The most comprehensive thread processing summary in history, decisively shared.

Thread is one of the common geometric features in mechanical engineering and is widely used. There are many thread processing technologies, such as thread rolling based on plastic deformation and thread rolling, turning, milling, tapping and tapping, thread grinding and thread grinding based on cutting.

I. Thread types

According to the tooth type, it can be divided into triangle, trapezoid, rectangle, sawtooth and arc thread;

It can be divided into left-handed and right-handed according to the thread direction;

According to the number of spirals, it can be divided into single line and multiple lines;

According to the shape of the thread matrix, it can be divided into cylindrical and conical.

Second, the elements of the thread.

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

1. Tooth type

On the cross section through the thread axis, the profile shape of the thread is called tooth profile. There are triangular, trapezoidal, sawtooth, circular arc and rectangular teeth.

Comparison of thread tooth types;

2. Diameter

Threads have large diameter (d, d), medium diameter (D2, d2) and small diameter (d 1, D 1), and the nominal diameter is used to indicate the thread size.

The nominal diameter of common thread is large diameter.

External thread (left)? Internal thread (right side)

3. Number of rows

Threads formed along spirals are called single threads, and threads formed by two or more spirals distributed at equal distances along the axial direction are called multi-head threads.

Single line (left) double line (right)

4. Pitch and lead

Pitch (p) is the axial distance between two corresponding points on two adjacent pitch lines.

Lead (ph) is the axial distance between two corresponding points on the pitch line of two adjacent teeth on the same helix.

Single thread, lead = pitch; When multiple threads are used, lead = pitch × number of threads.

5. Spin direction

The thread screwed in when rotating clockwise is called right-handed thread;

The thread screwed in when rotating counterclockwise is called left-handed thread.

Left-handed thread right-handed thread

Third, thread processing.

The method of machining various internal and external threads with thread machining tools.

1

Generally speaking, it refers to the method of machining threads on workpieces with forming tools or abrasives, mainly including turning, milling, tapping and grinding, grinding and cyclone cutting. When turning, milling and grinding threads, the transmission chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves a lead accurately and evenly along the axial direction of the workpiece every time the workpiece rotates. When tapping or tapping, the cutter (tap or die) moves relative to the workpiece, and the cutter (or workpiece) is guided by the pre-formed thread groove for axial movement.

2

Forming turning tool or thread combing tool can be used to turn threads on lathe (see thread processing tool). Because of its simple structure, forming turning tool is a common method to produce threaded workpieces in small batches. Using thread comb cutter to turn threads has high production efficiency, but the cutter structure is complex, which is only suitable for turning fine teeth and short threads in medium and large-scale production. Generally, the pitch accuracy of trapezoidal thread turning by ordinary lathe can only reach 8 ~ 9 grades (JB2886-8 1, the same below); Machining threads on a special thread lathe can significantly improve productivity or accuracy.

three

Milling with disc cutter or comb cutter on thread milling machine. Disc milling cutter is mainly used for milling trapezoidal external thread on screw, worm and other workpieces. Comb milling cutter is used for milling internal and external common threads and tapered threads. Because it is milled by a multi-edge milling cutter, the length of its working part is longer than the length of the thread to be machined, so the workpiece can be machined only by rotating 1.25 ~ 1.5 revolutions, and the productivity is very high. The pitch accuracy of thread milling can generally reach 8 ~ 9 grades, and the surface roughness is R5 ~ 0.63 microns. This method is suitable for mass production of threaded workpieces with average precision or rough machining before grinding.

