let me teach you some practical knowledge

      Mechanical engineering is a diversified subject and due to my ability of working in a single field ......
    I am only able to share with you the knowledge i gain in my daily life
    I really dont know where to start with you guys but have to start some where.
    In this post i like to explain what are the threads i see for the bolts we use in my job site.

      I mainly see only two type of threads namely Coarse series and fine series, may be either Metric screw threads specified by ISO or Imperial Standards specified by USA.
      So for Metric standards we have Metric coarse series threads and metric fine series threads and for unified standards we have Unified coarse series threads and unified fine series threads.

    ISO refers to International organization for standardization and UTS refers to Unified thread standards.

Often when we are talking about threads we cant stop talking about bolts.

so let me explain about thread using bolts.

       If you want to identify the type of thread used on a bolt just by seeing it - For a said length of bolt number of coarse threads are less than that of Fine threads. Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, whereas fine threads have a smaller threadform relative to screw diameter.

There are several parameters which we need to learn about threads like 

• Major diameter
• Minor diameter
• Pitch diameter
• Pitch
• Thread angle

Basic thread profiles for both the standards are the same. The differences between the two standards are the Major diameter, pitch and the thread has rounded crests and roots.

Imperial standards has an extra parameter known as TPI (threads per inch). They dont use the parameter "PITCH" so much but uses TPI. 
All Imperial bolts are measured in inches and all metric bolts are measured in mm.