I've long wondered why the construction of a crossbow does not require a mechanism, as it was the invention of primitive gears and ratchets that allowed heavier powered crossbows to be developed, historically speaking.
Additionally, if it is possible to require multiple skills for a task, it would be nice to see bowyer/metalsmith or even bowyer/metalsmith/mechanic skills affecting the ability to produce metal crossbows; there's an analogous anomaly with metal animal traps.
serving suggestions:
1 log + 1 mechanism (stone/metal) >> bowyer & mechanic skill, at bowyer's workshop >> wooden crossbow
1 metal bar + 1 mechanism (metal) >> bowyer & weaponsmith or metalsmith & mechanic, at forge >> metal crossbow
advanced serving suggestions:
(a 'prod' is the bow part, and a 'tiller' is the haft or stock part.)
1 log >> weighted composite of bowyer & woodcrafter skill @ bowyer's workshop >> [implicitly] small wooden prod & small wooden tiller >> practice/light game crossbow
1 log + 1 leather or 1 horn >> weighted composite of bowyer, woodcrafter and leatherworker skill @ bowyer's workshop >> a laminated composite prod
1 metal bar >> weaponcrafter skill with additional quality if bowyer @ forge >> a metal prod
1 log >> weighted composite of bowyer & woodcrafter skill @ bowyer's workshop >> a wooden tiller (add for bone & metal if desired, no effect on weapon quality)
composite prod + tiller + any mechanism >> bowyer's skill with additional quality if mechanic @ bowyer's workshop >> wooden crossbow
metal prod + tiller + metal mechanism >> bowyer's skill with additional quality if mechanic @ bowyer's workshop >> metal crossbow