The meaning of this word (screw) has changed a lot in the last few hundred years, at least in 1725, it means to mate.
The application of the thread principle can be traced back to 220 BC when the Greek scholar Archimedes created a spiral water lifting tool.
In the 4th century, the Mediterranean countries began to apply the principle of bolts and nuts to the press used in wine making. The outer threads of the time were made by winding a rope around a cylindrical bar and then marking it, while the inner threads were often hammered from a softer material around the outer threads.
Around 1500, Leonardo Da Vinci, an Italian, drew a sketch of the threaded device, which had the idea of using the female lead screw and the exchange gear to process different pitch threads. Since then, methods of mechanically cutting threads have developed in European watch manufacturing.
In 1760, English brothers J. and W. Wyatt received a patent for cutting wood screws with a special device. In 1778, British J. Ramsden made a screw cutting device driven by worm gear pair, which could process long threads with high precision. In 1797, H. Maudsley, an Englishman, used a lathe he improved to turn metal threads with different pitches using a female screw and a switching gear, and established the basic method of turning threads.
In the 1820s, Maudsley produced the first taps and dies for processing threads.
In the early 20th century, the development of automobile industry further promoted the standardization of thread and the development of various precision and efficient thread processing methods. Various automatic opening die heads and automatic shrinking taps were invented successively, and thread milling began to be applied.
In the early 1930s, thread grinding appeared.
Although the thread rolling technology was patented in the early 19th century, the development was slow due to the difficulty of mold manufacturing. It was not until the Second World War (1942 ~ 1945) that the precision of mold manufacturing was rapidly developed due to the needs of ammunition production and the development of thread grinding technology.
The thread is mainly divided into connection thread and drive thread
For the connection thread, the main processing methods are: tapping, sleeve wire, rolling wire, rubbing wire, etc.
On the transmission thread, the main processing methods are: rough finishing - grinding, cyclone milling - rough finishing - turning, etc.
The first category: thread cutting
Generally refers to the method of processing thread on workpiece with forming tool or grinding tool, mainly including turning, milling, tapping sleeve grinding, grinding and cyclone cutting. When turning, milling and grinding threads, the transmission chain of the machine ensures that the turning tool, milling cutter or grinding wheel moves accurately and evenly along the axis of the workpiece with each turn. When tapping or nesting, the tool (tap or die) rotates relative to the workpiece, and the first formed thread groove guides the tool (or the workpiece) to move in an axial direction.
01
Turning of Thread
Turning thread on lathe can use forming tool or thread comb. Because of the simple tool structure, it is a common method to produce single and small batch threaded workpiece. It has high production efficiency to turn the thread with the thread comb, but the tool structure is complex, which is only suitable for the short thread workpiece with fine teeth in medium and mass production. The pitch accuracy of ordinary lathe turning trapezoidal thread can only reach class 8 ~ 9 (JB2886-81, the same below). When threading is processed on a specialized thread lathe, productivity or accuracy can be significantly increased.
02
Milling of Thread
Milling is done on a thread milling machine with a disk or comb cutter.
Disc milling cutter is mainly used for milling the trapezoidal external thread on the workpiece such as screw and worm. Comb milling cutter is used for milling internal and external common thread and cone thread. Because it is milling with multi-edge milling cutter, the length of its working part is greater than the length of the thread being processed, so the workpiece only needs to rotate 1.25 ~ 1.5 turns to complete the processing, and the productivity is very high. The pitch precision of thread milling can reach 8 ~ 9, and the surface roughness is R5 ~ 0.63 micron. This method is suitable for mass production of threaded workpiece with general precision or roughing before grinding.
03
Tread Grinding
Thread grinding is mainly used to process the precision thread of the hardened workpiece on the thread grinding machine, which can be divided into single line grinding wheel and multi-line grinding wheel according to different cross-section shapes of the grinding wheel. Single-line grinding wheel can achieve pitch accuracy of 5 ~ 6, surface roughness of R1.25 ~ 0.08 micron, the grinding wheel dressing is convenient. This method is suitable for grinding precision lead screw, thread gauge, worm, small batch thread workpiece and shovel grinding precision hob. There are two kinds of grinding methods: longitudinal grinding and cutting grinding. The width of the grinding wheel in the longitudinal grinding method is less than the length of the thread to be ground, and the thread can be ground to the final size by moving the grinding wheel longitudinally for one or several strokes. In the cut-in grinding method, the width of the grinding wheel is greater than the length of the grinding thread, and the radial cut of the grinding wheel into the workpiece surface, and the workpiece can be grinded well at about 1.25 revolutions. The productivity is higher, but the precision is slightly lower, and the grinding wheel dressing is more complicated. The cut-in grinding method is suitable for scooping large batch taps and grinding some fastening threads.
