The Screw Thread
Feb 15, 2020
Threads are shapes made up of lines, and there are many types of threads. The most intuitive is the convex section of a spiral line formed on the surface of a cylinder or cone with a specific section. The thread is divided into cylindrical thread and conical thread according to its parent shape. According to its position in the matrix, it is divided into external thread and internal thread, and according to its section shape (tooth type), it is divided into triangle thread, rectangle thread, trapezoid thread, serrated thread and other special shape threads. Triangle thread is mainly used for connection, while rectangle, trapezoid and serrated thread are mainly used for driving. According to the direction of the helix is divided into left and right screw threads, generally with the right screw thread; According to the number of screw threads are divided into single-thread thread, double-thread thread and multi-thread thread; The connection is mostly single wire, and the transmission is double-wire or multi-wire; According to the size of the tooth is divided into coarse thread and fine thread, according to the different occasions and functions, can be divided into fastening thread, pipe thread, drive thread, special thread.
On the surface of a cylinder or cone, a continuous projection with a defined tooth shape formed along a helix. The bulge refers to the solid part on both sides of the thread. Also called teeth.
In machining, the thread is cut on a cylindrical shaft (or the surface of the inner hole) by a cutter or grinding wheel. At this point, the workpiece is rotated and the cutter moves a certain distance along the axial direction of the workpiece. The trace cut by the cutter on the workpiece is the thread. Threads formed on the outer surface of the circle are called external threads. Threads formed on the surface of the inner hole are called internal threads. The base of the thread is the helix on the surface of the axis. Usually, if the section of the thread is triangular, it is called triangular thread; The section is trapezoidal called trapezoidal thread; The section is serrated and is called serrated thread; The cross section is square is called square thread; The section is circular arc is called circular arc thread and so on.
The main classification
Threads are divided into triangular threads, rectangular threads, trapezoidal threads and serrated threads according to their cross section shape (tooth type). Triangular threads are mainly used for connection (see thread connection), while rectangular, trapezoid and serrated threads are mainly used for driving. Threads distributed on the outside of the mother are called external threads, and those on the inside of the mother are called internal threads. The thread formed on the cylinder matrix is called cylindrical thread, and the thread formed on the cone matrix is called conical thread. The thread is divided into two kinds according to the helical direction of the left turn and the right turn, generally with the right turn thread. Threads can be divided into single-line and multi-line, connection with the majority of single line; For the transmission of fast or high efficiency, using two or more lines, but generally not more than 4 lines.
Triangle thread is mainly used for connection, while rectangle, trapezoid and serrated thread are mainly used for driving. According to the direction of the helix is divided into left and right screw threads, generally with the right screw thread; According to the number of screw threads are divided into single-thread thread, double-thread thread and multi-thread thread; The connection is mostly single wire, and the transmission is double-wire or multi-wire; According to the size of the tooth is divided into coarse thread and fine thread, according to the different occasions and functions, can be divided into fastening thread, pipe thread, drive thread, special thread.
Triangle thread has good self-locking performance in cylindrical thread. It divides the coarse tooth and the fine tooth two kinds, the general connection multipurpose coarse tooth thread. The screw pitch of fine teeth is small, the raising Angle is small, the self-locking performance is better, often USES in the small part thin wall tube, has the vibration or the variable load connection and the fine adjustment device and so on. Pipe threads are used for tight fitting. Rectangular thread has high efficiency, but it is often replaced by trapezoidal thread because it is not easy to grind and the inner and outer threads are difficult to screw. Serrated thread teeth have working edges close to the straight rectangular edges and are often used to withstand unidirectional axial forces.
The tooth type of conical thread is triangular, which mainly depends on the deformation of teeth to ensure the tightness of the thread pair.
Main geometric parameters of cylindrical thread
(1) outside diameter (large diameter), the diameter of the imaginary cylinder that coincides with the top or the bottom of the thread. The nominal diameter of the thread is the large diameter.
(2) inside diameter (footpath), an imaginary cylinder diameter that coincides with the bottom or top of the outer thread.
