Zakariya Harris
Machine screw sizes are standardised, ranging from No. 0 to No. 14, with the former measuring .0600 in outside diameter and the latter, .14434. The pitch, or distance between the crests of each thread, is what distinguishes NC (National Coarse) from NF (National Fine) threads. Coarse threads have a larger pitch, resulting in fewer threads per inch and a bigger distance between them compared to fine threads. While NC and UNC (Unified National Coarse) are the same, NF and UNF (Unified National Fine) threads are interchangeable.
| Characteristics | Values |
|---|---|
| Thread type | NC (National Coarse), UNC (Unified National Coarse), NF (National Fine), UNF (Unified National Fine) |
| Pitch | The distance between the crests of each thread |
| Strength | NC threads are stronger in softer materials like aluminium; NF threads are stronger in steel |
| Use case | NC threads are used where jar and vibration are not important factors, and where parts need to be taken apart frequently; NF threads are used where jar and vibration are important factors, and where a fine adjustment is required |
| Tensile strength | NC threads are used where an average tensile strength is required; NF threads are used where the tensile strength required is above average |
| Assembly | NC threads are used where it is necessary to take parts apart frequently and under unfavourable conditions; NF threads are used where assembly is made without the use of wrenches |
| Hole material | NC threads are used where the holes in which cap-screws are assembled are tapped in metals other than steel; NF threads are used where the holes in which cap-screws are assembled are tapped in steel |
| Tap drill sizes | Listed in the order in which the threads per inch (tpi) are listed in the previous column |
What You'll Learn
Machine screw tap sets
When choosing between NC and NF, the rule of thumb is to use NC in soft materials and NF in higher-strength materials. For example, in the automotive world, aluminium would get NC and heat-treated steel would get NF.
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Unified National Coarse (UNC) threads
UNC threads have a 60-degree profile, similar to the ISO metric screw thread. However, the characteristic dimensions of each UTS thread, including the outer diameter and pitch, are chosen as inch fractions rather than millimetre values. This is a key difference between the UTS and ISO metric systems.
The major diameter (Dmaj) of a UNC thread is measured from the outer edge of the threads, while the minor diameter (Dmin) or root diameter is measured from the inner edge. The pitch (P) refers to the distance between thread peaks. UNC threads are typically designated by their major diameter and pitch in threads per inch (TPI).
UNC threads are the most common general fastener thread. They have a deeper and more generic fit compared to fine threads, making them easier to remove. UNC threads also generally have higher tolerance for manufacturing and plating and do not require cross-threading for assembly.
Unified threads, including UNC, come in three different classes:
- Class 1 (1A external, 1B internal): Used for applications requiring liberal tolerance, such as easy assembly or use in dirty environments.
- Class 2 (2A external, 2B internal): The most common class, used for general applications. It maximises strength while considering typical machine shop capabilities and practices.
- Class 3 (3A external, 3B internal): Employed for applications requiring closer tolerances and/or accuracy of thread elements.
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Unified National Fine (UNF) threads
UNF threads have a flank angle of 60 degrees and are parallel, with female and male thread forms. They are available in three different classes: 1A and 1B, which offer extremely loose tolerance thread fit for quick assembly and disassembly; 2A and 2B, which offer an optimum thread fit that balances performance, manufacturing, economy, and convenience, and are the most common class; and 3A and 3B, which are meant for close tolerance fasteners where accuracy and safety are critical.
UNF threads require less torque to develop equivalent bolt preloads compared to UNC threads. They also have less tendency to loosen due to their smaller thread incline. When choosing masking caps and plugs, UNF threads require more turns to tighten the plug into the hole compared to UNC threads.
Overall, UNF threads offer a range of advantages and are well-suited for applications requiring precision, strength, and accuracy.
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Tensile strength
The tensile strength of a machine screw is influenced by various factors, including the thread type, material, and length of engagement. National Coarse (NC) and National Fine (NF) threads offer different tensile strengths due to their distinct thread profiles.
NC threads, also known as Unified National Coarse (UNC) threads, have a larger pitch, resulting in fewer threads per inch or a bigger distance between threads. While NC threads provide adequate tensile strength for certain applications, they are typically used in situations where average tensile strength is required. They are well-suited for bolts and screws used in applications where jar and vibration are not critical factors and where frequent disassembly is necessary.
On the other hand, NF threads, also known as Unified National Fine (UNF) threads, have a finer pitch, resulting in a higher thread count per inch. The increased number of threads engaged results in a larger cross-sectional area, contributing to higher tensile strength. NF threads are commonly used in applications that demand above-average tensile strength and fine adjustments, such as with finished bolts and screws.
When choosing between NC and NF threads, it is essential to consider the specific requirements of the application. While NF threads generally offer higher tensile strength, the choice depends on factors such as the material being joined, the length of engagement, and the need for fine adjustments. Additionally, the ultimate tensile strength of the screw, which is the maximum stress it can withstand before breaking, should be considered in conjunction with the tensile strength requirements of the application.
In summary, the tensile strength of machine screws with NC and NF threads varies due to their pitch and thread count differences. NF threads generally provide higher tensile strength due to their finer pitch and increased thread engagement, making them suitable for applications requiring above-average tensile strength. However, NC threads are adequate for applications where average tensile strength is sufficient, and they offer advantages in specific scenarios, such as frequent disassembly.
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Threads per inch (TPI)
Threads per inch, commonly abbreviated as TPI, is a term used to identify the number of threads in an inch. To determine the TPI, an inch of the bolt is measured and the peaks on the fastener are counted. For example, a bolt with a measurement of 1/4"-20 has 20 threads per inch.
TPI is used to measure the threading of a bolt or nut to ensure that they will couple together properly. If the threading of a bolt and nut are different, they will either seize or strip the threading, resulting in an unusable connection.
The term "thread pitch" is used in place of TPI for metric fasteners, and the distance is measured in millimetres. For US fasteners, a higher TPI number indicates a fine thread, while a lower number indicates a coarse thread. For example, a fastener with measurements of 1/4"-20 would be coarse, while one with measurements of 1/4"-28 would be fine.
The number of TPI can also be used to determine the class of thread, which refers to the tolerances that control the closeness of fit between two threaded mating parts. There are three established classes of thread: Class 1, Class 2, and Class 3. Class 1 has the widest tolerances, while Class 3 has the closest tolerances.
In addition to TPI, the pitch of a thread can also be expressed as an angular measurement, such as degrees or minutes. However, this method is less common and may not be practical for determining TPI.
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Frequently asked questions
NC stands for National Coarse, while NF stands for National Fine. The difference between the two is the pitch, which is the distance between the crests of each thread. Coarse threads have a larger pitch, meaning there are fewer threads per inch and a bigger distance between them.
Coarse threads are suited to bolts and screws where jar and vibration are not important factors, where an average tensile strength is required, where it is necessary to take parts apart frequently and under unfavourable conditions, and where the holes in which the screws are assembled are tapped in metals other than steel.
Fine threads are suited to finished bolts and screws where jar and vibration are important factors, where the tensile strength required is above average, where a fine adjustment is required (especially when castle nuts are used), and where the holes in which cap-screws are assembled are tapped in steel.
NF threads are more likely to strip than their NC counterparts.
