What is CNC Thread Milling?

Updated: December 06, 2023


Thread milling is a conventional computer numerical control (CNC) manufacturing process. It is widely exploited for producing holes that adopt screws of varying sizes. Thread milling has numerous used in machine shop, sometimes even replaces CNC tapping. 


Let’s deepen into CNC mill machine, Threading process , types of thread cutters and learn more about its difference from tapping.

1. What is Thread Milling?

It is a metalworking process used to cut threads of varying sizes with the help of  machine spindles. In this case, the spindle is a rotatory part of a CNC thread mill that holds a cutting tool. By cutting off small chips of a previously made hole, CNC thread mills achieve threads of varying types.


A single CNC unit can produce numerous types of threads, and the essential considerations here are the following:


  • Left or right-handed threads. They differ in the direction in which a screw’s thread wraps around its shaft. Left-hand threads were made by clockwise spindle movement, and left-handed threads by counterclockwise spindle movement. 


  • Materials of workpieces. Traditionally, thread mills are metalworking CNC units, and even for wooden components, there is a need for a metal insert to accommodate the screw. Pay attention to types of metal if softer or harder ones require distinct tooling. 


  • Hole diameters. Considering that the thread mill is to be inserted in a pre-drilled hole, it is important that you choose the appropriate diameter of both a thread and a drill bit, so they match. 


  • Depth of thread engagement. Also called the height of thread engagement, this indicator is the radial distance by which coaxially assembled threads overlap each over. 


  • External or internal thread. It means a cutting tool that shapes either cylindrical or conical external or internal surfaces. The latter is for screws, while the first type is used, for example, to form bottles’ caps. 


  • Class of thread. It is simply a thread’s alphanumerical designation showing its grade of machining tolerance and allowance. 


  • Thread tolerance.Deriving from the previous consideration, the accuracy that can be achieved with a thread is also essential. 


  • Insert type. For a single row threading, there are two insert profiles, full profile and V-profile. They slightly change the approach to threading, as detailed at the end of the post. You may take a look at these profiles in the image below.
row threading insert type
2. How Does a Thread Mill Work?


Let’s provide an alternative definition of the process to reveal the machining aspect. A conventional, single-point threading is both a cutting and a forming operation done with a thread. It creates a helical groove having a specific shape with the help of a threading insert -– cutting tool, having a corresponding size and shape. This thread is to adopt a screw in it. Here are some aspects of threading:


  • Cutting operations. The main cutting procedure here is making a uniform advancement into a hole per spindle revolution. To achieve this, the CNC machine unit is to perform cutting in accordance with a set of commands. 


  • Machine units. CNC thread mills are machinery that has triaxle control and helical interpolation functions. Technically, though there are specific thread mills, such automated centers as CNC milling and turning ones can perform the operation as well.


  • Coding. In any case, CNC units require a G-coded file that would have all the operations defined precisely. For example, given a 3-axes thread mill, an engineer may code the command for the spindle: go 50 mm along X-axis, then go 20 mm along Y-axis, then lower down 10 mm along Z-axis. Then, he will code the command to start performing rotary movements while lowering the spindle down along Z-axis. In such a manner, a CNC thread mill is given exact instructions. 


  • Modeling. Typically, G-coded files are created automatically. It is done by converting computer-aided manufacturing (CAM) files having a 3D model of a workpiece into G-coded files. 


Eventually, a finished thread is obtained by preparing a file, setting a machine and its tooling, and performing the procedure. Take a look at the process of forming a left-handed, internal thread in the image below.

process of forming internal thread
3. What is a Thread?

You may already have an insight into what treads look like and how they operate. But your understanding of CNC threading would be incomplete without reviewing parts of a thread. At the end of the day, the specificities of a cutting tool are what define the final outcomes. 


So, a thread is a cutting tool that transmits motion in order to perform the procedure with its sharp edges, and its parts and significant measures are the following:


  • Root.  It is the part of a thread’s surface that joins the adjacent trad forms’ flanks. It also joins the thread overall to the cone from which the tool projects.


  • Flank. It is the part of a thread’s surface that connects the crest with the root. This intersection’s axial plane must be a straight line.


