Different Types of Threads

Different Types of Threads Basics and Practical Tips

Nuts and bolts are utilized everywhere in the manufacturing sector or in our daily life. Without threads, most machines would become incomplete and can’t serve us. Then, what are threads? The threads we discuss here indicate spiral grooves with equal pitch and depth formed on a parallel round cylindrical or conical round surface. 

The use of threads includes pneumatic components such as air cylinders, valves, and air preparation units. And multiple thread types are available for these pneumatic components. Selecting the proper thread type will assure optimal equipment compatibility and prevent malfunction. This article covers detailed information on threads to help you understand their types and correctly identify the most suitable threads for your project.

What is Thread?

A screw thread, often shortened to thread, is a profiled notch that wraps around a cylindrical bolt or pipes wave-like and continuously. Take a round stick as an example. We need to wind a wire in an angular direction around it, and in this case, the wire’s twist would advance along the round stick. The path that the wire takes is known as the helix. 

Screw threads are indispensable features of screw machines and thread fasteners. Here, a screw thread is a helical ridge on a cone or cylinder fastener, with the former referred to as a tapered thread and the latter as a straight thread (also called a parallel thread). Screw threads are utilized as fasteners for various reasons, including their strength, removability, simplicity of installation, and cost-effectiveness. 

Straight threads and tapered threads are the two most common types of thread. Straight threads have a parallel profile which keeps the same diameter throughout the whole part. However, tapered threads taper along with the thread profile, and their diameter gets thinner towards the end of the part.

In general, parallel threads include BSPP, UN/UNF, and metric parallel. And for tapered threads, covers metric tapered, BSPT, and NPT/NPTF.

geometric parameters of thread

What are the Key Geometric Parameters of the Thread?

To design and produce the thread, we must have some knowledge of the thread, particularly its geometric parameters. The following key parameters exist to obtain a whole picture of the thread type.

1. Major Diameter 

The thread’s major diameter is its largest diameter. The outside or external thread diameter refers to the diameter of a screw and encompasses a raised helix height around the thread. The screw shaft’s major diameter is the distance between two opposite crests. When it comes to the nut, its major diameter is the distance between two opposite roots.

2. Minor Diameter

The inner diameter is another name for the minor thread diameter. The thread’s minor diameter is its smallest diameter. The screw shaft’s minor diameter represents the distance between two opposite roots. However, the nut’s minor diameter is the distance between two opposite crests.

3. Pitch Diameter (Effective Diameter)

The pitch diameter is also known as the effective diameter. It is a diameter of an imaginary co-axial cylinder that can intercept the thread’s surface, which resides concentrically and roughly halfway between the major and minor diameters. The pitch diameter can cross, causing the intercept on a cylinder generator at the point where the pitch diameter meets the opposing flanks of the thread groove. The thread groove equals half of the screw thread’s nominal pitch. A thread pitch gauge tool can be employed to check and identify the diameter and size of the thread.

4. Pitch

The pitch of the thread, or thread pitch, is the distance between a point on a screw thread and its corresponding threshold. The pitch of the thread will be on the next thread that can be measured parallel to the axis. It should be in the same axial plane as corresponding points on an adjacent surface.

5. Lead

Lead means the distance a screw thread advances axially in one turn. On a single-threaded screw, the lead and pitch are the same, while on a double-threaded screw, the lead is twice the pitch. As for a triple-headed screw, the lead is three times the pitch.

6. Thread Starts

The number of thread starts indicates the number of independent threads that run around the thread’s length. The lead of a screw is calculated by multiplying the pitch by the number of independent threads. The lead of a single-start lead screw is equal to its pitch. A single-threaded screw has only one thread start, while a double-threaded screw owns two thread start.

7. Helix Angle

The helix angle is the angle created between the thread’s helix and a line perpendicular to the axis of rotation. A lead screw with a greater helix angle has fewer frictional losses and, thus, is more efficient.

8. Thread Angle

The thread angle is defined as the angle formed between two adjacent threads.

9. Tooth Angle

Tooth angle terminology involves a thread’s form and geometry. Threads come in a variety of forms and designs. The tooth angle can be trapezoidal, square, or triangle-shaped.

What are the Tools for Identifying Threads? 

Generally speaking, two typical tools for identifying threads include a caliper and a pitch gauge. Let us take a closer look at them.

Caliper

A caliper helps measure a male thread’s outside diameter and a female thread’s inside diameter. Using a caliper will provide the most precise and accurate measurements. In addition, it saves time and streamlines that process.

caliper used in thread

Pitch Gauge

A pitch gauge is useful for measuring the distance between thread crests. We measure the number of threads per inch for NPT, UN/UNF, and BSPP. And for metric threads, the pitch gauge determines the distance between each crest in millimeters.

pitch gauge used in thread

How to Identify Types of Threads?

