PARTS OF SCREW THREAD
                                                                                                            A screw threas has the folloeing parts ans shown in fig 1.1

 
                     fig. 1.1

1: PITCH


It is the distance between the same points of adjacent threads measured parallel to the axis of the screw.
2: LEAD

It is the distance that a nut will advance on its bolt along the axis of Thread in one turn or revolution.

I single start thread load and pitch are equal.

L=Lead of thread.

P=Pitch of thread.

In single start thread;

L=P

In double start thread;

L=2P

fig.1.2 

3: ANGLE OF THREAD

The angle included between the sides of the thread is known as angle of thread.

4: CREST

Crest is the top surface joining the two sides of a thread.

5: ROOT

It is the bottom the crest and the root of thread.

6: FLANK

It is the surface between the crest and the rot of thread.

7: DEPTH OF THREAD

It is defined as the distance measured perpendicular the the axis.

8: MAJOR DIAMETER

It is the largest diameter of the thread.
Major diameter = Minor diameter + 2depth of thread.

9: MINOR DIAMETER

It is the smallest diameter of the screw thread minor
Minor diameter = Major diameter - 2depth of thread.

10: PITCH DIAMETER

It is defined as the diameter of an imaginary cylinder which would pass through the threads at such points that the width of the grooves and the width of threads cut by the surface of cylinder are the same
Pitch diameter = Major diameter - Depth of thread.