four

It is mainly used to process the precision thread of hardened workpiece on thread grinder, and it can be divided into single-line grinding wheel and multi-line grinding wheel according to the different cross-sectional shapes of grinding wheels. Single-wire grinding wheel can achieve pitch accuracy of 5 ~ 6 and surface roughness of r 1.25 ~ 0.08 micron, which is convenient for grinding wheel dressing. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and shovel grinding precision hobs. Multi-line grinding wheel grinding is divided into longitudinal grinding method and cut-in grinding method. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the ground thread, and the thread can be ground to the final size by moving the grinding wheel longitudinally for one or several strokes. The width of grinding wheel in plunge grinding method is greater than the length of grinding thread. When the grinding wheel cuts into the surface of the workpiece radially, the workpiece can be ground in about 65438 0.25 revolutions, with high productivity, but slightly lower accuracy, and the grinding wheel dressing is more complicated. The cut-in grinding method is suitable for large-scale shovel grinding of taps and grinding of some fastening threads.

five

Nut-type or screw-type thread grinding tool is made of soft materials such as cast iron, and the part of the workpiece with pitch error is rotated back and forth to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy.

six

Tapping is to screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process internal threads.

Turning thread is to cut the external thread on the rod (or tube) workpiece with a mold. The machining accuracy of tapping or tapping depends on the accuracy of tap or die. Although there are many ways to process internal and external threads, small-diameter internal threads can only be processed with taps. Tapping and tapping can be done by hand, or by lathe, drilling machine, tapping machine and tapping machine.

seven

The processing method of plastic deformation of the workpiece by forming a rolling die to obtain thread rolling is generally carried out on a rolling machine or an automatic lathe with an automatic rolling head, which is suitable for mass production of external threads of standard fasteners and other threaded connectors. Generally, the outer diameter of the rolled thread is not more than 25mm, the length is not more than 100mm, the thread accuracy can reach Grade 2 (GB 197-63), and the diameter of the blank used is roughly equal to the middle diameter of the processed thread. Generally, the internal thread cannot be processed by rolling, but for soft workpieces (the maximum diameter can reach about 30 mm), the internal thread can be cold extruded by slotless extrusion tap, and the working principle is similar to tapping. The torque required for cold extrusion of internal thread is about 1 times that required for tapping, and the machining accuracy and surface quality are slightly higher than tapping.

The advantages of wire rolling are as follows: ① the surface roughness is less than that of turning, milling and grinding; ② Cold working hardening can improve the strength and hardness of the rolled thread surface; ③ High material utilization rate; (4) the productivity is doubled than that of cutting, and automation is easy to realize; ⑤ The rolling die has long service life. However, rolling thread requires that the hardness of workpiece material should not exceed HRC40, which requires high dimensional accuracy of blank; The precision and hardness of the rolling die are also high, and it is difficult to manufacture the die; Not suitable for asymmetrical thread of hobbing.

According to the different rolling dies, rolling threads can be divided into thread rolling and thread rolling.

Wire rolling: Two thread-tooth-shaped wire rolling plates are arranged oppositely, and the staggered spacing is 1/2. The static plate is fixed, and the moving plate moves along a reciprocating straight line parallel to the static plate. When a workpiece is fed between two plates, the moving plate moves forward to rub and press the workpiece, so that its surface is plastically deformed into a thread.

There are radial rolling, tangential rolling and rolling head rolling. Three kinds.

① Radial thread rolling: two (or three) thread rolling wheels are installed on parallel shafts, and the workpiece is placed on the bracket between the two wheels, and the two wheels rotate in the same direction and at the same speed, and one of them also performs radial feed movement. The workpiece is driven by the threaded roller to rotate, and the surface is radially extruded to form threads. For some lead screws with low precision requirements, a similar method can also be used for roll forming.

② Tangential thread rolling: also known as planetary thread rolling. The rolling tool consists of 1 roller with rotating center and 3 fixed arc wire plates. During wire rolling, the workpiece can be continuously fed, so the productivity is higher than that of wire rolling and radial wire rolling.

(3) Thread rolling: it is carried out on an automatic lathe and is generally used to process short threads on workpieces. There are 3 ~ 4 wire rollers evenly distributed around the workpiece in the rolling head. During wire rolling, the workpiece rotates, and the rolling head feeds axially to roll the workpiece into threads.

Common problems and solutions in thread processing