If you want to know how to identify the types of threads, you can refer to the following steps.

Step 1 – Male Thread vs. Female Thread

First, you must determine whether the thread is male or female. Examine the strands to see where they are set. If they are on the exterior of the thread, it is a male thread. If they are located inside the thread, it is a female thread. The gender of the thread does not inherently affect its function. It merely helps to differentiate between the two connections.

Step 2 – Tapered Thread vs. Parallel Thread

Next, you should determine whether the thread is parallel or tapered. Tapered threads cover NPT/NPTF and BSPT, while parallel threads have UN/UNF and BSPP. Metric tapered and metric parallel are self-explanatory. Sometimes, this step can be completed solely through an eye examination. Tapered threads become smaller in diameter as they approach the end of the fitting, whereas parallel threads maintain the same diameter throughout. If this isn’t apparent from observing the fitting, you can compare them with a pair of calipers. Parallel threads are those that contact the full length of the calipers; tapered threads are those that rock.

Furthermore, the tapered threads seal through metal-to-metal wedging or minor thread distortion. Parallel threads frequently require an o-ring or thread tape to guarantee a secure seal.

Tapered Thread vs. Parallel Thread Comparision

Step 3 – Determine the Pitch of the Thread

Thread pitch size refers to the distance between threads. This can be determined by comparing it to a pitch gauge or by precisely measuring and figuring out the number of threads within a particular distance. Because some thread pitches are relatively similar, try several gauges before choosing which suits you best. 

Step 4 – Measure Thread Size 

When it comes to deciding on thread size, you have two methods. Before you begin, determine whether your thread is a pipe thread (NPT/NPTF, BSPT, BSPP) or not (UN/UNF, metric parallel, metric tapered). And remember that a tapered thread can be either a pipe thread or not.

If you have a pipe thread (NPT/NPTF, BSPT, BSPP), compare its size to a nominal size profile. For non-pipe thread (UN/UNF, metric parallel, and metric tapered), use a caliper to determine the outside diameter of the thread.

Measure Thread size

Step 5 – Designate thread type standard 

Determining your thread type standard is the final step in identifying your type of thread. Compare your measurements to the size tables provided below.

NPT/NPTF Thread Type – National Pipe Tapered (Fuel)

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)2710.30.419.40.37
-04 (1/4)1813.70.5412.40.49
-06 (3/8)1817.30.6815.70.62
-08 (1/2)1421.30.8419.30.76
-10 (5/8)1422.90.9021.10.83
-12 (3/4)1426.91.0624.90.98
-16 (1)11½33.31.3131.51.24
-20 (1 ¼)11½42.21.6640.11.58
-24 (1 ½)11½48.31.9046.21.82
-32 (2)11½60.42.3857.92.29

*O.D. = Outside Diameter I.D. = Inside Diameter

G/BSP Thread Type – British Standard Pipe

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)289.70.388.90.35
-04 (1/4)1913.20.5211.90.47
-06 (3/8)1916.50.6515.20.60
-08 (1/2)1420.80.8219.10.75
-10 (5/8)1422.40.8820.30.80
-12 (3/4)1426.41.0424.60.97
-16 (1)1133.01.3031.01.22
-20 (1 ¼)1141.91.6539.61.56
-24 (1 ½)1147.81.8845.51.79
-32 (2)1159.72.3557.42.26

*O.D. = Outside Diameter I.D. = Inside Diameter

PT – JIS Tapered Pipe Thread Type

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)289.40.378.10.32
-04 (1/4)1913.70.5312.40.49
-06 (3/8)1917.20.68160.62
-08 (1/2)1421.50.8419.80.77
-10 (5/8)1423.10.9120.60.81
-12 (3/4)1426.91.0625.41
-16 (1)11341.3431.81.25
-20 (1 ¼)1142.61.6840.41.59
-24 (1 ½)1148.51.946.21.81
-32 (2)1160.42.3758.22.29

*O.D. = Outside Diameter I.D. = Inside Diameter

SAE Thread Type – Straight Thread O-Ring Boss

Dash Size

(Nominal Size)

