Calipers, Combs, And Guides Help Make Sense of Threads
by Alecia Robinson, on Thu, Nov 09, 2017 @ 12:11 PM
Scratching your head about threads? We have three tools that solve the mystery
|The Pipe and Tube Sizes board (pictured above) is at our back counter. Bring in your fittings and we can help determine the sizing.|
|The thread pitch gauge is another tool we use to help with thread size - it's even available for purchase! To get more info on sizing, download the Thread and End Connection ID Guide PDF.|
Can you tell the thread size and thread pitch on a component just by looking at it? Neither can we. That's why we have tools that eliminate the guesswork.
When you look at a thread, you'll see peaks (called crests) and valleys (called roots). The part in between the crest and the root is called the flank. The first thing you want to find out is whether the crests are all the same diameter. That would mean you have straight threads. So we get out our calipers and measure the first, fourth and last crests. If the measurements are the same, you have straight threads. If you get three different measurements, you have tapered threads.
There's another way to tell, provided that you are working with Swagelok parts. Other than standard NPT, Swagelok fittings are uniquely marked to indicate ISO tapered or ISO parallel threads.
Some people are surprised to learn that straight threads are not designed to seal on their own. They require a gasket, O-ring or some kind of metal-to-metal contact to finish the job.
Tapered threads are designed to seal as the mating threads are drawn together. In addition, some kind of sealant is necessary to prevent leaks. That's usually PTFE tape or a product such as SWAK.
Here comes the "pitch"
Next you need to know how close together the crests are. That's the pitch, usually expressed as the number of threads per inch.
Straight threads come in three standard kinds of measurements. The most common are SAE (Society of Automotive Engineers), ISO 228/1 (also known as British Standard Pipe Parallel), and metric. Tapered threads also have several standard forms of measurement: NPT (National Pipe Thread), ISO 7/1 (also known as British Standard Pipe), and metric.
The flanks can come in different angles, known as Whitworth, unified, or metric.
To discover the pitch, we use a thread comb, also known as a pitch gauge. It looks a bit like a pocketknife, but the blades have saw-tooth edges that fit into the thread roots. We simply try different blades until we find an exact match. Some fractional and metric thread forms are very similar, so we take the time to make sure it's a true fit.
You also can turn to the Swagelok Thread and End Connection Identification Guide, which you can download for free. It has all the terminology, thread ID reference tables, and step-by-step instructions for identifying threads.
Lastly, Edmonton Valve & Fitting has a board with various sizes of threads. If you come in with a part, we can see which sample it fits.
Obviously, the threads on two parts have to have the same size and type of threads if you want to connect them. But you don't have to guess. Check in with Edmonton Valve & Fitting and we'll work with you to get the right answer. Give us a call at 780.437.0640 or get in touch below.