Thread processing method and technology

Thread is a commonly used connection method in mechanical engineering. As a basic and necessary element in mechanical design, the majority of designers must learn and master the thread in depth, not only to be able to select the thread correctly and reasonably, but also to understand Thread processing method and technology in order to solve the problems that may be encountered in production.

Thread processing methods mainly include: thread cutting: generally refers to the method of processing threads on a workpiece with a forming tool or abrasive tool, mainly including turning, milling, tapping, threading, grinding, grinding and whirlwind 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 along the axis of the workpiece accurately and uniformly for each revolution of the workpiece. When tapping or threading, the tool (tap or die) rotates relative to the workpiece, and the first formed thread groove guides the tool (or workpiece) to move axially. Thread rolling: A processing method that uses a forming rolling die to plastically deform a workpiece to obtain a thread.

1. Introduction to rolling

Compared with cutting methods, rolling is not a metal cutting process. It does not form the required contour by removing the metal, but forms the thread according to the mirror image of the roller drawing die, which is a non-cutting thread forming process. When rolling the thread, the roller mold presses the thread shape into the workpiece blank by moving the base material. As the roller displaces the material that will form the root, the material will flow out from the root in the radial and axial directions, and the shape of the roller will form a thread shape in the workpiece blank. The following figure shows the change of the material microstructure of rolling thread and cutting thread. (Guide: Material selection requirements for high-strength bolts)

Rolling is mainly used for the processing of external threads. Before processing, the diameter of the blank should be roughly equal to the pitch diameter of the thread. For higher-precision threads, the selection of the blank diameter should be determined according to material performance calculations and process tests. Exceeding or lowering the proper diameter will produce unqualified threads. If the outer diameter of the blank is too small, it will not be able to completely flow into the roller mold; if the size of the blank is too large, unnecessary pressure will be exerted on the roller and the roller frame, which may cause damage to the thread rolling equipment. Most of the rolled threads are 60° crest angle threads, and the rollers used have relatively sharp crests, usually with arc edges to facilitate penetration of the material. The flank of each tooth is 30°, which ensures the force required to make the material flow in a controlled and predetermined manner in the axial and radial directions. The picture below shows the threaded rolling wheel.

2. Rolling method

According to different rolling molds, thread rolling can be divided into thread rolling and thread rolling.

Two thread rolling boards with threaded teeth are arranged relative to each other with a staggered 1/2 pitch, the static plate is fixed, and the moving plate makes a reciprocating linear motion parallel to the static plate. When the workpiece is fed between the two plates, the movable plate advances and rubs the workpiece to plastically deform its surface into a thread. The thread rolling is carried out on the thread rolling machine or on the automatic lathe with automatic opening and closing thread rolling head. It is suitable for mass production of external threads of standard fasteners and other threaded connections. The efficiency is quite high, and it can reach 50 per minute. . The outer diameter of the thread processed by this method is generally no more than 25mm, and the length is no more than 100mm.

According to different feeding methods, thread rolling is divided into three types:

The axial threaded roller starts from the end of the turning center tailstock and moves along the center line of the workpiece blank to form a thread. The general processing range of axial rollers is 1.5~228mm in diameter.

The axial roller is usually installed in one of the turning center turret tool sleeves. One pass can feed 3 (or at most 6) rollers to the blank and start it by the rotation of the workpiece blank. The arrangement of the roller allows the blank to pass, so that a thread longer than the width of the roller can be formed. Some of the threads on these rollers are progressive, just like taps or broaches, and can be machined to the shoulder or other workpiece features, but the life of the rollers may be affected by additional stress.