Thread PitchMale Thread O.D. mmMale Thread O.D. inchesFemale Thread I.D. mmFemale Thread I.D. inches
-02 (1/8)243.90.316.90.27
-03 (3/16)249.60.388.60.34
-04(1/4)2011.20.449.90.39
-05(5/16)2012.70.511.40.45
-06(3/8)1814.20.5612.90.51
-08(1/2)16190.75170.67
-10(5/8)1422.30.8820.30.8
-12(3/4)1226.91.0624.90.98
-14(7/8)12301.1827.71.09
-16(1)1233.31.31311.22
-20(1 ¼)1241.41.6339.11.54
-24(1 ½)1247.71.8845.51.79
-32(2)1263.52.561.22.41

*O.D. = Outside Diameter I.D. = Inside Diameter

Metric Tapered/Parallel Thread Type

SI Metric Port Size mmThread Pitch mmMale Thread O.D. mmMale Thread O.D. inches
M5 × 0,8.850.1968
M8 × 1,0180.3150
M10 × 1,01100.3937
M12 × 1,51.5120.4724
M14 × 1,51.5140.5512
M16 × 1,51.5160.6299
M18 × 1,51.5180.7087
M22 × 1,51.5220.8661
M27 × 2,02271.063
M33 × 2,02331.299
M42 × 2,02421.654
M50 × 2,02501.969
M60 × 2,02602.362

*O.D. = Outside Diameter

What are the Different Types of Threads?

Threads are utilized in a variety of industrial and commercial applications worldwide. Although tiny, they are essential to many different kinds of products. There are numerous thread types to choose from. The following are some of the most important threads:

1. Right-Hand Threads

These threads are leaning to the right-hand side and tightened in a clockwise direction. When viewed from a position on the axis through the center of the helix, it moves away from the observer when turned in a clockwise movement and towards the observer when turned in a counterclockwise movement. This is called a right-hand (RH) thread because it adheres to the right-hand grip rules. 

Right hand threads and left hand threads

2. Left-Hand Threads

These threads are inclined towards the left-hand side and tightened in an anticlockwise direction. In one instance, on a screw with left-hand threads, the threads are cut such that the bolt, screw, or nut must be rotated counterclockwise to engage or secure it. However, the usage of left-hand threads is relatively low.

3. Taper Threads

Taper threads are a form of thread that is carved on a tapered surface. These threads can be either right- or left-handed and often used for shafts like buffing spindles. 

The taper threads serve the following function:

  • For precise instruments such as micrometers.
  • To join two machine parts, such as a nut-bolt and stud.
  • To obtain mechanical components for lifting weight, such as chain pulleys, jacks, etc.
  • To control or minimize the speed.
  • In power transfer tasks.

4. “VShape Threads

This thread form is shaped like the letter “V” in the English alphabet. V-shape threads are the most commonly used threads that can be cut or manufactured using a tap, die, lathe machine, or milling machine, among other methods. They can be made up of various standards.

V shape threads

5. Metric or International Threads

Metric or international threads are the most prevalent form of general-purpose screw thread. It was one of the first international standards. 

In these threads, the screw is flat, and the root is round. Its angle is at 60°. Generally speaking, the number written after “M” represents the pitch. Take a -M20 X 2.5 as an example. It indicates that the number of threads is 20 per inch, and the pitch is 2.5 mm.

Metric or International Threads

6. British Standard Threads

1) British Standard Whitworth Threads

British Standard Whitworth (BSW) thread originated in Great Britain. In 1841, Sir Joseph Whitworth, a citizen of Great Britain, devised and specified this type of thread. The purpose of this thread was to facilitate interchangeability. It is commonly utilized in ordinary nut bolts and other tasks. It has an angle of 55°. Its crest and root are both round. It is referred to as BSW Thread.

British Standard Whitworth Threads

2) British Standard Fine Threads

British Standard Fine (BSF) thread resembles BSW thread in shape. This thread’s angle is also 55°, but the number of threads per inch is more, implying that the thickness of threads is less. This will result in a more strong grip. The nut bolts with this form of thread are usually applied to areas where there is more vibration.

British Standard Fine Thread

3) British Association Threads

British Association (BA) thread is found in small and delicate electrical and mechanical devices such as radios, televisions, electrical products, meters, and watches. These threads are angled at 47 1/2° and suitable for 1/2′′ or 6 mm nut bolts.

British Association Threads

4) British Standard Pipe Threads

British standard pipe thread has a 55° angle and is cut at 3/4 taper per foot. It is commonly found in sanitary pipe fittings, steam conduits, and gas pipelines. 

7. Seller Threads

The seller thread is referred to as the American national thread. The American Standards Institute has acknowledged it. It is a thread as a “V”. Its angle is 60°. It has flat roots and crests. The same standard institute invented the national fine thread. This thin thread is utilized in automobiles, aircraft, etc.

