Understand The Type of Thread and Identify It

What is Thread?

A screw thread often abbreviated to just ‘thread’ is a helical ridge that is used to translate rotational motion into linear motion. Suppose we have a round stick and coiled a wire around it in the form of a spiral. This spiral is like the thread on a cylindrical bolt or pipe. It forms a wave-like notch which is known as a helix.

The thread spirals a ridge around a cone or cylinder. The ‘lead’ of a screw thread is a very important factor. It is the amount of advance made by the screw in one revolution. This lead is about the screw’s mechanical advantage. It determines the efficiency of the car in transforming the rotational force into linearity.

There are two forms of screw threads as below；

• Straight Threads: These threads have the same cross-sectional dimension throughout their length. They are most suitable for applications where there is a requirement of equal strength along the full length. Some of the examples include BSPP, UN/UNF and metric parallel threads.
• Tapered Threads: These threads are tapered; that is, they become thinner at one end than at the other. This, in turn, assists in developing better sealant joints. They are normally employed where the connection has to be airtight, as in the case of pipes. Some of the common types of threads are the metric tapered, the BSPT as well as the NPT/NPTF threads.

Threads are important in machines and fastenings. They are robust, can be easily unbolted, very easy to fix and are relatively inexpensive. Their design is such that they can be easily assembled and disassembled. This is important in performing maintenance and repair services of industrial equipment.

What are the Main Geometric Parameters of the Thread?

To design and analyze threads, it is necessary to know their main geometric characteristics. These parameters describe various kinds of mechanical thread sizes and their performance characteristics. It could help you to understand how to read thread dimensions. Here’s an overview of the essential elements:

1. Primary Diameter: Screw major diameter is defined as the largest diameter of the thread. It is also referred to as the outside or external diameter. It means the space between two threads on a screw or the space between the base of the threads on a nut. For instance, calipers or thread pitch gauges are used to measure this diameter.
2. Minor Diameter: The minor diameter, also known as inner diameter, is the smallest diameter of the thread. That is the space between the roots of the thread on a screw or the crests on a nut. Instruments are normally needed to measure this dimension of the business.
3. Pitch Diameter (Effective Diameter): It is the distance between the major and minor diameters. The diameter of an imaginary cylinder touches the surface of the thread at a point which is mid way between the two opposite flanks of the thread groove. This measurement is important in ascertaining how well the thread fits and the tolerance that is present and this is normally done using a thread pitch gauge.
4. Pitch: The pitch of the thread refers to the axial distance between corresponding points on adjacent threads. This measurement is parallel to the thread axis. It is essential for defining thread spacing.
5. Lead: Lead is the distance a thread advances axially in one full rotation. For a single-threaded screw, the lead is equal to the pitch. In multi-threaded screws, like double or triple-threaded screws, the lead is two or three times the pitch, respectively.
6. Thread Starts: The number of thread starts refers to the number of independent threads that spiral around the screw. A single-threaded screw has one start, while a double-threaded screw has two starts, directly affecting the screw’s lead.
7. Helix Angle: The helix angle is the angle formed between the helix of the thread and a line perpendicular to the screw’s axis. Screws with a steeper helix angle are more efficient. They have less friction.
8. Thread Angle: The thread angle is the angle between the thread flanks, measured in an axial plane. This angle influences the thread’s strength and load-bearing capacity.
9. Tooth Angle: The tooth angle describes the geometric shape of the thread. Common forms include trapezoidal, square, and triangular shapes. Each serves different mechanical purposes and applications.

What are the Different Types of Threads?

Threads are a fundamental component in various industrial and commercial applications globally. Though small, their importance in the functionality of numerous products cannot be understated.

Common Types of Threads

• Right-Hand and Left-Hand Threads: Right-hand threads tighten clockwise. They are the standard in most applications. Conversely, left-hand threads tighten counterclockwise. They are used less often, mainly where reverse movements are needed to prevent loosening.
• Taper Threads: Taper threads are carved on a tapered surface. They are vital for joining machine parts, like nuts, bolts, and studs. These threads are vital in precision instruments like micrometers. They are critical to detailed mechanical work.
• “V” Shape Threads: Shaped like the letter “V,” this thread type is the most commonly utilized. It can be manufactured using various methods, including taps, dies, lathes, or milling machines. V-shape threads are renowned for their versatility and reliability across numerous applications.

