Machine Design
A bolt of M 24 × 2 means that

The effective diameter of bolt is 24 mm and there are 2 threads per cm
The pitch of the thread is 24 mm and depth is 2 mm
The nominal diameter of bolt is 24 mm and pitch is 2 mm
Cross-sectional area of the threads is 24 mm²

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Machine Design
In worm, gears, pitch lead angle is

Half the angle between two inclined faces in axial plane
The angle between the tangent to the pitch helix and the plane of rotation
None of these
The angle between the tangent to the pitch helix and an element of the cylinder

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