04
Grinding of Thread
A nut or screw thread tool made of soft materials such as cast iron is used to rotate and grind the parts with pitch errors on the processed threads in positive and negative directions to improve the pitch accuracy. The hardened internal threads are also often ground to eliminate deformation and improve accuracy.
05
Tapping and Nesting
Tapping
It is to screw the tap into the bottom hole pre-drilled on the workpiece with a certain torsion pitch to process the internal thread.
Set of silk
The die is used to cut out the external thread on the bar material (or tube material) workpiece. The precision of tapping or nesting depends on the precision of the tap or die.
Although there are many ways to process internal and external threads, small diameter internal threads can only rely on tap processing. Tapping and bushing can be manually operated, as well as lathes, drill presses, tapping and bushing machines.
The second category: thread rolling
A process in which the workpiece is plastic deformed by forming a rolling die to obtain a thread. The thread rolling is usually done in a wire rolling machine or on an automatic lathe equipped with a self-opening and closing thread rolling head. External thread suitable for mass production of standard fasteners and other threaded connectors. The outer diameter of the rolling thread is generally not more than 25 mm, the length is not more than 100 mm, the precision of the thread can reach 2 level (GB197-63), the diameter of the blank used is roughly equal to the middle diameter of the processed thread. Generally, the internal thread can not be processed by rolling, but the slot-free extrusion tap can be used for the soft workpiece (the maximum diameter can be up to about 30 mm). The working principle is similar to tapping. Cold extrusion of internal thread requires about 1 times more torsion than tapping, machining accuracy and surface quality is slightly higher than tapping.
The advantages of thread rolling: ① the surface roughness is less than that of turning, milling and grinding; ② The surface of the thread after rolling can improve the strength and hardness because of cold work hardening; ③ High material utilization rate; (4) The productivity increases exponentially compared with the cutting process, and it is easy to realize automation; The rolling die has a long life. But the hardness of the workpiece material is not more than HRC40; The size accuracy of the blank is high. The precision and hardness of the rolling die are also high, making the die more difficult; Not suitable for rolling threads with asymmetric dentition.
According to the different rolling die, thread rolling can be divided into two categories: rolling wire and rolling wire.
06
Thread Rolling
07
Thread Rolling
There are three kinds of radial rolling wire, tangential rolling wire and rolling head rolling wire.
(1) Radial wire rolling: 2 (or 3) screw tooth rolling wheels are installed on the axis parallel to each other, and the workpiece is placed on the support between the two wheels. The two wheels rotate in the same direction and at the same speed (FIG. 7 [radial wire rolling]), one of which also makes radial feed movement. The workpiece rotates under the drive of the wire roller, and the surface is radial extruded to form a thread. A similar method can also be used to roll the lead screw which does not require high precision.
(2) Tangential rolling wire: also known as planetary rolling wire, the rolling tool is composed of a rotating central rolling wheel and three fixed arc wire plates (FIG. 8[Tangential rolling wire]). When rolling wire, the workpiece can be continuously fed, so the productivity is higher than the rolling wire and radial rolling wire.
(3) Wire rolling head Wire rolling: on the automatic lathe, generally used for processing the short thread on the workpiece. In the rolling head, there are 3 to 4 rolling wheels arranged around the workpiece (FIG. 9[rolling head rolling wire]). When the wire is rolling, the workpiece rotates and the rolling head feeds axially to roll the workpiece out of the thread.
08
Edm Thread machining
Common threads are usually processed by machining centers or tapping equipment and tools, sometimes by hand. However, in some special cases, the above method is not easy to obtain good machining results, such as the need to process threads after heat treatment of parts due to negligence, or because of material constraints, such as the need to tap directly on the hard alloy workpiece. At this time, we need to consider the way of EDM processing.
Compared with the machining method, the sequence of EDM is the same. It is necessary to drill the bottom hole first, and the diameter of the bottom hole should be determined according to the working condition. The electrode needs to be machined into a thread shape and the electrode needs to be able to rotate during machining.
Contact Person: Mr. Frank Li
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