Basic knowledge of thread and introduction of various thread processing methods

Basic knowledge of thread and introduction of various thread processing methods

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Elements of a thread

Thread includes five elements: tooth type, nominal diameter, number of threads, pitch (or lead), and direction of rotation.

1. Tooth type

The profile shape of the thread on the cross-sectional area through the thread axis is called the profile. There are triangular, trapezoidal, zigzag, arc and rectangular tooth shapes. Thread profile comparison:

2. Diameter

Threads have major diameters (d, D), middle diameters (d2, D2), and minor diameters (d1, D1). The nominal diameter is used to represent the thread, and the nominal diameter is the diameter that represents the size of the thread. The nominal diameter of ordinary threads is the major diameter.

3. Number of lines

A thread formed along a helix is ​​called a single-thread thread, and a thread formed by two or more helixes distributed equidistantly in the axial direction is called a multi-thread thread. The following figure shows single-thread thread (left) and double-thread thread (right):

4. Pitch and lead

The pitch (p) is the axial distance between the corresponding two points on the pitch line of two adjacent teeth.

The lead (ph) is the axial distance between the corresponding two points on the center diameter line of two adjacent teeth on the same helix.

For single-thread thread, lead = thread pitch; for multi-thread thread, lead = thread pitch × number of threads.

5. Rotation

Threads screwed in clockwise are called right-hand threads; threads screwed in counterclockwise are called left-hand threads.

Classification of threads (by use):

There are many forms of threads, which can be divided into the following two categories according to different uses:

1. Fasten the thread

It is used for fixed connection between parts. Common threads and pipe threads are commonly used, and the thread profile is mostly triangular. The main requirements for ordinary threads are the reliability of screwability and connection; the main requirements for pipe threads are sealing and connection reliability.

2. Transmission thread

It is used to transmit power, motion or displacement, such as the thread of lead screw and micrometer screw, and its tooth shape is mostly trapezoidal or serrated. The main requirements for the transmission thread are accurate and reliable transmission, good thread contact and wear resistance.

Thread processing method

Commonly used thread cutting methods include thread turning, thread milling, thread grinding, tapping and threading, etc.; few cutting methods include thread rolling and thread rolling, etc.; special processing methods include EDM and EDM conjugate synchronous rotation processing, etc.

When choosing a thread processing method, there are many factors to consider, the main ones are the workpiece shape, thread profile, thread size and precision, workpiece material and heat treatment, and production type. The following is an introduction to common thread processing methods:

1. Thread cutting

It generally refers to the method of machining threads on workpieces with forming tools or abrasives, mainly including turning, milling, tapping, threading, grinding, grinding and whirling. When turning, milling and grinding threads, the drive chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves exactly and evenly one lead along the axis of the workpiece for each revolution of the workpiece. When tapping or threading, the tool (tap or die) rotates relative to the workpiece, and the previously formed thread groove guides the tool (or workpiece) to move axially.

Thread turning Thread turning on a lathe can be done with a forming turning tool or a thread comb (see Threading Tools). Thread turning with a forming turning tool is a common method for single-piece and small batch production of threaded workpieces due to the simple tool structure; thread turning with a thread tool has high production efficiency, but the tool structure is complex, and is only suitable for turning in medium and large batches. Short thread workpieces with fine pitch. The pitch accuracy of ordinary lathes for turning trapezoidal threads can generally only reach grades 8 to 9 (same as JB2886-81 below); for thread processing on specialized thread lathes, productivity or accuracy can be significantly improved.

2. Thread milling

Milling with a disc or comb cutter on a thread mill. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screw and worm. Comb-shaped milling cutter is used for milling internal and external common threads and taper threads. Since it is milled with a multi-blade milling cutter and the length of its working part is greater than the length of the thread to be processed, the workpiece only needs to be rotated 1.25 to 1.5 turns to be processed. Done with high productivity. The pitch accuracy of thread milling can generally reach 8 to 9 grades, and the surface roughness is R 5 to 0.63 microns. This method is suitable for mass production of threaded workpieces of general precision or for roughing before grinding.

3. Thread grinding

Mainly used for precision threads of hardened workpieces on thread grinding machines. According to the different shape of the grinding wheel section, it is divided into two types: single-line grinding wheel and multi-line grinding wheel. The pitch accuracy that can be achieved by single-line grinding wheel grinding is 5 to 6 grades, and the surface roughness is R 1.25 to 0.08 microns, which is more convenient for grinding wheel dressing. This method is suitable for grinding precision lead screws, thread gauges, worms, small batches of threaded workpieces and relief grinding precision hob. Multi-line grinding wheel grinding is divided into longitudinal grinding method and plunge grinding method. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the thread to be ground, and the grinding wheel moves longitudinally once or several times to grind the thread to the final size. The width of the grinding wheel of the plunge grinding method is larger than the length of the thread to be ground. The grinding wheel is cut into the surface of the workpiece radially, and the workpiece can be ground well after about 1.25 revolutions. The productivity is high, but the accuracy is slightly lower, and the grinding wheel dressing is more complicated. Plunge grinding is suitable for relief grinding of large batches of taps and for grinding certain threads for fastening.

4. Thread grinding

The nut-type or screw-type thread grinding tool is made of soft materials such as cast iron, and the part of the processed thread on the workpiece with the pitch error is rotated and ground in the forward and reverse directions to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy.

5. Tapping and threading

Tapping is to screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process the internal thread. Threading is to cut the external thread on the bar (or pipe) workpiece with a die. The machining accuracy of tapping or threading depends on the accuracy of the tap or die. Although there are many ways to process internal and external threads, small-diameter internal threads can only be processed by taps. Tapping and threading can be done manually, as well as lathes, drilling machines, tapping machines and threading machines.

6. Teeth rolling and rolling process

The processing method for mass production of external threads has high efficiency and good quality. Two threaded rods are used for thread rolling to squeeze the workpiece to be processed, so that the surface of the part is pressed out of threads. The tooth rolling uses two plate-like tools with inclined teeth to move toward each other to squeeze the workpiece, and form threads on the surface of the workpiece.

Hope the above can help you.

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