Metal threading Explained: Manufacturing Process

Manufacturing complex designs generally involves assembling simpler parts to form a complex geometry.

Assembling these parts generally require fasteners such as screws and bolts that are manufactured by the metal threading process.

So what is metal threading? What are the techniques to perform metal threading?

Metal threading is a process used to create helical grooves on the workpiece, enabling it to be fastened to another workpiece. Depending on the application, these threads can be made by subtractive, formative, or additive manufacturing techniques. Generally, metal threading involves cutting male and female threads on the mating parts.

This article discusses the metal threading process in detail by going through its various aspects such as its process, application, and advantages.

What is Metal Threading? Guide

Metal threading is a metal processing technique that involves making uniform, continuous helical threads on the surface of a cylindrical or conical workpiece. 

These threads act as a coupling and transmit motion, which can help achieve a mechanical advantage. The screw thread transforms rotational motion into linear motion, allowing various applications. 

Metal threading is used for various applications that require high load capacity and the ability to transform load with high accuracy and precision, such as screws, bolts, and lead screw drives.

The process of metal threading generally involves material removal from the surface of the workpiece by using metal threading tools with machines such as lathes, mills, dies, etc.

Depending upon the type of thread required, such as acme threads, circular threads, single-start or multi-start threads, etc., a suitable method of metal threading can be implemented.

Metal thread cutting is also used for manufacturing of transmission drives like ball screw and lead screw drives.

Apart from these, modern 3D printers provide the ability to print metal workpieces with threads, eliminating the need for secondary threading operation.

Therefore, the process to generate screw threads is selected based on time, cost, precision requirement, equipment availability, application, etc.  

Based on your application, the threaded rod might require the machining of screw-driving heads like Torx and Torx Plus.

Different Techniques for Metal Threading

Metal threading processes can be classified into three main categories: thread cutting, thread rolling, and additive manufacturing.

1. Thread Cutting

Thread cutting is a subtractive method of generating threads on the internal or external surfaces of cylinders and cones using tools and dies.

This process removes material with each successive pass to obtain desired thread depth using a tool of the required thread pattern.    

Thread Milling

In thread milling, threads are made using a high-speed, rotating milling cutter to remove material from the surface of the workpiece to produce threads.

A hole is drilled in the workpiece to insert the milling tool for internal threads, where the circular motion of the tool carves out the thread.

Similarly, external threads are carved by feeding the milling tool to the outer surface of the workpiece to carve out the threads.

Thread milling imparts comparatively lower forces resulting in improved tool life and making it suitable for brittle and thin-walled parts. 

A thread mill cutter can cut internal and external threads of varying diameters and produce right- and left-handed threads, allowing it to perform multiple operations with one tool.

This method is suited for high-volume production where threads are cut on different sizes of workpieces.

Its high precision and control make it suitable for cutting threads on blind holes, which otherwise is challenging to perform with a single-point threading tool.

Tapping & Threading

This process uses taps and dies to create threads. A tap is used to cut or form threads on the internal surface, while a die is used to cut threads on the external surface. 

The process of using a tap to cut threads is called tapping, while the process involving a die to form threads is called threading. 

Tapping can be done using a drill machine or tap wrench. A tap is a drill bit with grooves cut to make threads.

There are three types of commonly used taps: bottom taps, plug taps, and taper taps.

Bottom taps have a flat bottom with negligible chamfer, making them difficult to insert into a hole.

Therefore, these taps require an already threaded hole for easy insertion.

However, it has a continuous cutting edge without taper to facilitate easy thread cutting up to the bottom of a blind hole. 

Whereas plug taps have tapered cutting edges, making it easier to start and align the tap into a hole. As a result, these taps do not require an already threaded hole.

Taper taps are similar to plug taps and have even more tapered cutting edges giving them a gradual and less aggressive cutting action.

These taps are used to cut threads on hard materials with very small diameters. Usually, a taper tap has 8 to 10 tapered threads.

On the other hand, threading dies are used to cut male threads on cylindrical material such as rods. The dies can be solid or adjustable. 

The accuracy with which a solid die cuts threads is subject to the precision of the die and the effects of wear.

Adjustable dies can be calibrated using screws to account for wear or for getting a different thread fit.

Generally, a die having a slightly larger diameter than the workpiece is used for threading, and chamfering the edges of the workpiece facilitate easy centering of the die and reduces the required force.

Single Point Thread Cutting

Single-point thread cutting technique uses a single-point cutting tool to cut threads on a rotating workpiece by moving the tool linearly and radially.

A CNC lathe is generally used to perform CNC threading operations.

The movement of the tool along the surface of the rotating part removes material to create threads, where the feed rate and the depth of cut determine the pitch and thread depth, respectively.

This is achieved using a lathe machine in which metal stock is held in the chuck and rotated while the cutting tool is mounted on a carriage and is moved along the X and Z axes to get the desired thread. 

Single-point threading is a versatile process that can cut both internal and external threads and is not limited by the size of the part to be threaded. 

Apart from that, the ability to precisely control the single cutting edge provides the ability to cut unique threads with multi-start and other specific requirements.

However, using a single-point cutting tool reduces the material removal rate, making it unsuitable for applications where high productivity is desirable.

