Thread processing is the method of processing various internal and external threads with thread processing tools. Today, we will elaborate on seven thread processing methods. Let’s take a look together!
1. Thread cutting:
Generally refers to the method of processing threads on workpieces with forming tools or grinding tools. The main methods include turning, milling, tapping, threading, grinding, lapping and whirlwind cutting, etc.
When turning, milling and grinding threads, the machine tool’s transmission chain ensures that the turning tool, milling cutter or grinding wheel moves accurately and uniformly along the workpiece’s axial direction by one lead for each revolution of the workpiece.
When tapping or threading, the tool (tap or die) and the workpiece perform relative rotational motion, and the tool (or workpiece) is guided by the previously formed thread grooves to move axially.
2. Thread Turning:
On a lathe, thread turning can be done using a form tool or a thread comb tool. Turning threads with a form tool is a common method for single-piece and small-batch production of threaded parts due to its simple tool structure.
Turning threads with a thread comb tool has high production efficiency but a complex tool structure, making it suitable for medium and large-batch production of short fine-threaded parts.
The pitch accuracy of trapezoidal threads turned on a general lathe is generally only 8 to 9 grades (JB2886-81, the same below); processing threads on specialized thread lathes can significantly improve production efficiency or accuracy.
3. Thread Milling:
Milling is performed on a thread milling machine using a disc milling cutter or a comb milling cutter. Disc milling cutters are mainly used for milling trapezoidal external threads on lead screws, worms, etc.
Comb milling cutters are used for milling internal and external common threads and conical threads. Due to the use of multi-edge milling cutters and the length of the working part being greater than the length of the thread to be processed, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the processing, achieving high production efficiency.
The pitch accuracy of thread milling is generally 8 to 9 grades, and the surface roughness is R5 to 0.63 microns. This method is suitable for batch production of general-precision threaded parts or rough machining before grinding.
4. Thread Grinding:
It is mainly used for grinding precision threads on hardened workpieces on specialized thread grinding machines. According to the different cross-sectional shapes of the grinding wheel, it can be divided into single-line grinding and multi-line grinding.
Single-line grinding can achieve a pitch accuracy of 5 to 6 grades, with a surface roughness of R1.25 to 0.08 microns, and the grinding wheel is relatively easy to dress.
This method is suitable for grinding precision lead screws, thread gauges, worms, and small-batch threaded parts, as well as dressing precision hobs.
Multi-line grinding is further divided into longitudinal grinding and plunge grinding. In longitudinal grinding, the width of the grinding wheel is less than the length of the thread to be ground, and the grinding wheel moves longitudinally one or several times to achieve the final size.
In plunge grinding, the width of the grinding wheel is greater than the length of the thread to be ground, and the grinding wheel plunges radially into the workpiece surface.
The workpiece only needs to rotate about 1.25 turns to complete the grinding, achieving high production efficiency but slightly lower accuracy, and the grinding wheel dressing is more complex. Plunge grinding is suitable for dressing large-batch taps and grinding certain fastening threads.
5. Thread Lapping:
Thread lapping tools made of cast iron or other soft materials in the form of nuts or screws are used to lap the parts of the already processed threads on the workpiece where there are pitch errors in both forward and reverse directions to improve pitch accuracy. Hardened internal threads are usually also ground to eliminate deformation and improve accuracy.
6. Tapping and Threading:
Tapping is the process of using a certain torque to screw a tap into a pre-drilled bottom hole in the workpiece to produce an internal thread. Threading is the process of cutting an external thread on a bar or tube workpiece using a die. The processing accuracy of tapping and threading depends on the accuracy of the tap or die.
Although there are many methods for processing internal and external threads, small-diameter internal threads can only be processed using taps. Tapping and threading can be done manually or using lathes, drilling machines, tapping machines, and threading machines.
A method of obtaining threads by causing plastic deformation of the workpiece with a forming rolling die. Thread rolling is generally performed on a thread rolling machine, a thread rolling and forming machine, or an automatic lathe equipped with an automatic opening and closing thread rolling head. It is suitable for large-batch production of standard fasteners and other threaded connection parts with external threads.
7. Thread Rolling:
The outer diameter of the rolled thread is generally no more than 25 millimeters, and the length is no more than 100 millimeters, with a thread accuracy of up to grade 2 (GB197-63). The diameter of the blank is approximately equal to the pitch diameter of the thread to be processed.
Generally, internal threads cannot be rolled, but for workpieces with relatively soft materials, internal threads can be cold extruded using a slotless extrusion tap (with a maximum diameter of about 30 millimeters), and the working principle is similar to tapping.
The torque required for cold extrusion of internal threads is approximately twice that of tapping, and the machining accuracy and surface quality are slightly higher than those of tapping.
The advantages of thread rolling are as follows:
① The surface roughness is smaller than that of turning, milling and grinding;
② The surface of the rolled thread is hardened due to cold working, which increases its strength and hardness;
③ The material utilization rate is high;
④ The productivity is several times higher than that of cutting processing and it is easy to achieve automation;
⑤ The service life of the rolling die is long. However, the hardness of the workpiece material for thread rolling should not exceed HRC40; the dimensional accuracy of the blank is required to be high; the precision and hardness of the rolling die are also high, making the manufacture of the die difficult; it is not suitable for rolling threads with asymmetric tooth profiles.
Thread rolling can be divided into two types according to the different rolling dies: threading and rolling.
>>> Threading is carried out by two threading dies with thread profiles offset by half a pitch and arranged opposite to each other. The static die is fixed, while the dynamic die moves back and forth parallel to the static die. When the workpiece is fed into the space between the two dies, the dynamic die advances and presses the workpiece, causing its surface to undergo plastic deformation and form a thread.
>>> Rolling includes radial rolling, tangential rolling and rolling head rolling.
① Radial rolling: Two (or three) rolling dies with thread profiles are installed on parallel shafts. The workpiece is placed on the support between the two dies. The two dies rotate at the same speed in the same direction, and one of them also performs radial feed motion. The workpiece rotates under the drive of the rolling dies, and its surface is radially extruded to form a thread. For some screws with lower precision requirements, a similar method can also be used for rolling.
② Tangential rolling: Also known as planetary rolling, the rolling tool consists of a rotating central rolling die and three fixed arc-shaped threading dies. During rolling, the workpiece can be continuously fed, so the productivity is higher than that of threading and radial rolling.
③ Rolling head rolling: It is carried out on an automatic lathe and is generally used for processing short threads on workpieces. The rolling head contains 3 to 4 evenly distributed rolling dies around the workpiece. During rolling, the workpiece rotates and the rolling head performs axial feed, rolling t
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