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Thread milling can present a machinist many challenges. While CNC Threading Machines are capable of producing threads with relative ease, there are a lot of considerations that machinists must make prior to beginning the job in order to gain consistent results. To conceptualize these features and choose the right tool, machinists must first understand basic thread milling applications.

What is a Thread?
The primary function of a thread is to form a coupling between two different mechanisms. Think of the cap on your water bottle. The cap couples with the top of the bottle in order to create a water tight seal. This coupling can transmit motion and help to obtain mechanical advantages. Below are some important terms to know in order to understand threads.

• Root – That surface of the thread which joins the flanks of adjacent thread forms and is immediately adjacent to the cylinder or cone from which the thread projects.

• Flank – The flank of a thread is either surface connecting the crest with the root. The flank surface intersection with an axial plane is theoretically a straight line.

• Crest – This is that surface of a thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects.

• Pitch – The pitch of a thread having uniform spacing is the distance measured parallelwith its axis between corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis. Pitch is equal to the lead divided by the number of thread starts.

• Major Diameter – On a straight thread the major diameter is that of the major cylinder.On a taper thread the major diameter at a given position on the thread axis is that of the major cone at that position.

• Minor Diameter – On a straight thread the minor diameter is that of the minor cylinder. On a taper thread the minor diameter at a given position on the thread axis is that of the minor cone at that position.

• Helix Angle – On a straight thread, the helix angle is the angle made by the helix of the thread and its relation to the thread axis. On a taper thread, the helix angle at a given axial position is the angle made by the conical spiral of the thread with the axis of the thread. The helix angle is the complement of the lead angle.

• Depth of Thread Engagement – The depth (or height) of thread engagement between two coaxially assembled mating threads is the radial distance by which their thread forms overlap each other.

• External Thread – A thread on a cylindrical or conical external surface.

• Internal Thread – A thread on a cylindrical or conical internal surface.

• Class of Thread – The class of a thread is an alphanumerical designation to indicate the standard grade of tolerance and allowance specified for a thread.

Types of Threads & Their Common Applications:
• ISO Metric, American UN: This thread type is used for general purposes, including for screws. Features a 60° thread form.

• British Standard, Whitworth: This thread form includes a 55° thread form and is often used when a water tight seal is needed.

• NPT: Meaning National Pipe Tapered, this thread, like the Whitworth Thread Form, is also internal. See the above video for an example of an NPT thread.

• UNJ, MJ: This type of thread is often used in the Aerospace industry and features a radius at the root of the thread.

• ACME, Trapezoidal: ACME threads are screw thread profiles that feature a trapezoidal outline, and are most commonly used for power screws.

• Buttress Threads: Designed for applications that involve particularly high stresses along the thread axis in one direction. The thread angle on these threads is 45° with a perpendicular flat on the front or “load resisting face.”

Thread Designations
Threads must hold certain tolerances, known as thread designations, in order to join together properly. International standards have been developed for threads. Below are examples of Metric, UN, and Acme Thread Designations. It is important to note that not all designations will be uniform, as some tolerances will include diameter tolerances while others will include class of fit.
Liuzhou Biaohuang Machinery Equipment Co., Ltd. is located in Liuzhou, a beautiful city in Guangxi Zhuang Autonomous Region in China.We integrate production and sales as a whole, with self-management import and export rights

Our overseas offices are located in Singapore and Dubai. Our group has seven factories in China.

These factories are located at the cities of China at Liuzhou, Shanghai, Jiangsu, Shandong, Tianjing, Hebei and Liaoning.


Liuzhou Biaohuang Machinery Equipment Co., Ltd. is now one of the regular suppliers to the international market. During the past few years, our company secured the flow of several supplies from different countries to the final destination. We have more than 15 years of experience in export sales. Our products are sold to United States of America, Dubai, Saudi Arabia, Yemen, Egypt, Indonesia, Vietnam, Malaysia, Singapore, Thailand, Africa etc.

Our products cover thread protector for OCTG & drill pipe, bucking unit, plastic pipe, bucking machine, CNC pipe threading machine all for tubing & casing pipe processing etc.

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