2. Thread Rolling

This is a deformative process of threading, where threads are formed by deforming ductile metals using a hardened steel thread rolling die.

Unlike thread cutting, this process does not involve material removal and therefore eliminates material wastage.

Thread rolling refers to the process of making external threads, while the process of making internal threads by this method is termed as "thread forming."

As this method does not remove material from the stock, no chips are produced, leading to minimal wastage.

Rolled threads have high yield strength as the increased pressure during forming results in grain conformance.

Furthermore, formed threads have high dimensional accuracy, good surface finish, and low friction coefficient. 

There are mainly three types of rolling dies used for thread rolling: flat dies, cylindrical dies, and planetary dies.

Flat dies consist of two flat die surfaces, one stationary and the other sliding.

The piece to be threaded is placed on one end of the fixed die, and the other die slides over the blank. 

This causes the blank to roll between the dies, forming the threads.

Cylindrical rolling dies can be either a two-die or a three-die arrangement in which the rolled threaded profile is generated by rotating the die surfaces along the surface of the workpiece.

This results in plastic deformation of the material, producing the required thread design.  

Planetary thread rolling dies can work on a range of workpieces and are one of the most effective thread-forming tools for headed screws.

They are best suited for mass production of threads having a diameter of less than 25mm. 

This die consists of a central thread rolling wheel and one or more stationary curved dies.

The metal stock is fed between the dies at one end of the stationary die and a threaded piece is ejected from the other end.  

Thread Forming

The process of thread forming requires a cutting tool that is similar to a tapping tool, known as fluteless tap or roll tap.

This process is performed to form internal threads on an already drilled hole, where the diameter of the hole is slightly smaller than the diameter of the roll tap.

Spaced lobes in the roll tap form the thread on the surface of the workpiece as it is moved into the hole.

However, forming can generate high frictional heat, making it necessary to use cutting fluid to provide lubrication.

This process is best suitable for cutting threads on a blind hole as it does not involve the accumulation of chips during the threading process.

3. Additive Manufacturing

With the advancement in technology, 3D printers can be directly used to print metal workpieces with the required thread pattern.

This provides the ability to produce highly precise threads with tight tolerances.

However, this method is limited by the resolution of the printer and cannot produce threads smaller than the diameter of the printing nozzle.

Thread Rolling vs Thread Cutting- Which one to Choose?

Parameter	Thread Rolling	Thread Cutting
Chip formation	Chips are not formed	Chips are formed
Precision	More precise thread pattern	Less Precise
Strength	Higher yield strength	Requires heat treatment
Production Rate	High	Low
Tool Life	Longer tool life	Shorter tool working life
Tools Required	Separate dies required for each type of thread	Single tool can perform all thread cutting
Setup Cost	High due to variety of dies	Low due to limited tools
Production Cost	Lower cost of production	High cost of production
Material	Suitable for ductile metals	Used on most metals
Labor	Lower labor requirement	Labor Intensive

Thread rolling provides various advantages over thread cutting. Since forming involves no material removal in the form of chips, it requires a smaller blank size leading to material savings.

Moreover, rolling gives more precise control of the process and penetration rates, but is not suitable for threading hard metals.

Thread cutting can leads to overcutting threads, but rolling eliminates the possibility of overcutting as no material is removed in the threading process. 

The production rates of thread forming can be up to 10 times faster than cutting processes. Forming also provides greater dimensional accuracy along with improved tool life.  

However, thread rolling requires a high initial investment for rollers and dies. A separate set of dies is needed for each thread type.

With the thread-cutting process, threads of any form, size, or shape can be cut. It does not require multiple tools for different parameters, as a single tool can perform all operations. 

Thread cutting can be used for threading almost any metal without the risk of cracks on the workpiece, but the threaded workpiece might require a heat treatment process such as annealing to regain the tensile strength.

This process is cost-effective and has a lower initial cost, making it suitable for most production systems.

Various tools such as lathes, mills, etc., can be used for cutting threads, and appropriate procedures can be selected based on the requirement.

Final Thoughts

Threading is a basic fabrication process, and screw threads have a variety of applications ranging from mechanical actuation to measurement. 

They are most widely used in the form of fasteners, allowing us to create high-strength, non-permanent joints that can be dismantled without damaging the joined components.

Either cutting or forming can generate threads. Each has its benefits and shortcomings.

Thread cutting suits applications where initial cost is essential and production volumes are low. It is also suited for shops that require a wide variety of threads to be produced. 

While thread forming requires a high initial investment, it can compensate for those costs with the quality and rate of threaded parts produced along with improved tool life.

Frequently Asked Questions (FAQ)

How do I choose a screw thread?

Screw threads are chosen based on three primary considerations: the head design, material, and size. Generally, it is advised that the screw should be long enough to reach half the thickness of the bottom piece of the assembly, and the diameter or gauge of the screw would depend on the fastening material.

What are the classes of threads?

Thread classes define the degree of tightness or looseness of mating threads. The higher class number refers to a tighter fit, while A & B represent external and internal threads. The most commonly used thread class is 2A & 2B. 

Can we use metal threading techniques to cut threads on plastics?

Yes, you can use metal threading techniques to cut threads on plastics. However, plastic is a soft material and requires comparatively less cutting and forming force. Therefore, appropriate process parameters should be maintained when threading plastics.