A Screw thread is a spiral ridge that wraps around a screw. It is the thread that makes the srew a screw. A screw thread is needed for so many purposes. It is a fastener that helps with motion transfer and load distribution. Screw threads are mostly applicable in construction, but they are also heavily relied upon in the automotive and aerospace sector, and the making of medical devices. An important feature of a screw thread is that it is easily detachable, while still maintaining it’s strength.
This article is aimed at providing you with an understanding of the types of threads, helping you select the most appropriate and cost-effective type of thread, and making your task easier
What are Screw Threads?
A screw thread is the helical ridged pattern on a screw that determines how it fits into a threaded hole. This ridge converts a rotational movement into linear motion. It bites into a corresponding thread, thereby securing the fastening. Threads are of different types: Internal thread or External thread.
In any case, Screw threads have different standards, depending on which country you are from, these standards is to ensure uniformity. So we have standards like the ISO metric screw thread and the Unified Thread Standard.
External Thread
A screw thread is the helical ridged pattern on a screw that determines how it fits into a threaded hole. This ridge converts a rotational movement into linear motion. It bites into a corresponding thread, thereby securing the fastening. Threads are of different types: Internal thread or External thread.
In any case, Screw threads have different standards, depending on which country you are from, these standards is to ensure uniformity. So we have standards like the ISO metric screw thread and the Unified Thread Standard.
The external thread, as the name implies, is on the outside of the cylindrical material. An example of this is the ridges on a regular screw or bolt.
Internal Thread
The Internal thread is formed on the inside of the cylinder. The inside of a nut is an example of an internal thread.
This works because the internal and external threads fit together. The lid of a jar only remains fastened to the jar because the external thread on the jar and the internal thread on the lid fit in perfectly together
Componenets and Geometric Parameters of a Screw Thread
The Crest
The Crest is the very top of the thread. This is the highest point in the thread.
Root
Unlike the crest, the root of a thread is the lowest part of the thread profile, where the thread transitions to the core of the workpiece. It is the lowest point of the thread and is between two crests.
Thread Flank
The flank is the sides of the thread, it runs from the crest to the root, and connects them. The flank slopes at an angle, depending on the type and the size of the thread.
Pitch
The pitch is the distance from one crest to the next
Thread Angle
This is the angle formed between two flanks. The angle between the sides of the thread, which is typically measured in degrees, dictates the thread’s strength and fit
Major Diameter
Major Diameter is the largest diameter of a screw thread, measured from the crest (peak) of one thread to the crest of the opposite. It is the outside of the thread. The thread diameter is measured by a clipper rule or a thread pitch gauge.
Minor Diameter
This is the inside of the thread, it is the narrowest part of the thread. Picture scraping off the ridge of the thread. It is the measurement of the lowest part of the thread
Thread depth
This is the distance between the Crest and the root. The depth of a thread says a lot about the strength, grip, and reliability of the thread
Lead
This is the distance the screw can move in a full 360 degrees turn. The lead will determine the application of the screw. A screw with a high lead is used in objects like bottles, caps, clamps or jack. While a low lead is used for precise control, and when a strong holding force is required. An example of a low lead application is musical instruments like the violin or the guitar.
Overview of threading standards
Thread standards are the rules and formats established and used to ensure universal thread compatibility and facilitate its maintenance. Threading standards are usually based on metric or imperial measurement systems. Threading standards generally define the shape, tolerances, and sizes of the thread. They also establish a system of naming the thread.
The main reason for a threading standard is to ensure that parts can fit together and function well, even if they are from different manufacturers or different countries. Standards reduce the risk of thread failure. They ensure machines and tools have specific and exact specs. Common threading standards are listed below
ISO metric screw thread
This is the most widely used standard. Defined by the International Standards Organization, the ISO Metric Screw thread has its dimensions in millimeters. It has a thread angle that is V-shaped and is at 60 degrees. The ISO Metric thread is available in a wide range of standardized sizes and pitches, with the designation “M” followed by the major diameter and pitch (e.g., M10 x 1.5). M = Metric, 10 = Major diameter, 1.5 = the pitch.
The Iso Metric thread could either be a coarse thread or a fine thread. A coarse thread is the more common option. It is easier to assemble, and it hardly crosses threads. Where the pitch is not specified, it is assumed that it is coarse. A Fine thread, on the other hand, has a stronger grip and is more suitable for vibration-prone applications.
Unified Thread Standard (UTS)
The Unified thread standard is a System that is mainly used in the United States and Canada. It has its dimensions in inches, the unified thread form is also measured in Threads per inch. Its shape is similar to that of the ISO, it is shaped at a 60-degree angle. The UTS has the following thread series: The Unified National Coarse, The Unified Fine thread, and The Unified National Extra Fine. An example of the UTS designation is 3/8-20 UNC
- 3/8= Major Diameter
- 20 = 20 threads per inch
- UNC = Universal National Coarse (referring to the thread type)
British Standard Whitworth (BSW)
The British Standard Whitworth (BSW) is a standard developed in Britain. It is seen as the first national thread standard in the world, but it has, however, now been overtaken by the UTS. This thread form has a 55° V-thread with rounded roots and crests. It is also measured in Inches and the pitch, in Threads per Inch. An example of the BSW designation format is 1/2 in BSW(14 TPI)
- 1/2 In refers to the Nominal Diameter
- 14 TPI stands for 14 threads per inch
National Pipe Thread
This applies to the US. It is the standard for pipe threads used in plumbing, gas lines, sprinklers, and hydraulics. This thread form is different in the sense that unlike bolt and screw threads, they are designed to be water and fluid tight.