  • Crest. It is the part of a thread’s surface that joins the flanks of a thread. 


  • Pitch. It is simply the distance between corresponding points on adjacent thread forms. It lies in the same axial plane as other thread forms, and on the same side of the axis. Its value can be measured by dividing the lead by the number of thread starts.


  • Major diameter. Given a straight thread, it is the diameter of a major cylinder. 


  • Minor diameter. Given a straight thread, it is the diameter of a minor cylinder. 


  • Helix Angle. Given a straight thread, this angle is made by the thread’s helix and its relation to the thread’ axis. This angle complements the lead one. 
cnc machining thread
4. Types of Thread Mill You Need to Know

Thread mills or thread cutters are tools to make thread by milling. They may vary in forms and sizes, depending on a thread’s purpose and the intended design. There are two major types of thread milling tools:


  • Solid carbide thread mill. Such tools are more suitable for small-sized holes and are technically more precise and expensive than indexable ones. They offer single-pass thread generation, smoothest cutting action possible, and performing blind operations over multiple material types. 
  • Indexable insert thread mill. These ones are intended for holes that are over 0.625 inches in diameter. They are more cost-effective because each insert suits both right and left-hand machining, and tool life is longer. Such cutters offer single-tool-pass production as well, performing blind operations and reliable clamping. They also require less machine power.


Take a look at what thread mills look like in the image below.

thread mills profile
5. Thread Milling vs Tapping – What’s the Difference?

Both thread milling and typing can be used to create screw threads. But that’s the only major similarity. So, let’s review some distinct factors to outline a clear difference between these two manufacturing processes:


FactorCNC Thread millingCNC Tapping
Hole sizesCan be performed over holes of nearly any sizeTypically used for small-sized, pre-drilled holes
Thread qualityA thread mill is to be smaller than a hole helping metal chips to evacuate without internal surface deteriorationA tapper is to be the same size as a hole, so metal chips can scratch internal surfaces while evacuating
Cutting force requiredLow cutting force needed overall, especially when using indexable insert cuttersMore considerable cutting force is needed
The need for changing cuttersWhen using indexable insert cutters, a single tool can fit several hole sizesOne cutter suits only one hole size
Fit controlAllows minor adjustments of thread size and positionOnce the hole is tapped, it is not possible to adjust a thread size and position
Labor involvementIt can be done automatically onlyIt can be done automatically or manually
Surface finishGreat surface quality overall. Superior surface finish can be achieved by employing several passes of thread cuttersSurface quality is mediocre without the possibility of improvement without applying extra finish
6. How Do I Choose a Thread Mill?

When it comes to choosing a thread cutter for your needs, there are some aspects to consider.


  • Do not chase the most affordable options. Experience shows that cutting tools are something that worth every penny spent as cheap cutters deteriorate rapidly.
  • Insert type. As a rule of thumb, full profile inserts perform better overall and produce higher quality shapes at maximal accuracy. At the same time, V-profile inserts enable not to purchase much inventory because multiple pitches may fit the same thread. 
  • Cutting diameter. You can freely employ a thread mill of a smaller size than a hole. So, stick with versatile diameters rather than purchasing a distinct cutter for every hole size.
  • Assess the number of rapid passes. It is not obligatory to perform threading in one pass. In complex applications, it is better to revise the number of passes with cutters of different sizes. Several radial passes will help you to achieve finer internal surface quality. It will also improve the consistency of thread from batches to batches. 
  • Male or female thread.  Pay attention to the specificities of a threading operation to be performed. One clear example is the need for an external or male or internal or female thread, as mentioned previously. Take a look at what both look like in the image below. 
Male and female thread
thread mill tool holder recommendations
7. Summary

In essence, threading is one of the most straightforward manufacturing operations. CNC milling or truning center or lathe performs it with the help of thread mills and G-coded file specifying the instruction.


There are two major families of thread mills, which are indexable insert and solid carbide ones. They have varying costs and suit different design purposes. When choosing a thread mill, you should pay particular attention to its characteristics and type so the desired cutter operates as expected.

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