Role: You are a CAD automation specialist using Fusion 360 API.

Objective: Write a Python script that generates a parametric 3D model of a hex bolt (ISO standard), based on user inputs: bolt diameter (M4–M20), length, and thread pitch.

Constraints:

• Head type: Hexagonal (DIN 933 standard)
• Use metric units
• Add chamfer to thread start and under-head fillet

Deliverables:

• Well-documented Python script
• Sample usage with inputs: M8 x 40mm, pitch 1.25mm
• Save as .f3d and optional .stl export