/ sledge hammer can be crafted with the following recipes

Player Recipe
Primary skill used: fabrication(6)
Required skills: N/A
Time to complete: 420 minutes
Tools required:
> 1 tool with anvil quality of 3
> 1 tool with hammering quality of 3
> electric forge (640 charges) OR acetylene torch (640 charges) OR electric arc furnace (320 charges) OR charcoal forge (640 charges) OR basecamp forge (640 charges) OR demon forge (640 charges) OR rock forge (640 charges)
> metal fileset OR pair of flatjaw tongs
Components required:
> 1x plank
> 8x lump of steel OR 32x chunk of steel OR 160x scrap metal
Byproducts:
> 13x splintered wood
--
This recipe can be found in the following books when fabrication is at least the required level:
101 Crafts for Beginners (level 3)
DIY Compendium (level 4)