Trained Only
This skill allows you to build mechanical devices from scratch, including engines and engine parts, weapons, armor, and other gadgets. When building a mechanical device from scratch, the character describes the kind of device he or she wants to construct; then the Gamemaster decides if the device is simple, moderate, complex, or advanced compared to current technology.
Special: A character without a mechanical tool kit takes a -4 penalty on Craft (mechanical) checks.
A character with the Builder feat gets a +2 bonus on all Craft (mechanical) checks
Craft Ammunition: d20apoc
table for examples of crafting mechanical here