Errors in Threads

     Errors in screw threads are related to the five elements of the screw threads. They are major diameter, minor diameter, pitch diameter, pitch and thread angle. If any errors are taking place in these five elements the produced screw is rejected. So, these elements are also be checked with proper gauging system carefully. The threads are produced by a point cutting tools.
     The errors in major and minor diameter cause interference of the mating threads, less root section, less wall thickness and poor contact of the flanks, which ultimately cause the weak in strength of the component. The errors in effective diameter also cause the interference of the flanks.
     The errors in pitch and thread angle also cause the progressive tightening of the mating parts due to the interference of the flank surfaces.

     Let us discuss some important errors in thread forms. They are

1. Drunken error
2. Pitch errors 

    Drunken Error:  It is error due to the irregular form of helical groove on a cylindrical surface. In this case pitch measured parallel to the axis is always same, but problem is with the thread is not cut to its true helix.
Due to this flank surface will not be as a straight edge, it will be as curved form.

     Pitch errors: 
    The pitch errors are due to improper ratios of cutting tool velocity to rotating velocity of the workpiece. these pitch errors are again classified as

Progressive pitch erros
Periodic pitch errors
Irregular errors

     Progressive errors: In this the pitch error results increasing of major or minor diameter or decreasing of major or minor diameter. It means the error may either in increasing order or decreasing order.
     Periodic errors: In this the pitch error causes the errors to repeat at certain time of interval.
     Irregular errors: These are the errors randomly take place on threads without any specific reason. These are the combination of all the errors take place on threads.