(3) middle diameter, the bus bar passes through the imaginary cylinder diameter of the same width as the bulge and groove on the tooth shape.
(4) pitch, adjacent teeth in the middle diameter line corresponding to the axial distance between two points.
(5) lead, the axial distance between two adjacent teeth on the same spiral line and the middle diameter line.
6. Tooth Angle, the included Angle between two adjacent teeth on the thread tooth.
The Angle between the tangent line of the helix on the middle diameter cylinder and the plane perpendicular to the thread axis.
The height of the work today, the two phase matching thread on the coincident part of the thread in the direction of perpendicular to the axis of the distance. The nominal diameter of the thread is the nominal diameter of the outer diameter of the thread except the inner diameter of the pipe for the pipe thread. Threads have been standardized in metric (metric) and inch systems. The international standard USES metric system, China also USES metric system.
The outside diameter is the nominal diameter of all threads except the inside diameter of pipe. The screw pair whose rise Angle is less than the friction Angle will not loose under the action of axial force, which is called self-locking, and its transmission efficiency is low.
In cylindrical threads, triangle threads have good self-locking performance. It divides the coarse tooth and the fine tooth two kinds, the general connection multipurpose coarse tooth thread. The screw pitch of fine teeth is small, the raising Angle is small, the self-locking performance is better, often USES in the thin wall pipe of small part, has the vibration or the variable load connection, and the fine adjustment device and so on.
The tooth type of taper thread is triangular, which mainly depends on the deformation of teeth to ensure the tightness of the thread pair.
The standard drawing
In mechanical drawing, the view drawing of thread and thread fastener is more complicated. In order to improve the efficiency of drawing, the prescribed drawing method is usually adopted (see figure). The international standard ISO and the Chinese standard for mechanical drawing both specify that the top of the thread shall be represented by a thick solid line, and the bottom of the tooth shall be represented by a thin solid line. In the view perpendicular to the projection plane of the thread axis, the thin solid line shall only be represented by 3/4 turns, and the termination limit of the thread shall be represented by a thick solid line. Tooth type, diameter, pitch of thread...... And so on in the marking size marked with thread code. In the sectional view of the assembly drawing, it is stipulated that the threaded fasteners are drawn according to the uncut, and a simplified drawing method is stipulated for common threaded fasteners, such as a set of hexagon bolts, hexagon nuts and washers can be drawn according to the drawing method in the drawing.
In the standard thread marking method stipulated by the national standard, the first letter stands for the thread code, such as: M for common thread, G for pipe thread sealed without thread, R for pipe thread sealed with thread, Tr for trapezoid thread, etc. The second number represents the thread's nominal diameter, the larger diameter of the thread. It represents the maximum diameter of the thread in millimeters. The following symbols are pitch, rotation, middle diameter tolerance code, top diameter tolerance code, screw length code.
M6 means ordinary thread with nominal diameter of 6mm.
The method of machining the thread directly by means of a die
A method of obtaining threads by plastic deformation of a workpiece by forming a rolling die. Thread rolling is usually done on a thread rolling machine. Thread rolling machine or on automatic lathes with automatic open and close thread rolling head, suitable for mass production of external threads of standard fasteners and other threaded couplings. The outside diameter of the rolled thread shall not exceed 25mm, the length shall not exceed 100mm, and the thread accuracy shall be up to grade 2 (gb197-63). The diameter of the blank used shall be approximately equal to the middle diameter of the processed thread. Rolling generally can not process the internal thread, but for soft materials can be used to cold extrusion tap tap internal thread (maximum diameter up to 30 mm or so), the working principle is similar to tapping. The torque for cold extruding internal thread is about 1 times larger than tapping, and the machining precision and surface quality are slightly higher than tapping.
Advantages of thread rolling: less surface roughness than turning, milling and grinding; The surface of the rolled thread can be hardened by cold work to improve the strength and hardness. High material utilization rate; Productivity is doubled than cutting, and it is easy to achieve automation; Rolling die life is very long. However, the rolling thread requires that the hardness of the workpiece material does not exceed HRC40; Higher requirement on blank size precision; The precision and hardness of the rolling mould are also high, making the mould more difficult; Not suitable for rolling thread with asymmetrical tooth shape.