International and Standard Threads

ISO thread types

• ISO Metric Thread (M): Standardized globally, used primarily in Europe, with a 60-degree flank angle. Notable for use in various mechanical assemblies, and for its dimensions and pitch in millimeters. It is detailed under DIN 13 and DIN 14 standards.
• ISO Metric Fine Thread (MF): It is like the ISO Metric Thread but with a different pitch. It is often used in precision applications like watchmaking.
• Steel Conduit Thread (Pg): It was used in electrical engineering. Defined in DIN 40430, it has an 80-degree flank angle. It is being replaced by metric threads.
• Trapezoidal Thread (Tr): It has a symmetrical trapezoidal profile with a 30-degree flank angle. Multiple DIN standards, including DIN 103, DIN 380, and DIN 30295, govern it.

American thread types

• Unified National Coarse Thread (UNC): The American version of the ISO Metric Thread. It has inch dimensions and a 60-degree flank angle. It is mainly used in the USA.
• Unified National Fine Thread (UNF): It’s a finer version of UNC. It has the same 60-degree flank angle. It’s used for applications that need finer threading.
• Unified National Special Thread (UNS): It has a varying pitch but a standard 60-degree flank angle. This is used in specialized applications, like microphone stands.
• National Taper Pipe (NPT): A tapered pipe thread used in pipe fittings, has a 60-degree flank angle and is self-sealing at low pressures.
• National Taper Pipe Dryseal (NPTF): It’s like NPT but for high-pressure, self-sealing use, without sealants.

British Whitworth threads

• British Standard Whitworth Coarse (BSW/WW): Sir Joseph Whitworth introduced it. It uses inches and has a 55-degree flank angle. It’s commonly used in the British Kingdom.
• British Standard Fine (BSF): The fine-thread counterpart to BSW also uses inches and features a 55-degree flank angle.
• British Standard Pipe (BSP/G): A non-self-sealing, cylindrical Whitworth pipe thread. Its inch specs are odd and don’t match the actual diameters.
• British Standard Pipe Taper (BSPT/R): It has a tapered external thread and a cylindrical internal thread. It’s self-sealing. Abbreviated as R for male threads and Rp for female threads.

Specialized and Industrial Use Threads

• Square Threads: They are very efficient and have low friction. So, they are mainly used for power transmission. This includes jackscrews and leadscrews in machine tools. Their design, however, makes them less robust compared to V-threads.
• Acme Threads: Acme threads are a modified square thread. They are easier to cut and stronger. So, they are good for bench vices and brass valves. Their unique shape allows for efficient power transmission with minimal wear.
• Knuckle Threads: are used for rough work and ready for easy casting and rolling. Knuckle threads are in glass bottle necks, coupling gears, and large molded insulators in the electrical trade. Their half-round root and crest help withstand damage and dirt.
• Buttress Threads: They combine the benefits of square and V-threads. They can withstand significant loads in one direction. So, they are ideal for applications like bench vices, where unilateral pull is common.
• Worm Threads: Like Acme threads but deeper, worm threads are used in vehicle worm wheels. They provide a durable solution for angular power transmission.

What are the Tools for Identifying Threads?

Identifying threads accurately is crucial in various applications, including manufacturing, maintenance, and repair. Several tools are available to help identify thread types, sizes, and pitches. Below is a detailed overview of the most commonly used tools for thread identification:

1. Thread Pitch Gauge

A thread pitch gauge is a tool used to measure the pitch or lead of a screw thread. It consists of multiple blades, each with teeth of different pitches, allowing the user to match the thread profile quickly.

• Usage: Align the teeth of the gauge with the threads of the screw or bolt to determine the pitch.
• Types: Available in both metric and imperial units.

2. Thread ID/OD Caliper

A caliper measures threads’ internal diameter (ID) and external diameter (OD). This tool is essential for determining the size of the threaded part.

• Usage: Place the caliper jaws on the thread’s inner or outer edges and read the measurement.
• Types: Digital, dial, and vernier calipers are commonly used.

3. Thread Plug and Ring Gauges

These gauges are used to check the accuracy of internal and external threads, respectively. They ensure that the threads conform to specific standards.

• Usage: Insert the plug gauge into the internal thread or screw the ring gauge onto the external thread to check for a proper fit.
• Types: Available for thread standards, including metric, UN, and BSP.

4. Thread Measuring Wires

Thread measuring wires are used in conjunction with micrometers to accurately measure the pitch diameter of threads. They are handy for precision measurements.

• Usage: Using a micrometer, place the wires in the tread grooves and measure the distance between them.
• Types: Sets typically include wires of different diameters to match thread pitches.

