Inch and Metric Thread Standards

Inch and metric threads are widely used in the field of nozzle manufacturing. We often make different nozzle threads to match the spray system from different countries.
Sometimes we may make 1/8" British-inch thread with 55° thread angle, and sometimes we also do 1/8" US-inch thread, because the two types are different.

1. Difference of Thread Sizes Explained

Inch and metric thread sizes are specified differently. Metric threads are indicated by the pitch in mm, and the US/British-inch threads are specified by the nominal major diameter and number of threads per inch.
   For example, a 1/4-20 (UNC coarse) thread has a 1/4" nominal major diameter and 20 threads per inch, while a 1/4-28 (UNF fine) thread has the same nominal major diameter but 28 threads per inch.

In the inch threads, the thread pitch measurement is calculated by dividing a inch (25.4mm) by the number of threads per inch (TPI).
   For example, RELAB BB series Full Cone Nozzles (Model: BB1/4-SS10), the nozzle thread is 1/4 BSPT, so thread pitch=25.4mm÷19 threads per inch=1.3368mm

In the Metric threads, the thread callouts are fundamentally different, because the thread pitch is defined by the distance between each thread in mm instead of threads per inch.
   For example, RELAB FD series Mist Nozzles (Model: FDM5-SS304+CE2), the nozzle has an M5x0.8 (coarse) thread with a 0.8mm pitch, while an M8x0.75 (fine) thread has a 0.75mm pitch.

Therefore, when our engineers use the CNC Machines of Metric version to lathe the thread of our nozzles, we only need to input the Metric thread pitch into the system, and don't need to do any conversion.

Both DIN and ISO standards for metric threads ignore the pitch callout on coarse threads. 
   For example, a thread diameter defined as "M12" indicates a coarse pitch (M12-1.75).

2. Difference of Thread Angles

Metric thread has common thread (thread angle 60°); Trapezoidal thread (thread angle 30°); Buttless thread (thread Angle 33°); Square teeth thread and so on. In RELAB nozzles, we only use the common Metric thread with 60° tooth type.

Inch threads are classified by thread shape divided: parallel and taper thread; classified by thread angle divided: 55° and 60° thread. The British thread is isosceles 55° tooth type (BSPT, BSPP), and US-made thread for the waist 60° tooth angle (NPT, NPS).

People usually used to call thread sizes, one inch equals eight points, and 1/4 inch is 2 points, 3/8 is 3 points, and so on.

In addition, there are:  

• ISO-metric thread standard 60 degrees;
• UN-uniform thread standard 60 degrees;
• API-American oil pipe thread standard 60 degrees;
• W-British Wyeth thread standard 55 degrees.