Threading is an important part of understanding fasteners. There are many types of screw threads. When tapping bolts or threading screws, it’s essential to know the proper thread size.
There are two main kinds of threads in the Unified Screw Thread (UST) Standard—UNF and UNC. UNF is the designation for Unified Fine Threads. Meanwhile, UNC denotes Unified Coarse Threads.
UNC threads are a type of coarse thread that has an unbroken series of right-hand turns with uniform geometry. Certain threads cover a specific range. It’s vital to understand that range so that you can use the correct sized tap and die when needed.
To learn more about UNC threads, keep reading.
An Overview of The Unified Screw Thread Series
The Unified Thread Standard defines standard thread form and series. It also defines allowances, tolerances and designations.
This standard helps to make sure that screws, bolts and other threaded fasteners are compatible. They’ll work with each other regardless of manufacturer or country.
UNC and UNF are both symbols in the Unified Screw Thread Standard. They’re also both defined based on the number of threads.
Operators use both types of screws for fastening. However, they’re usually each applied in different fields.
Coarse threads have higher strength. Meanwhile, fine threads are better at sealing.
The UTS is the main standard for bolts, nuts and many other threaded fasteners. It’s managed by two organisations. Those organisations are:
• The American Society of Mechanical Engineers
• The American National Standards Institute
All fasteners have the same 60° profile, whether Metric or Imperial. With Imperial fasteners, however, the UTS thread is in inch fractions instead of millimetre values.
Nevertheless, the basic profile of UTS threads is the same. The only differences are the values used for measuring the major diameter and the pitch of the threads.
UNF vs UNC Threads
The load capacities of UNF and UNC threads are close. Sometimes, however, operators use threads as fasteners. In these instances, UNF thread fasteners are better at preventing loosening compared to coarse (UNC) threads.
In other instances, operators use fine threads for adjustments. In this case, fine threads also have the advantage. However, these aren’t the only differences between UNF and UNC threads.
UNF threads have a tighter helix angle. This characteristic is what gives them improved tightness. They’re also more conducive to self-locking.
Also, UNF threads have a smaller pitch. In other words, they have more teeth screwed in the same thread length.
This characteristic can reduce fluid leakage. Operators use screws with UNF threads to seal holes for this reason.
Meanwhile, UNC threads have fewer teeth of the same length. They have a larger cross-section size of each tooth. They’re more suitable for bearing larger force and impacts for this reason.
Overall, UNC threads have higher strength. They’re also highly interchangeable.
However, UNC threads also have greater tolerance. They can handle manufacturing and plating better compared to their UNF counterparts.
As you can see, UNF and UNC threads have varying characteristics. Accordingly, they have different uses.
The Unified Screw Thread Series Designations
UTS uses a standardised naming system. For UTS threads, a number indicates the major diameter of the thread. Another number will indicate the pitch measure and threads per inch.
In some cases, the diameter of the thread is smaller than a quarter inch. If so, the manufacturer indicates the diameter using an integer.
For all other diameters, the manufacturer will use an inch figure. In some cases, the manufacturer will also follow the number by a designation. For example, the figure might include:
• UNC (unified coarse)
• UNF (unified fine)
• UNEF (unified extra fine)
The manufacturer might also include a tolerance class in the thread figure.
For instance, imagine that a manufacturer classifies a bolt as #6-32 UNC 2B. In that case, the bolt has a major diameter of 0.1380 inches.
It also has a pitch of 32 threads per inch. Meanwhile, it has a tolerance class of 2—more on tolerances in a moment.
In some cases, you may need to repair an older piece of equipment. If so, it helps to understand that operators have used non-standard 14 thread per inch nuts and bolts for years. Now, however, the standard is 12 threads per inch.
As a result, it’s much easier to find fasteners using this older standard. 12 thread per inch fasteners are also more expensive for this reason.
Screw Thread Gauges
There are several characteristics that you must assess to find the right fastener characteristics. You can assess those characteristics using a screw thread gauging system.
A screw thread gauging system will help you to establish the dimensional acceptability of screw threads. Here, it’s important to use the right thread gauge when examining threads.
ASME/ANSI B1.2-1983 is a standard for gauges and gaging unified inch screw threads. It provides essential specifications and dimension for the gauges used for unified Imperial screws. This standard applies to UN and UNR external threats.
Meanwhile, ASME/ANSI B1.3-2007 applies to both Imperial and metric screws. This standard applies to UN, UNR, UNK, M and MJ threads. It’s suitable for gauging all UN, UNJ, M and MJ threads and external UNR threads.
ASME/ANSI standards establish the criteria for screw thread acceptance when using a gauging system. It encompasses the screw thread characteristics that you must inspect. You must use the proper gauge to inspect those characteristics.
In other words, you can’t use a UNJ thread gauge to inspect UN threads. Thread gauges are not interchangeable. They have different properties, and the features that you must measure differ as well.
Inspection of screw threads is an art as well as a science. For some applications, you must also possess knowledge of how to read data about tolerances.
Screw Thread Tolerance
Tolerance is part of a classification system to support the interchangeability fasteners. Most threaded fasteners comply with UST standards. This system is equivalent to the fits required for assembled parts.
For instance, Class 1 threads fit loosely. They’re intended for ease of assembly or use in a dirty environment.
Class 2 threads fit freely. They’re the most common type of thread. Manufacturers produce Class 2 threads to maximise strength.
Class 3 threads have a medium fit. They’re also a common screw thread. However, they have a closer tolerance and are used for high-quality work.
Class 4 threads were designated as close-fitting for even tighter tolerances. However, this classification is now obsolete.
Finally, Class 5 is considered an interference thread. It requires a wrench to turn. You might see this kind of thread used for spring shackles on a vehicle.
Finding The Right Tap and Die Size
When you cut a thread into a hole, it’s called tapping. Conversely, you’d use a die to cut threads onto a cylinder or bolt. Before you can use a tap or die, however, you must determine the threads per inch for the part needing repair.
With a thread gauge, you can measure the size of a nut or bolt. With the proper measurement, you can choose the corresponding tap and die needed for the repair.
With a die, you’d use the tapered side to begin the repair. The die fits into a special wrench that holds the die and guides your work.
You can also calculate threads per inch manually.
For example, you may need to measure metric threads. Here, you’ll need to subtract the pitch from the diameter of the thread. Thread charts may guide you in your assessment.
For instance, imagine that you have a bolt marked with M5x0.75. In that case, you’d subtract 0.75mm from 5, coming up with 4.25mm. You’ll need a 4.25mm die for the repair.
You can calculate the tap or die size for an Imperial fastener in the same way. For example, imagine that you have a 3/4.10 UNC bolt with a pitch of 0.1 and a diameter of 0.75.
You’d subtract the 0.1 pitch from 0.75 diameters. As a result, you’ll need a 0.65, or 16.5mm, die for the repair.
Again, you may need to refer to thread charts to make the right assessment. A size chart makes the process a little easier by clearly highlighting fastener characteristics.
However, it’s easier—and much more accurate—to use a professional thread gauge to assess thread size.
Expert Tools for Precision Thread Work
Now you know more about UNC threads. If you need professional thread working tools, we’ve got you covered.
Without the right tools, threading can prove difficult and time-consuming. What’s worse, you could end up paying more to replace fasteners when you could just repair them. Wiseman Threading Tools makes working with threads easier, faster and more accurate.
We offer a wide range of threading tools. You can save money and increase productivity with our gauges, dies, taps and chasers.
We can even customise threading tools. We offer custom engineering services and can design a solution that works best for your company’s unique needs.