5. Thread Identification Kits

These kits contain tools to identify thread types and sizes. They include thread pitch gauges, calipers, and ID cards.

• Usage: Use the tools to measure thread dimensions. Compare them to the reference charts.
• Types: Kits are available for specific thread standards, such as NPT, JIC, ORFS, BSP, and metric threads.

6. VisionGauge® Thread Inspection Tools

Advanced thread inspection tools like VisionGauge® can measure threads without contact. They provide fast, accurate results.

• Usage: Use the software to select the measurements to report. Then, draw a region of interest around the threads for automatic measurement.
• Types: Suitable for many threaded parts, not limited to screws3.

7. Screw Checkers

Screw checkers are simple tools. They help users quickly find the size and pitch of screws and bolts. Users match them to pre-drilled holes or threaded sections.

• Usage: Insert the screw or bolt into the corresponding hole or thread section to determine its size.
• Types: Available in both metric and imperial versions.

8. Seat Angle Gauge

A seat angle gauge measures the angle of a threaded part’s seating surface. This is vital for sealing fluid connections.

• Usage: Place the gauge on the seating surface and read the angle measurement.
• Types: Typically included in comprehensive thread identification kits.

How to Identify Types of Threads?

It’s vital to identify thread types in fields like engineering, manufacturing, and maintenance. Correct identification ensures compatibility in fittings and connections. It prevents leaks and mechanical failures.

Step 1: Male vs. Female Threads

Examine where the threads are located to determine whether a thread is male or female. If the threads are on the outside of a fitting, it’s a male thread. If they’re on the inside, it’s a female thread. 

As one text explains, “If they are on the exterior of the thread, it is a male thread. It is a female thread if they are located inside the thread.” This distinction is crucial. It helps identify the right counterparts for connection.

Step 2: Classify Thread Shape

Threads can be classified into tapered or parallel types based on physical characteristics. Tapered threads narrow as they extend outward. You can usually spot them by eye or with a caliper. One text notes, “Tapered threads get smaller in diameter at the end of the fitting. “Parallel threads have the same diameter throughout.”

These differences affect how each type seals. Tapered threads seal through metal-to-metal wedging or minor thread distortion. Parallel threads often need an o-ring or thread tape to ensure a secure seal.”

Step 3: Measure Thread Pitch

The pitch of a thread refers to the number of threads per inch or the distance between threads. You can use a pitch gauge, a tool designed to measure this. It’s important to try several gauges before choosing. Slight pitch differences can cause misidentification.

Step 4: Determine Thread Diameter and Size

You can use a caliper to measure the thread size,. For male threads, measure the outside diameter. For female threads, measure the inside diameter. One guide suggests, “If you have a pipe thread, compare its size to a nominal size profile.” Knowing whether a thread is a pipe or a non-pipe thread is vital. It affects the measurement and its uses.

Step 5: Designate Thread Type and Standard

After taking all the measurements, the last step is to find the thread type standard. This involves comparing your findings against standardized-size tables. “Determining your thread type standard is the final step in identifying your type of thread,” as stated in one text. This helps ensure that the threads will function correctly in their designated applications.

What are Practical Tips for Choosing the Right Types of Threads?

Choosing the right screw thread is crucial. It ensures optimal performance and strength in various uses. The selection process involves knowing the threads’ specific traits. This includes the material, design, and the application’s needs. As tasks grow more complex and varied, informed thread selection is key.

Compatibility and Customization 

It is essential to recognize that not all screw threads are compatible with all uses. Each thread type is designed for a distinct function. It is made of materials suited for that purpose. Some threads are customized for durability. Others are optimized for easy installation in soft materials. This customization is vital. It meets the diverse needs of various industries.

Design Requirements and Variations 

When selecting a screw thread, one must consider the design requirements. Some threads have sharp designs that can be drilled directly into wood. They are suitable for quick, direct applications without pre-drilled holes.

Conversely, some screw threads may need pre-drilled holes. This is to ensure proper installation and function. It is important in complex materials where direct drilling could be ineffective or damaging.

Critical Role of Thread Spacing 

Thread spacing is another significant factor to consider. Different screw threads have varying spacings. This can affect their performance and suitability for specific tasks. For example, the spacing must be matched precisely with bolts or nuts to ensure a secure fit and proper functionality. Incorrect spacing can lead to loose fittings, compromising the assembly’s structural integrity.

Conclusion

We have slimmed down the various types of threads and their standard ones. We believe you can choose the right screws for your project after reading this. If you still have any questions about screws, welcome to inform us.