Thread Machining

Thread processing is to use thread processing tools to process a variety of internal and external thread methods.

First, thread cutting

Generally refers to the method of thread processing on the workpiece with forming tool or abrasive, mainly including turning, milling, tapping sleeve wire grinding, grinding and cyclone cutting, etc.When turning, milling and grinding threads, the drive chain of the machine ensures that the turning tool, milling cutter or grinding wheel moves one lead accurately and evenly along the axial direction of the workpiece at each revolution of the workpiece.

When tapping or setting the wire, the cutting tool (tap or die) and the workpiece for relative rotation, and formed by the first thread groove guide the cutter (or workpiece) for axial movement.

Two. Thread turning

Thread cutting on a lathe can be done with a forming tool or a thread comb.Turning thread with forming turning tool is a common method for single piece and small batch production of threaded workpiece because of its simple structure.Using the thread comb to turn the thread, the production efficiency is high, but the tool structure is complex, only suitable for medium, mass production of turning fine teeth of short thread workpiece.The pitch accuracy of turning trapezoidal thread on the general lathe can only reach 8 ~ 9 grades (JB2886-81, the same below).Threads can be machined on a specialized threading lathe with significantly higher productivity or accuracy.

Three, thread milling

Milling on a thread milling machine with a disk cutter or a comb cutter.Disc type milling cutter is mainly used for milling screw, worm and other workpiece on the trapezoidal external thread.Comb type milling cutter used for milling internal and external common threads and taper threads, because of the use of multi-edge milling cutter, the length of the working part is greater than the length of the thread is processed, so the workpiece only need to rotate 1.25 ~ 1.5 revolution can be processed, high productivity.

The pitch accuracy of thread milling can reach 8 ~ 9 grades, and the surface roughness is R5 ~ 0.63 micron.This method is suitable for mass production of general precision threaded workpiece or rough machining before grinding.

 

Four, thread grinding

It is mainly used for machining precision thread of hardened workpiece on thread grinder. It can be divided into single wire grinding wheel and multi-wire grinding wheel according to different section shape of grinding wheel.The single wire grinding wheel can reach the pitch precision of 5 ~ 6, the surface roughness of R1.25 ~ 0.08 micron, the grinding wheel dressing is more convenient.This method is suitable for grinding precision screw, thread gauge, worm, small batch thread work piece and shovel grinding precision hob.Multi – wire grinding wheel grinding is divided into longitudinal grinding method and cutting grinding method of two.In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the thread to be ground. The grinding wheel can grind the thread to the final size by moving the grinding wheel longitudinally once or several strokes.In the cutting grinding method, the width of the grinding wheel is larger than the length of the grinding thread. The grinding wheel is cut into the surface of the workpiece in the radial direction, and the workpiece can be grinded at about 1.25 revolutions. The productivity is higher, but the precision is slightly lower, and the dressing of the grinding wheel is more complex.The cutting – in grinding method is suitable for the tapping of large batch and the grinding of some fastening threads.

Five, thread grinding

In order to improve the precision of screw pitch, the screw grinding tool with nut or screw type is made of cast iron and other soft materials, and the parts with screw pitch error on the workpiece are rotated and ground in positive and negative direction.Hardened internal threads are also usually ground to eliminate distortion and improve accuracy.

Tapping and setting

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

The set of wire is used to cut the external thread on the bar (or pipe) workpiece with the die.The machining accuracy of tapping or sleeve depends on the precision of tap or die.Although there are many methods for processing internal and external threads, small diameter internal threads can only be processed by tapping.Tapping and blanking may be done by hand, or by lathes, drill presses, tapping machines and blanking machines.Thread rolling is generally carried out on the thread rolling machine or on the automatic lathe equipped with automatic thread rolling head. It is suitable for mass production of the external thread of standard fasteners and other thread connections.

 

The outer diameter of rolling thread is generally not more than 25 mm, and the length is not more than 100 mm. The precision of the thread can reach class 2 (GB197-63). The diameter of the blank is roughly equal to the middle diameter of the thread to be processed.Rolling generally can not process internal thread, but the material is soft workpiece can be used without groove extrusion tap cold extrusion internal thread (up to about 30 mm in diameter), the working principle is similar to tapping.The torsion required by cold extrusion of internal thread is about 1 times larger than tapping, and the machining accuracy and surface quality are slightly higher than tapping.

The advantages of thread rolling are:

(1) Surface roughness is less than turning, milling and grinding;

(2) the thread surface after rolling can improve the strength and hardness due to cold work hardening;

High material utilization rate;

(4) the productivity is doubled compared with the cutting process, and easy to achieve automation;

⑤ The rolling die has a long life.However, rolling thread requires that the hardness of the workpiece material does not exceed HRC40;The requirement of blank size precision is higher;The precision and hardness of the rolling die are also high, so it is difficult to manufacture the die.Not suitable for rolling thread with asymmetrical profile.According to the different rolling die, thread rolling can be divided into two categories: screw rolling and wire rolling.

The screw rolling is two screw rolling boards with screw tooth shape staggered 1/2 pitch relative arrangement, the static plate is fixed, the moving plate for parallel to the static plate reciprocating linear motion.When the workpiece is sent between the two plates, the moving plate forward rubbing the workpiece, so that its surface plastic deformation into threads.

There are three kinds of rolling wire: radial rolling wire, tangential rolling wire and rolling head rolling wire.

(1) Radial wire rolling: 2 (or 3) screw rolling wheels with thread tooth shape are installed on the shaft parallel to each other, the workpiece is placed on the support between the two wheels, the two wheels rotate in the same direction and at the same speed, and one of the wheels is also used for radial feed movement.The workpiece rotates under the drive of the wire roller, and the surface is radial extruded to form threads.A similar method can also be used for some lead screws with low accuracy.

(2) Tangential rolling wire: also known as planetary rolling wire, the rolling tool is composed of a rotating central rolling wire wheel and three fixed curved wire plates.During wire rolling, the workpiece can be continuously fed, so the productivity is higher than that of screw rolling and radial wire rolling.

③ Thread-rolling head: it is carried out on the automatic lathe, which is generally used to process the short thread on the workpiece.There are 3 ~ 4 rolling rollers distributed around the workpiece in the rolling head.When rolling the wire, the workpiece rotates, the rolling head feeds axially, and the workpiece is rolled out of the thread.


Post time: May-08-2021