The try-square is a precision instrument which is used to check the squareness of a surface and the flatness of surfaces.

Witness marks The line marked on metal surfaces is likely to be erased due to handling. To avoid this, permanent marks are made by placing punch marks at convenient mark intervals along the marked line. Punch marks act as a witness against inaccuracies in machining and hence, they are known as witness marks.

The type of drill used for general purpose drilling work is Type N.

The helix angles vary according to the material being drilled. According to indian standards, three types of drills are used for drilling various materials.
Type N - For normal low carbon steel.
Type H - For hard and tenaceous materials.
Types S - For soft and tough materials.

The angle between the flanks in trapezoidal thread is 30 degree.