What should be the included angle of the cutting tool used for machining metric threads in a thread-cutting operation?

Explanation:

Metric Thread:

The Bureau of Indian Standards has adopted a unified screw thread profile based on the metric system as the standard thread profile for use in India and designated it as the Metric Screw Thread.

• It also recommends the pitch of the thread instead of threads per inch as one of the standard parameters in specifying a screw thread.
• Metric threads are specified in a drawing by the letter M followed by the nominal size (Basic Major Diameter) and pitch. both expressed in millimetres.
• The included angle of the cutting tool used for machining metric threads in a thread-cutting operation is 60°.

For example a 20 mm diameter 1.25 mm pitch fine screw thread is expressed as M20 × 1.25.

A 20 mm diameter 1.5 pitch a count screw thread is expressed as M20. 

• It is customary not to mention the pitch of a coarse thread.
• If thread length is required to be specified it is added after the pitch and an x is used to separate the length of the thread from the rest of the designation. 

For example M10 × 1.25 × 25 indicates a thread length of 25 mm.