Different thread gauges for different types of threads

  • Industries all over the world use different types of threads. You have the metric threads in Europe whereas the US and the Canadian industries opt for the UN threads. Similarly, the Acme threads are used in the US while a similar thread but with a slight variation, the trapezoidal threads are popular in Europe. In the event of having different types of threads, it becomes imperative to use different types of thread gauges for measuring their tolerance levels. In order to understand this concept better, we shall look at the different types of threads used in the US and other parts of the world.



    The position in the US and Canada


    In North America, and Canada in particular, there are two main thread series in use. The oldest of these uses Imperial fractional measurement and is often referred to as SAE which stands for The Society of Automotive Engineers. Back in 1916 when the association was formed, the term ‘automotive’ was derived from Greek where ‘auto’ means “self” and from the Latin word ‘motivus’ which means “of motion”. In other words, any form of self-powered vehicle. As the Society developed it came to incorporate the standards of the various other associations of self-powered vehicles, everything from tractors to aircraft.


    The SAE standard thread pitches are measured in TPI or threads per inch. For each diameter of SAE threads, there are also two pitches, commonly referred to as UNC or Unified National Coarse (commonly called coarse thread) and UNF or Unified National Fine (fine thread).


    The other, more recently adopted standard is known generally as metric but actually encompasses a number of different standards, from different countries. DIN, ISO, JIS, these are all metric standards. Metric thread pitches are expressed as the distance between threads, so a thread pitch of 1.0mm would mean it was 1.0mm between the tops of two adjacent threads. For the most part, the various metric standards agree on a dimensional standard and a common coarse thread pitch, which is the most widely used. Fine pitch metric fasteners do exist and often come in more than one fine thread pitch —some as many as 4 or 5! This can cause some confusion and one should really have a thread gauge if they are in doubt. 


    Use the right thread gauges: 


    Since the US industries use the UN threads, you need the UN thread gauges for measuring their tolerance levels. Similarly, you need metric thread gauges to determine the pitch of the metric threads. Hence, each type of gauge is important in its individual way.


    The thread taps have to be different too


    Naturally, when you have threads of varying sizes, you need different kinds of thread taps as well to carve out these threads. The metric thread tap and the trapezoidal thread tap are the most commonly used thread taps in Europe whereas the ACME thread taps have their importance in the US industries.


    We shall look into more of these aspects in some of the blogs to follow.