What is meant by a screw thread?

Screw threads are a series of ridges having the profile of a helix. This helix profile runs continuously along the length of a uniform solid or a hollow bar of a circular cross-section at periodic intervals. These helix ridges are also formed over a conical bar. These continuous helical ridges are termed threads.

Screw threads that run along a uniform circular bar are called straight threads, and screw threads that run along a conical bar are called tapered threads. Threads run over either the external surface of the bar or at the internal surface of the bar. Screw threads that are found at the external surfaces are called external screw threads, and the ones found at the internal surfaces are called internal screw threads.

Application of screw threads

There are various applications of screw threads,

• Fastener: Nuts and bolts are two of the most common components used in machinery and construction works that work as a fastener. A bolt has an external screw thread, while a nut has an internal screw thread. Both of these (nuts and bolts) are used to form a temporary hold between two or more different components. Usually rotating the bolt or nut clockwise, tightens the joint, while rotating them anticlockwise loosens the joint.
• Joining: When two members such as a hollow pipe or a solid rod are to be joined co-axially, such as, in plumbing works, the task can be accomplished by using screw threads, which are produced on both the components.
• Clamping: A clamping device is used in mechanical workshops for thread building on movable parts (jaws). For example, vice is a clamping device. Such devices are used to provide a solid grip to a component, which is being clamped.
• Linear movement: The lead screw that is present in a lathe machine and other machine components, such as the sliding worktable of a milling machine, all have an inbuilt screwing mechanism. The linear motion of the components is achieved by the rotation of lead screws.
• Power presses: Screw threads find extensive use in power presses, such as in mechanical presses and automobile screw jacks. These screw threads have special profiles that can withstand high axial loads.

Screw thread manufacturing

The manufacturing of threads can be broadly classified into, thread cutting, thread forming, and thread-making by additive manufacturing. All these manufacturing processes are used to create multiple threads on multiple components including various thread sizes.

Thread cutting

This kind of thread cutting generally leads to the loss of materials. The following are the thread cutting process,

• Screw thread cutting by using a tap and a die: The taps are the internal thread-making tool. A tap is a cylindrical bar having screw thread profiles formed at its outer surface. To use a tap for thread cutting, first, a hole has to be made using a drill, then the taps are rotated inside the hole to get the desired profile. A die is used for making an external thread over the outer surface of a cylinder.
• Screw thread cutting by a single point cutting tool: A single point cutting tool is a tool that has a single cutting edge. This kind of tool is generally used to form screw threads over the external surface of a solid cylindrical bar.
• Screw thread cutting through wire Electro Discharge Machining or wire EDM: Electrodes in the shape of the threaded profile are made to contact the raw material (stock) where the threads are to be manufactured. The whole setup is dipped inside a chamber having a dielectric fluid. Due to the electric discharge, the material is removed from the stock through erosion. The cutting process is such that the path of the motion of the electrode intersects the horizontal axis of the stock at complementary angles.
• Thrilling: Thrilling is a term used when there is a combination of threading and drilling. This operation is performed by using a computer numerical control (CNC) machine that makes the use of a special tool known as a thrilling tool. The tools are mounted on the spindle of the CNC tool holder. The cutting operation initiates as the tool moves longitudinally along the z-axis.

Thread forming

In this thread manufacturing process, instead of material removal, the material is formed or plastically deformed to get the required profile of the screw thread. Based on this, there are various thread forming processes. They are listed below:

• Thread rolling: In the thread rolling process, the stock is placed in between two roller mills. One of the mills is used to support the stock while the other has the profile of the screw thread engraved on it. The roller having the profile of the screw thread is made to touch the stock; the rollers are rotated by simultaneously applying the force. A similar profile of the screw thread then gets formed on the surface of the stock.
• Thread casting: In this process, a mold in the form of the required screw thread profile is used. Molten metal in the liquid form is poured into the mold cavity. Upon solidification, the required profile of the screw thread is formed.

Thread forming by additive manufacturing: Threads can also be formed with the aid of additive manufacturing. Layer by layer, the addition of a suitable material can result in the formation of the desired screw thread profile. This method of manufacturing is also known as the reciprocal of the manufacturing process that removes materials by subtraction, also called subtractive manufacturing.

Basic terminology of a screw thread

Some of the basic terminologies used for the screw threads are summarized below.