Acme Thread
The Acme thread form is mostly used for brass valves, bench vices, jacks, and screw-cutting lathes. It has a trapezoid shape. The thread angle is 29 degrees, and it has flat crests and roots, making it stronger under load. It is designed to withstand wear, tear, and friction in the long term.
The different types of Acme threads
| Type | Description | Application |
| General-purpose Acme | It is the most common type, it has moderate strength | Screws and vises |
| Stub Acme | It is shorter and applicable for short threading | CNC machine Precision lead screw |
| Multiple start Acme | It has more than one thread and faster movement | Jacks |
Types of Screw Threads
Screw threads can be classified based on the following factors:
Function
Under this classification, we have: Fastening threads, Motion Threads, and Sealing threads. Fastening threads are used to hold or join parts together.
E.g., Bolts, nuts, Screw, Motion threads on the other hand convertrotary motion to linear motion. Sealing threads create air and liquid-tight joints.
Shape
| Shape or Angle | Type |
| 60 degrees | V-Thread |
| Square | flat top or bottom |
| Trapezoida | Acme |
Thread direction
This could either be right-handed or left-handed
Thread Pitch
It could be either a single-start thread or a multi-start thread
Gender
We have the Male and Female threads. These types of threads refer to how two different srew threads form fit together. The male threads are external, they are designed to screw into something, which could be another screw like componenet. The female threads, on the other hand, are internal; they are designed to be screewed into. They receive the male thread
Manufacturing of Screw Thread
Manufacturing materials
There are different kinds of materials used in manufacturing different kinds of screw. Generally, screws and bolts are made from steel. This is because steel is strong, has a great load-bearing capacity, and is inexpensive.
However, screws and fasteners can also be produced from other materials depending on the screw thread sizes, the intended application, and the laid down manufacturing standards. Some of these other materials include:
Titanium
Stainless steel
Hardened steel
Plastic:
Molybdenum:
Copper alloy
Brass
Aluminum
Superalloys
Manufacturing Methods
Screw threads are made using different manufacturing methods:
Thread Cutting
Thread cutting is a process where cutting tools or machines are used to carve out the screw thread. Machines like the lathe or the CNC are usually used in this process. Thread cutting is accurate and precise. It is used to produce threads in small quantities and batches. It is, however, slower and produces more waste by-products.
Thread Milling
Thread milling is performed on a thread milling machine with a disc milling cutter or a comb milling cutter. The idea is for the miller to move across a workpiece, milling threads into it. This process applies to both internal and external threads. An advantage of the thread milling process is that it is versatile and has a good surface finish. It is, however, slow, and it causes wear to the tool used faster than other methods.
Thread rolling
This is a cold forming process where hardened dies are pressed into and rolled against a blank shaft with high pressure; the metal flows into the thread profile of the die, producing a smooth and strong thread.
Thread rolling has three types: Flat die rolling, top-die cylindrical rolling, and three-die cylindrical rolling.
Thread rolling is advantageous in that it strengthens the material, it is fast and ideal for high-volume production, and it wastes no materials. Threads produced by this method can withstand fatigue.
This method is only suitable for ductile materials
Thread Grinding
Here, a precision grinding wheel is used to grind the thread. This process is used when very tight tolerances are required or when a hard material is being worked. Thread grinding produces accurate and smooth grinds. It is, however, time-consuming and expensive.
Thread Casting
This is also called molding. A mold of the screw is formed, it is then cast with the intended material. It is more applicable for plastic screw
Other methods include: 3D Printing
Applications of Screw Threads
Screws are widely used as fasteners to hold objects together. They are also used in our everyday products like screw tops for containers.
| Application | Examples |
| Motion Transmission | Jacks, Vises, Lead Screws |
| Fastening & Joining | Bolts, nuts, screws, sheet metal screws |
| Decorative and Specialized applications | Bulbs, bottle covers, and Camera stands |
| Sealing | Pipe threads, gauges, plugs, and valves |
| Measurement | Calipers |
| Assembly/Disassembly of Devices | Phones, Home appliances |
Thread Measurement and Inspection
There are a few thread identifying tools and instruments that is used to measure a screw thread. They include:
Thread Pitch Gauge Tool
A thread pitch gauge helps to measure the pitch of a thread, which is the thread spacing between two crests.
The gauge has a set of blades. The blades each have teeth, whose measurement corresponds to each pitch. The pitch is measured by simply aligning the gauge and checking which of the teeth fits.
Thread Caliper
The thread caliper is used to measure the major diameter and minor diameter of a threaded part.
Go/No-Go Thread Plug Gauge
This is used to measure internal screw, it checks the acceptability.
Other measuring instruments include:
Optical Comparator
Thread micrometer
Profile projectors
Thread scanners
The last two options are used for precision work
Conclusion
Screw threads, though simple, are fundamental and indispensable to the operation of all machines and devices. Having an understanding of them will help users (engineers, technicians, operators) make better decisions on the construction and maintenance of their devices. In other words, an understanding about the right screw thread is an essential skill that affects the quality of work of a person and the performance of their devices.