The concept of UNF and UNEF thread gages

By: Frank Xu


Screws, nuts, bolts, and pipes are very common tools in any manufacturing industry. Each of these tools has threads for fixing them properly. Imagine what happens if you do not use the right combination. The screws or the nuts fall off thereby creating lots of problems. In case of heavy machinery, there are chances of accidents taking place. Hence, there is a need to check the tolerance levels of these threaded tools. You have an instrument known as the gauge that is able to check these tolerance levels and determine the pitch of the threads. There can be coarse threads or fine threads depending on their usage. This brings the concept of UNEF thread gages and UNF thread gages.

Different types of threads

There are many types if thread series depending on their usage. They are the Metric, Unified, National Taper Pipe, Aeronautical taper pipe, and the British Standard Whitworth.

In the industry level, the usage is mostly restricted to the first two types alone. Countries like the US and Canada use the Unified Thread System whereas the other countries all over the world use the Metric System. The US and Canadian industries are also gradually making the switch over to the metric system over a period. The principal difference between the two systems is that the Unified Thread system uses the ‘inch’ as a unit of measurement and the metric system uses the ‘centimeter’ or the ‘millimeter’ as its unit of measurement.

We shall now concentrate on the Unified System that is further classified into UNC (coarse), UNF (fine), UNEF (extra fine), UNS (special) and UN (constant pitch). The major difference is in the presence of threads per inch (tpi). It is usually fixed for each category.

The 5/16 UNC has 18 tpi whereas 5/16 UNF has 24 tpi, and 5/16 UNEF has 32 tpi.

Difference between coarse and fine threads

Let us see the difference between the two types of gauges, coarse and fine threads.

The Coarse threads: The advantages of coarse threads are as follows.

1. Easy to assemble with minimum chance of cross threading
2. Minor damage to threads when you handle of ship them
3. Does not easily strip when you fasten into lower strength materials
4. Possible to have thicker plating as there is less chance of corrosion
5. Exhibit better fatigue resistance because of less concentration of stress at thread root radius

The fine threads: These are the advantages of using finer threads.

1. They have a larger tensile area and thus exhibit higher strengths in tension
2. Shorter thread depth allows for threading in thin wall applications
3. The smaller helix angle and finer threads enable precise adjustment
4. When there is restriction of length, the finer threads provide greater strength.
5. Easier to tap into harder materials

There is not much difference between extra fine and fine threads. You use the extra fine threads in sophisticated instruments and tools.

You have two different gages to determine the pitch of the fine and the extra fine threads. They are the UNF thread gages and UNEF thread gages respectively.

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Frank Xu is the author of this article on ACME thread tap.Find more information, about Trapezoidal thread tap here

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