• The major diameter of a screw thread: Major diameter is the outward diameter of a screw thread. In other words, the measurement from one tip of the helix profile to the other tip of the helix profile present in the opposite direction gives the major diameter of the screw thread.
• The minor diameter of a screw thread: Minor diameter is simply the diameter of the cylindrical bar over which the helix profile is formed. It is termed as minor diameter for being the smallest diameter of a thread.
• The pitch diameter of a screw thread: If a fictitious cylindrical profile is created over the screw thread profile, such that the cylinder exactly divides the helix profile into two equal parts, then the diameter of that fictitious cylinder is known as the pitch diameter of the screw thread.
• Pitch: It is simply the peak-to-peak distance between two helix profiles.
• Helix angle: The profile of the screw threads is in the form of helixes. These helixes are at certain angles. The angle formed by the helix is known as helix angle.
• Flank: The flank of a thread is the height of the helix. In a different sense, the height is the slant height because the helix tilts at an angle from the axis of the cylindrical bar.

Basic profiles of screw threads

The profile of a screw thread depends upon the purpose for which the screw thread is being used. The profile largely depends on the screw thread which is used for a fastening purpose or for a power transmission purpose. Also, the profile of the screw threads may be fine or coarse depending on the smoothness of the traverse required. 

The different profiles of the screw threads are discussed below:

• V-thread: These types of screw threads have been recommended by ISO for their industrial applications. These kinds of screw threads are also known as unified threads. This thread has a thread angle of ${60}^{\circ }$.
• Square thread: The profile of these kinds of threads is in the shape of a square. The flank of the thread remains extruded upwards (straight) from the surface of the cylindrical bar.
• ACME thread: These kinds of threads are a modified version of the square thread. The profile is tapered in the case of the ACME thread. In a different sense, it can be said that the flank of the thread is inclined at some angle. This kind of thread has a profile angle of ${29}^{\circ }$.
• Buttress thread:  Buttress threads are the threads that are used in machinery that requires the application of vertical lift; for instance, in screw jacks. The profile of the buttress threads is asymmetrical, which makes it durable to sustain heavy load applications in the single axial direction. 
• Unified national coarse thread (UNC): These are the kinds of standardization that are comparable to ISO. They are specifically used in the United States (US). 

Context and Applications

The study of the metrology of screw threads is extensively used in domains like

• Bachelors of Technology (Instrumentation Engineering, Civil Engineering, and Mechanical Engineering)
• Bachelors of Science (Applied Science)
• Master of Science (Applied Science)

Practice Problems

Q1) Which of the following is the reason for the inclusion of space between the mating surfaces of an internal thread of a nut and the external thread of a bolt?

a) To provide smooth translational motion

b) To avoid jamming and tightness

c) To include the conditions of thermal expansion

d) All of these

Correct option: d

Explanation: Certain amount of tolerances is provided between the mating surfaces of the internal thread of a nut and the external thread of a bolt. These tolerances are provided to avoid jamming during functioning, take into account the thermal expansion, and provide a smooth relative motion between the components (nut and bolt).

Q2) Which of the following thread angle represents an ACME thread?

a) ${60}^{\circ }$

b) ${29}^{\circ }$

c) ${30}^{\circ }$

d) None of these

Correct option: b

Explanation: ACME threads are the type of square thread, whose profile is tapered. The thread angle of such thread is ${29}^{\circ }$.

Q3) Which of the following instruments is used to measure the effective diameter?

a) Vernier calipers

b) Micrometer

c) Screw gauge

d) Profilometer

Correct option: b

Explanation: The effective diameter or the pitch diameter of a thread is measured by a micrometer.

Q4) Which of the following threads is used in machinery that requires powered vertical lift?

a) ACME threads

b) Square threads

c) Buttress threads

d) UNC threads

Correct option: c

Explanations: The buttress threads are asymmetrical profiled threads, that are used in applications that require strong vertical lifts, for instance, in power screws.

Q5) Which of the following is true for the multiple start threads?

a) The profile is tapered with a thread angle of ${29}^{\circ }$.

b) The profile is created by single helical grooves with a helix angle of ${60}^{\circ }$.

c) When two or helical grooves are equally spaced in the axial cylinder.

d) None of these

Correct option: c

Explanation: Multiple start threads are produced by creating two or more helical grooves, placed equally on an axial cylinder.