What is Thread Machining?
The process of threading in mechanical design involves forming helical or spiral grooves on a workpiece, enabling screw threads to be created for fastening purposes.
In thread machining, several key terminologies are pivotal for understanding and executing the process effectively. The pitch refers to the distance between adjacent threads, which is crucial for fitting components together seamlessly. The thread angle, another vital term, is the angle formed by the thread’s sides, influencing the threaded connection’s strength and load-bearing capacity. Additionally, major and minor diameters play a significant role; the primary diameter is the largest diameter of a screw thread, while the nominal diameter is the smallest. These dimensions are critical in ensuring the threads fit perfectly, providing a secure and reliable assembly in various mechanical applications.
Type of Thread
Threads, the helical structures that facilitate the assembly of components, come in various types to suit different applications and requirements in manufacturing. Understanding the types of threads is crucial for selecting the appropriate threading method and ensuring compatibility between parts.
External threads and internal threads are the primary categories, with external threads found on bolts and screws and internal threads inside nuts or tapped holes. This distinction is fundamental in mechanical design, allowing parts to interlock securely.
Metric and Unified National (UN) threads are among the most common thread types. Metric threads, defined by their pitch in millimetres, are prevalent in most parts of the world, adhering to international standards for dimensions and tolerances. They are characterized by their simplicity and uniformity, making them suitable for various applications.
On the other hand, Unified National threads are used primarily in the United States and Canada. These threads are identified by their pitch in threads per inch (TPI), with UN threads further subdivided into UNC (coarse), UNF (OK), and UNEF (extra fine) categories, each serving different purposes based on the required strength and precision.
Another necessary type is the National Pipe Thread (NPT), specifically designed for sealing pipe joints. NPT threads are tapered, allowing for a tighter and more fluid-resistant seal, essential in plumbing and fluid transfer applications.
Thread Machining Methods
Thread machining encompasses a variety of methods and tools designed to produce precise threads on workpieces. The choice between these methods, primarily thread cutting and thread forming, depends on the material, application, and desired finish of the thread.
Thread cutting involves removing material from a workpiece to form threads, utilizing tools such as taps and dies for internal and external threads, respectively. The main threading methods are thread milling, thread grinding, thread lapping, tapping and socketing, and thread rolling.
Thread Milling
Thread milling is performed on a milling machine or a CNC machining center. It involves using a single or multi-point cutting tool to create threads in a spiral motion. Milling on a thread milling machine is carried out with a disk or comb milling cutter. The disk milling cutter is mainly used for milling trapezoidal external threads on screws, worms, etc. The comb milling cutter is used for milling internal threads.
The comb milling cutter is used for milling internal and external common threads and taper threads because it is milling with a multi-flute milling cutter, and the length of its working part is greater than the length of the processed thread, so the workpiece is only used for milling. Since it is a multi-flute milling cutter and the size of its working part is longer than the length of the thread being processed, the workpiece only needs to be rotated by 1.25 to 1.5 revolutions and can be processed, which has high productivity. This method is suitable for batch production of threaded workpieces of general accuracy or roughing before grinding.
Thread Grinding
Thread grinding uses a rotating abrasive wheel to remove material and create precise threads. It is used primarily for processing precision threads of hardened workpieces on thread-grinding machines. It is divided into two kinds of grinding wheels according to the different cross-sectional shapes of grinding wheels: single-thread grinding wheels and multi-thread grinding wheels.
The pitch accuracy achieved by single-line grinding is 5-6 levels, the surface roughness is R1.25-0.08 micron, and it is convenient to dress the grinding wheel. This method is suitable for grinding precision screws. This method is ideal for grinding precision screws, thread gauges, worm gears, small batches of threaded workpieces and grinding precision hobs.
Tapping
Tapping is the process of cutting internal threads in a hole with a tool called a tap. It’s commonly used for nuts, threaded holes, and other internal threads. Taps come in various types, including straight flute, spiral point (for through holes), and spiral flute (for blind holes), each designed to eject chips and prevent jamming efficiently.
The machining accuracy of tapping or threading depends on the accuracy of the taps or plate teeth. Machining Although there are many ways to machine internal and external threads, internal threads of small diameters can only be processed by taps. Tapping and threading can be done manually or by hand. Lathes, drilling machines, tapping machines and threading machines.
Die Threading
Die threading is used for cutting external threads on cylindrical parts, such as bolts and rods. Dies are circular tools with internal thread patterns that cut into the workpiece as they’re turned around it. Adjustable dies allow for minor size adjustments, enabling a tight fit for the threaded part.
Thread Rolling
This is a method of obtaining threads by plastic deformation of the workpiece with a shaped rolling die. Thread rolling is generally performed on thread-rolling machines. Thread rolling machine or in the automatic lathe attached to the automatic opening and closing thread rolling head on the On, suitable for mass production of standard fasteners and other threaded joints of the external thread.
The outer diameter of the rolling thread is generally not more than 25 mm, the length is not more than 100 mm, and the thread accuracy can be up to grade 2 (GB1000). Thread precision can be up to level 2 (GB197-63); the diameter of the blank used is roughly equal to the centre diameter of the processed thread.
The advantages of thread rolling
- Surface roughness is less than turning, milling and grinding.
- Rolling the thread surface due to cold hardening can improve the strength and hardness.
- High material utilization.
- Productivity than cutting doubled and easy to automate.
- Rolling die life is long. and hardness.
- High utilization rate of materials.
- The productivity of the cutting process has increased exponentially, and it is easy to automate.
How to Machining Thread?
Selection of the Correct Tool
Choose the appropriate thread-cutting tool based on the thread type (internal or external), material, and desired finish. For example, taps for internal threads dies for external threads, or milling tools for thread milling.
Material Preparation
Prepare the workpiece material, ensuring it is suitable for the type of thread and machining process. This may involve selecting a material with the right thread forming or cutting properties.
Machine Setup
Set up the machine, whether a lathe, milling machine, or specialized thread-cutting machine, with the correct tooling and parameters for machining the thread. This includes the pitch, depth, and diameter of the thread.
Cutting Process: Proceed with the thread-cutting process, which could involve tapping, die threading, single-point threading, thread milling, or another suitable method. A hole is first drilled to the tap drill size for internal threads. For external threads, the workpiece is prepared to the correct diameter.
Inspection and Finishing
After machining, inspect the thread for accuracy, dimensions, and finish. Use gauges or precision measuring tools to ensure the thread meets the required specifications. Any necessary finishing processes, such as deburring or polishing, should be performed to achieve the desired quality.
Tips For Machining Thread
Machining threads involves a series of steps and considerations to ensure the production of precise, functional threads that meet specific application requirements.
-When tapping, chip control is a big problem that should not be ignored, especially when machining workpiece materials that are less hard, denser, and prone to long swarf.
-The male end of the part should be chamfered.
-The end of the female thread should have a countersunk hole.
-Standard thread sizes are more economical.
-Determine whether the threaded connection is a one-time event or requires frequent disassembly, which affects the need for thread durability.
-Coarse threads are generally easier to machine and have high tensile strength, while fine threads are suitable for applications subject to higher loads and vibration.
Conclusion
Manufacturing high-quality parts requires a deep understanding of various manufacturing processes. Among many processes, the cutting of machining threads is particularly critical. While this process may seem complex initially, it is essential to any product that requires joining or coupling and should never be lightly ignored.
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