Seller Threads Type

8. Square Thread

A square thread is a prevalent screw thread that gets its name due to its square cross-section. Because of their high efficiency, square threads are extensively used for power transmission in either direction. Such threads are commonly found on the feed mechanisms of machine tools, valves, spindles, screw jacks, and so on.

Square threads are not as strong as V-threads but provide less frictional resistance to movement than Whitworth threads. The square thread pitch is frequently twice that of a BSW thread of the same diameter.

Square Threads type

9. Acme Threads

Acme threads are a transformation of the square threads, but their shape is tapered, which means they are flat at the bottom and thin at the top. Compared to square threads, it is relatively simpler to cut on a job and is also thought to be stronger. Because the ends of these threads are inclined, it is easier to install or remove the split nut from them and does not have a backlash. The angle of this thread is 29°. Acme thread is often used on brass valves, bench vices, and screw-cutting lathes. 

Acme Threads

10. Knuckle Threads

Knuckle thread is a type of screw thread that may be used for both ready and rough work. It features a half-round root and crest. It is easily damaged or dirty. Knuckle thread is a modification of square thread that can be readily cast and rolled. 

The knuckle thread has a 30° angle. Knuckle threads are one of several thread types found in necks of glass bottles, coupling gears, valves, fittings, slides, hydrants, huge molded insulators used in the electrical trade, and railway carriage couplings.

Knuckle Threads

11. Buttress Threads

Buttress thread is a screw thread with a triangular cross-section, but only one face is at right angles to the axis of the screw. The second face is only inclined. Buttress thread boasts more pulling power than square thread. Therefore, it is utilized when there is excessive pull on one side.

The buttress thread unit has both square and V-thread benefits. It possesses the low frictional resistance of the square thread and the same strength as the V-thread. Typically, bench vices have buttress threads on their spindles.

Buttress Threads type

12. Worm Threads

Worm threads are a type of thread that is similar to acme threads in that their angle is also at 29°. One distinction is that they are deeper than acme threads. Worm threads are used in the worm wheel of motor vehicles. Because three teeth of the worm wheel may fit into the shaft when utilizing a shaft with worm threads, it can transfer at 90°. They can fit and function well.

Worm Threads

13. Single and Multi-Threads

Some independent and separate threads can run along with a piece of work. As a result, single-threaded screws and multiple or multi-start threaded screws are utilized for some pieces of work.

A single-threaded screw means a screw when one completely turns around the bolt or screw. And there is a movement of one thread. It is also known as starts, and we could have a single, two, or three starts.

When there is a moment of more than one thread, it is referred to as a multiple-threaded screw. Multi-start threads can be used when quick motion or movement is necessary.

Single and Multi Threads

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

When choosing the right types of threads, you should consider the following characteristics of threads. In this way, the threads can offer proper and suitable strength and performance for the application.

  1. Not all screw thread is compatible with all uses. Each one is customized to certain requirements, purposes, and material sizes.
  2. Some screw threads require pre-drilled holes, but others have sharp designs intended to drill directly into softer materials such as wood. 
  3. Screw threads have different thread spacing, which is significant for diverse screw features. If you use bolts or nuts, ensure that they match the spacing of the screws.

Summary

Threads are spiral grooves of equal size or shape created on a cylindrical or cone item or pipe. There are multiple thread kinds, each with its design styles and levels of accuracy necessary. Understanding the different types of threads and how to incorporate them into your designs and final parts is critical. Getting the right thread may help you successfully make a final assembly. 

LEADRP has in-depth knowledge of various types of fasteners used in CNC machining. Our professional and experienced engineering expert can offer thread solution that suits your project. In addition, we provide you with a one-stop prototyping service. Suppose you have any need, you are welcome to contact us. Do not hesitate, and upload your CAD files today!

References

Thread – From Wikipedia

13 DIFFERENT TYPES OF THREADS [COMPLETE GUIDE] WITH IMAGES – From The Engineers Post

Thread ID Made Easy. Learn All About Different Types of Threads – From The Hope Group

Thread Basics: The key geometric parameters Of The Thread – From Fidelity Tools

FAQ

A screw thread, also known as a thread, is a helical structure used to convert rotational to linear motion or force. A screw thread is a ridge wrapped around a cylinder or cone in the shape of a helix, with the former referred to as a parallel thread and the latter referred to as a tapered thread.

There are six common types of threads:

 

1. UN/UNF.

2. NPT/NPTF.

3. BSPP.

4. BSPT.

5. Metric parallel.

6. Metric tapered.

In the Unified Thread Series, there are UNC (coarse pitch), UNF (fine pitch), and UNEF(extra fine pitch).

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