Even with advancements in the industry, screws and threads still play a key part in fastening. The primary function of threads in any design is to facilitate the assembly of parts using two different mechanisms. Thread making requires skill, precision and consistency. Luckily, CNC boasts of a healthy appetite for fabricating complex parts. In today’s article, we explore how CNC machines are used to make threads on workpieces through an array of machining techniques.
Methods such as cutting, milling, grinding and turning can all be used to make functional threads. Threads may also be internal or external. Read on to understand each threading process and the benefits of using CNC machines to make your threads.
Internal & External Threading
The difference between internal and external threading is very basic. Internal threading remains within the main part. They may be considered as the receptive threads, accepting fasteners like bolts and screws. External threads, in contrast, are found on the fasteners themselves. They are external to the hardware, and found on screws and bolts. For a complete fastening process, the external threads must fit and lock into the internal threads.
There is an existing international standards for threads. There are UN, Metric and Acme thread designations, each denoting parameters such as diameter tolerances and class of fit. Below is an example of all 3 types of designations:
A 2 050 15-X
Where A – Acme
2 – Number of thread starts
050 – The basic major diameter (0.5” or ½”)
15 – Number of threads per inch
X – Denotes the purpose of the thread (1 denotes screw, 2 denotes nuts and 3 denotes flange)
M 12 x 1.50 – 2h - RH
Where M – Metric
12 – Nominal diameter
1.50 – Pitch
2h – Class of fit
RH – Right hand
¼ 10 UNC 2B RH
Where ¼ – Thread major diameter
10 – Number of threads per inch
UNC – UNC thread series
2B – Class of thread (A – External threads, B – Internal threads)
RH – Right hand
Thread Machining with CNC
Below is a brief overview of the most common thread manufacturing methods in CNC:
Tapping is a simple threading solution that is used to make internal threads. It uses a torque to screw the tap into an existing bottom hole in the workpiece to make the internal thread. The process is very economic, making screw thread profiles well suited for small diameters and moderate accuracy operations.
Tapping can be done manually or with lathe machines. It has a variety of applications, and can be executed with lower cost of cutting tools and overall production costs.
Thread milling uses the circular ramping motion of cutting tools to make screw threads on CNC machines. The process engages the milling cutter and multi-axis machining center of the x, y and z axis linear feed in a circular interpolation to make the thread. Thread milling can be used for processing threads into large parts. It is very fast and highly efficient. Thread milling processes are suited for asymmetric/non-rotating parts, thin-walled parts, CNC components made from materials likely to experience chip breaking and CNC components that require tough/high cutting forces. They also suit machining of blind holes with short-bottom holes.
Lathe threading can make high quality screw threads with numerous pitches, tapered threads and multiple lead threads. For turning threads on a lathe, the CNC machinists may also use a thread chaser.
Thread chasers are suitable for use with small, single piece, simple structure workpieces at high production efficiency.
This CNC machining method uses a forming rolling die to deform the workpiece and obtain the desired external threads. Thread rolling is well suited for high volume operations with standard fasteners and other threaded couplings. The working concept can be likened to tapping, in that both processes uses a torque. Thread rolling, is however, more accurate, and produces better suffer quality than tapping.
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