The application of the screw thread principle can be traced back to the screw water-lifting tool created by the Greek scholar Archimedes in 220 BC. In the 4th century AD, the principle of bolts and nuts began to be applied in wine presses along the Mediterranean coast.
At that time, the external threads were made by winding a rope around a cylindrical bar and then cutting according to the marks, while the internal threads were often formed by hammering a softer material wrapped around the external threads.
Around 1500, Leonardo da Vinci, an Italian, had already sketched a design for a thread processing device that included the idea of using a lead screw and exchange gears to machine threads of different pitches. Since then, the method of mechanically cutting threads has developed in the European watchmaking industry.
In 1760, the British brothers J. Wyatt and W. Wyatt were granted a patent for a special device to cut wood screws. In 1778, the British J. Ramsden made a screw thread cutting device driven by a worm gear pair, which could produce very precise long threads. In 1797, the British H. Maudslay, on a lathe improved by himself, used a lead screw and exchange gears to turn metal threads of different pitches, laying the foundation for the basic method of turning threads.
In the 1820s, Maudslay produced the first taps and dies for threading. At the beginning of the 20th century, the development of the automotive industry further promoted the standardization of threads and the development of various precise and efficient thread processing methods. Various automatic opening die heads and automatic contracting taps were invented one after another, and thread milling began to be applied. In the early 1930s, thread grinding emerged.
Although the thread rolling technology was patented in the early 19th century, it developed very slowly due to the difficulty in manufacturing molds. It was not until the period of World War II (1942-1945), when the demand for military production and the development of thread grinding technology solved the precision problem in mold manufacturing, that it achieved rapid development.
Screws are mainly divided into connecting screws and transmission screws.
- For connection threads, the main processing methods are: tapping, threading, turning, rolling, and die threading, etc.
- For transmission threads, the main processing methods are: rough and finish turning – grinding, whirlwind milling – rough and finish turning, etc.
The first category: Thread cutting generally refers to the method of processing threads on workpieces using forming tools or grinding tools, mainly including turning, milling, tapping, threading, grinding, lapping and whirlwind cutting, etc.
When turning, milling or grinding threads, for each revolution of the workpiece, 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. When tapping or threading, the tool (tap or die) and the workpiece perform relative rotational motion, and the previously formed thread grooves guide the tool (or workpiece) to move axially.
01 Thread Turning
Threads can be turned on a lathe using a form tool or a thread comb tool.
Turning threads with a formed 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 its tool structure is complex and is only suitable for medium and large-scale production of short fine-threaded parts.
The pitch accuracy of trapezoidal threads machined on a general lathe is usually only up to grade 8 to 9 (JB2886-81, the same below); when threads are processed on specialized thread lathes, productivity or accuracy can be significantly improved.
02 Thread Milling
Milling is carried out on a thread milling machine using a disc cutter or a comb cutter.
The disc milling cutter is mainly used for milling trapezoidal external threads on workpieces such as lead screws and worm gears.
Comb-shaped milling cutters are used for milling internal and external common threads and conical threads. As they are milled with multi-edge milling cutters and the length of their working parts is greater than that of the threads to be processed, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the processing, which results in a very high productivity.
The pitch accuracy of thread milling can generally reach grade 8 to 9, 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.
Internal threads are machined with thread milling cutters.
03 Thread Grinding
It is mainly used for grinding precision threads on hardened workpieces on thread grinding machines. According to the different cross-sectional shapes of the grinding wheel, it can be divided into single-thread grinding wheel and multi-thread grinding wheel grinding.
Single-line grinding wheels can achieve pitch accuracy of grade 5 to 6, with surface roughness ranging from R1.25 to 0.08 microns. The dressing of grinding wheels is relatively convenient. This method is suitable for grinding precision lead screws, thread gauges, worms, small-batch threaded workpieces, and grinding precision broaches.
Multi-line grinding with grinding wheels can be 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 once or several times to achieve the final size of the thread. In plunge grinding, the width of the grinding wheel is greater than the length of the thread to be ground.
The grinding wheel plunges into the workpiece surface radially, and the thread can be ground in about 1.25 rotations of the workpiece. This method has a higher production rate but slightly lower precision, and the dressing of the grinding wheel is more complex. Plunge grinding is suitable for grinding large batches of taps and grinding certain types of fastening threads.