According to the different rolling mould, the thread rolling can be divided into two types: rolling thread and rolling thread.
Screw thread two screw thread plates with thread tooth shape staggered 1/2 pitch relative arrangement, static plate fixed fixed, moving plate parallel to the static plate reciprocating linear motion. When the workpiece is sent between the two plates, the moving plate forward to rub the workpiece, so that the surface plastic deformation into threads.
There are three kinds of rolling wire: radial rolling wire, tangential rolling wire and rolling head rolling wire.
Radial wire rolling: 2 (or 3) thread thread shaped wire rolling wheels are installed on parallel shafts, the workpiece is placed on the support between the two wheels, the two wheels are rotating in the same direction and at the same speed, one of which also performs radial feeding motion. The workpiece is driven by the roller, and the surface is squeezed to form thread. A similar method can also be used to roll the lead screw with low precision.
Tangential rolling wire: also known as planetary rolling wire, the rolling tool consists of a rotating central roller and three fixed arc wire plates. When rolling, the workpiece can be continuously fed, so the productivity is higher than rolling and radial rolling.
Thread rolling: used in an automatic lathe to process short threads on a workpiece. There are 3 ~ 4 roller wheels arranged around the workpiece in the roller head. When the thread is rolled, the workpiece is rotated and the roller head is fed axially to roll the workpiece out of the thread.
A method of machining threads on a workpiece with a forming tool or abrasive.
Thread milling: milling on a thread milling machine with a disc or comb cutter. Disk milling cutter is mainly used for milling screw, worm and other workpiece
Trapezoidal external thread. Comb milling cutter for milling internal, external common thread and taper thread, because it is milling with a multi-edge milling cutter, the length of its working part is greater than
The length of the thread is processed, so the workpiece only needs to be rotated 1.25 ~ 1.5 to complete the processing, high productivity. The screw pitch precision of thread milling can reach 8 ~ 9 grades and the surface roughness is R5 ~ 0.63 micron. This method is suitable for batch production of threaded workpieces of general accuracy or for roughing before grinding.
In science and technology developed technology advanced today's processing center has become an irreplaceable tool for the production enterprises, so more and more thread processing is milling processing, high efficiency and high precision to bring more benefits to the enterprise. In order to meet this demand, many companies should be born. Provide professional solutions for special thread requirements.
Thread grinding: it is mainly used for machining precision threads of hardened workpiece on the thread grinding machine. There are two kinds of grinding wheels: single-thread grinding wheel and multi-thread grinding wheel according to different shapes of grinding wheel section. The grinding precision of single-thread grinding wheel is 5 ~ 6, the surface roughness is R1.25 ~ 0.08 micron, and the dressing of grinding wheel is more convenient. This method is suitable for grinding precision screw screw, thread gauge, worm, small batch of threaded workpiece and 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 being ground. The width of the grinding wheel cut into the grinding method is larger than the length of the thread being ground, and the grinding wheel is cut into the surface of the workpiece radially. The workpiece can be ground after about 1.25 turns. The productivity is higher, but the accuracy is lower, so the dressing of the grinding wheel is more complicated. The cut-in grinding method is suitable for shoveling larger taps and grinding some fastening threads.
Thread grinding: using soft materials such as cast iron to make nut or screw type thread grinding tools, the workpiece has been processed screw pitch error parts of the positive and negative rotation grinding, in order to improve the pitch precision. Hardened internal threads are also usually ground to eliminate deformation and improve accuracy.
Tapping and bushing: tapping is to screw the tap into the pre-drilled bottom hole of the workpiece with a certain torque to produce the internal thread. The thread is cut with die on rod (or tube) workpiece. The machining accuracy of tapping or bushing depends on the accuracy of tap or die. Although there are many ways to process internal and external threads, the internal thread with small diameter can only rely on tap processing. Tapping and threading can be done by hand or by lathes, drillers, tapping machines and threading machines.