyparts back up. Note that whole corpses usually yield 5-8 bones upon death (avg 6), arms only yield 1-4 (avg 2). You may also use this system with or without necromancers and pit live goblins into it, they usually yield 6 bones and some body parts. Though if you use necromancers its suggested to either stop the repeater before pitting any live creatures or make another pitting area under the hatches and pit from there as groups of creatures tend to survive all fall better than 1 creature at a time.
2. The second way is much more efficient than the first, but requires 1 or more artifact mechanisms to make it work. Instead of using height to kill the corpses, a weapon trap with a artifact mechanism and 10 serrated blades of any material can be used instead (since artifact mechanisms never jam). Only 1 necromancer is needed for this method, and is positioned 3 tiles away from the weapon trap, overlooking it behind 2 glass windows with a mechanism door in between to control it's vision. Your 1x1 pit should still be 5 tiles deep at least though, to prevent dwarves being spooked by the revived corpses. When your ready, link up the route to the minecart and watch body parts revive and slowly get mowed down. Its recommended you have more than 1 of this small pit set up so you can grind more corpses and clear out 1 pit at a time while the others keep grinding.
Note: to clear out pits, turn off all refuse stockpiles that accept anything other than bones and skulls by turning on "accept from links only" so your dwarves only haul out the bones and not the trash.
Note: try to use raising bridges as the door for each pit, kobold body parts tend to get mixed into the grinders which can lock-pick its way out of doors and result in doors with "door taken by intruder" and a couple hundred zombie body parts overrunning your fortress from the inside (a.k.a fun).
Note: i had a minecart system in place in my fort to help haul the bones and body parts back up.
Note: i didn't try this with many building destroyers, but i'm pretty sure the glass windows are safe. Fortifications are not usable since corpses and body parts tend to get tangled up in them and are hard to get out, and spook dwarves trying to clean out the pits.
*Bonus: Use water to clean out the contents of the pits and wash them onto a 1x1 refuse stockpile.
'''Difficulty:''' Hard
'''Usefulness:''' High, and becomes higher the more corpses you have; especially useful for getting something out of necromancer sieges than just useless corpses. Can also be used to recycle dead stray animals and your own dwarves that your dwarves refuse to butcher (don't forget slabs).
Note: the repeater i used for the first method was a creature logic one with 2 plates linked to hatches and a kobold thief, so i only needed 1 repeater for the alternating hatches. Be aware different kobolds have different speeds, you may need 2 or 3 hatches depending on what speed the kobold moves at. I used 3 since my kobold moved fast and the second hatch openned before the first one closed.
EDIT: Note: i modded my kobolds and removed their trapavoid. This also works with goblin thieves; i used kobolds since they tend to yield less bones than goblins.
EDIT: WARNING: do not use the second method on un-armored, fleshy, non-skeleton corpses, especially those of larger creatures. After some testing it seems artifact mechanisms DO get jammed after all, and quite alot. Up until now the only corpses i ground up using this second method were body parts i hauled up from splattered goblins and zombies i threw down my chute. I suspect 1 of 2 things is at work here, either non-skeletal corpses weigh more and therefore jam more OR more likely that larger un-mutilated corpses have a tendency to get "struck down" instead of "collapsing" (i noticed this when i used a slicing weapon on zombies in adventure mode).
I don't think the mechanism needs to be artifact anymore, i havent tested any non-artifact mechanisms. It is safe however, to pit armored non-stripped goblins on the disk trap if you use glass disks or lower (i use glass) which cant slice through their helmet and breastplate. They usually lose all their limbs and their reanimated corpses cant be "struck down" in the case that your trap does get jammed and there are undead in the pit, minecart stop some items down the pit and the falling items should eventually kill the undead (i used sand bags and mined out minerals).
EDIT2: after more testing, i've concluded that skeletons are absolutely safe to grind using serrated discs and axe blades, but rotten or freshly killed corpses are a no-go. Size of corpses does not seem to matter any more, just the fact that they must be skeletal. This is presumably because a skeleton's vital parts such as brain, lungs, and heart have rotted away, and thus they can never be "struck down" and disc attacks that would have otherwise hit such parts would "pass right through".
i've also tried axe blades in place of serrated discs, but they were only a little slower and did not sever parts more often despite the greater impact mass. Probly because mass only affects armor penetration and the axe blade only had 1 attack with 1.6 more mass than the serrated discs' 3 attacks.
im also sure mechanism quality doesnt matter now, but higher quality artifacts grind more thoroughly since they get more hits in.