04 Thread grinding
Screw thread grinding tools made of relatively soft materials such as cast iron in the form of nuts or screws are used to grind the parts of the already machined threads on the workpiece that have pitch errors in both forward and reverse rotations to improve pitch accuracy. Hardened internal threads are usually also ground to eliminate deformation and improve accuracy.
05 Tapping and Threading
Tapping: It is the process of using a certain torque to screw a tap into a pre-drilled bottom hole in a workpiece to create an internal thread.
Threading: It is the process of cutting external threads on a bar (or tube) workpiece with a die. The machining accuracy of tapping or 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 by taps. Tapping and threading can be done manually or by using lathes, drilling machines, tapping machines and threading machines.
The second category: Thread Rolling
The processing method of making the workpiece undergo plastic deformation to obtain threads by using a formed rolling die is called thread rolling. Thread rolling is generally carried out on a thread rolling machine, a thread forming machine or an automatic lathe equipped with an automatic opening and closing thread rolling head. It is suitable for the mass production of external threads of standard fasteners and other threaded connection parts.
The outer diameter of the rolled thread is generally no more than 25 millimeters, and the length is no greater than 100 millimeters. The thread accuracy can reach grade 2 (GB197-63). The diameter of the blank used is approximately equal to the pitch diameter of the thread to be processed.
Rolling generally cannot be used to process internal threads, but for workpieces with relatively soft materials, internal threads can be cold extruded with a slotless extrusion tap (with a maximum diameter of about 30 millimeters), and its working principle is similar to that of 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 better than those of tapping.
Advantages of thread rolling:
① The surface roughness is smaller than that of turning, milling and grinding.
② The surface of the rolled thread can increase strength and hardness due to cold working hardening.
③ High material utilization rate.
④ The productivity is several times higher than that of cutting processing and is easy to achieve automation.
⑤ The service life of the rolling die is very long.
However, the hardness of the workpiece material for rolling threads should not exceed HRC40; the dimensional accuracy of the blank is required to be relatively high; the precision and hardness of the rolling die are also high, and the manufacturing of the die is relatively difficult; it is not suitable for rolling threads with asymmetrical thread profiles.
According to the different rolling dies, thread rolling can be divided into two types: thread forming and thread rolling.
06 Thread Rolling
Two threading dies with thread profiles are arranged opposite each other with a pitch offset of 1/2. The static die remains stationary while the dynamic die moves back and forth in a straight line parallel to the static die. When the workpiece is fed between the two dies, the dynamic die advances and extrudes the workpiece, causing its surface to undergo plastic deformation and form a thread.
07 Thread Rolling
There are three types: radial thread rolling, tangential thread rolling and thread rolling with a rolling head.
① Radial threading: Two (or three) threading wheels with thread profiles are installed on parallel shafts. The workpiece is placed on the support between the two wheels. The two wheels rotate at the same speed in the same direction, and one of the wheels also performs radial feed motion. The workpiece rotates under the drive of the threading wheels, and the surface is radially extruded to form threads. For some lead screws with lower precision requirements, a similar method can also be used for rolling forming.
② Tangential threading: Also known as planetary threading, the rolling tool consists of one rotating central threading wheel and three fixed arc-shaped threading plates. During threading, the workpiece can be continuously fed, so the productivity is higher than that of thread rolling and radial threading.
③ Thread rolling with thread rolling head: This process is carried out on an automatic lathe and is generally used for processing short threads on workpieces. The thread rolling head contains 3 to 4 thread rolling wheels evenly distributed around the outer periphery of the workpiece. During thread rolling, the workpiece rotates while the thread rolling head feeds axially, thereby rolling the thread onto the workpiece.
08 Electrical Discharge Thread Machining
The processing of common threads is generally carried out using machining centers or tapping equipment and tools. Sometimes, manual tapping can also be done.
However, in some special circumstances, the above-mentioned methods are not easy to achieve good processing results. For instance, due to negligence, it is necessary to process threads after the heat treatment of parts, or due to material limitations, such as directly tapping on hard alloy workpieces. At such times, the processing method of electrical discharge machining needs to be considered.
Compared with the mechanical machining method, the sequence of electrical discharge machining is the same. Both require the creation of a pilot hole first, and the diameter of the pilot hole should be determined based on the working conditions. The electrode needs to be processed into a threaded shape, and during the processing, the electrode needs to be able to rotate.
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