Common machining methods of internal thread include tapping, turning, extrusion molding and thread milling. Now let's briefly understand the advantages and disadvantages of various process methods and their use limitations.
Tapping process
Tapping is a widely used internal thread processing method, which obtains the required thread contour with the help of the special geometry of the tap, so the processing is relatively simple, and does not need a special machine tool, but only needs to be operated on an ordinary machine tool or machining center.
During machining, the tap is first transmitted into the workpiece and cut, and then reversely leaves the workpiece after reaching the machining depth. In this way, the machining can be completed in a very narrow space and the chips can be discharged. The selection of taps depends on different processing materials and processing conditions. Tap tapping is often used in small diameter and mass production.
Turning
In internal thread machining, indexable inserts are often used to turn the workpiece to obtain the desired thread profile. This process is often used in triangular thread machining, because the axial section shape of the thread is very consistent with the shape of the large cutting edge. During processing, the turning tool must move a pitch distance longitudinally every time the workpiece rotates. If it is a multi head turning tool, the product of the pitch distance and the number of heads will be moved, so that the correct thread can be processed.
There are three common methods for turning triangular thread, including straight cutting method, oblique cutting method and left-right cutting method.
The straight feed method is a processing method that adopts the radial direction perpendicular to the axis of the workpiece after the trial cutting and inspection of the workpiece and the thread pitch meet the requirements, and repeats it for many times until the thread is turned. The biggest advantage of this method is that the tooth shape is relatively accurate, but cutting the two edges of the turning tool at the same time will cause poor chip removal, large cutting force, the turning tool is easy to be damaged, and the chip is also easy to scratch the thread surface.
The oblique feed method is a processing method that makes the turning tool feed along the thread profile side in the radial direction and at the same time, completes the thread processing after multiple tool walks, and then uses the straight feed method to eat the tool. This method is usually used when the workpiece pitch is greater than 3mm. The final straight cutting method can effectively ensure the tooth angle accuracy of the thread.
The left-right tool feeding method is a processing method that uses the horizontal carriage scale to control the vertical feed of the thread turning tool, and then uses the small carriage scale to control the left-right micro feed of the turning tool. It should be noted that when the thread is close to the completion of processing, the nut or thread gauge should be used to check whether the thread size and processing accuracy are qualified. This method is only used in ordinary lathes. It is easy to operate and widely used. It is generally used to process holes with larger diameter.
Extrusion processing
Extrusion processing is similar to tapping processing, first extruding the tap to screw it into the pre drilled hole, and then extruding the material in the axial and track directions, so as to form a unique tooth profile internal thread. This is a chip free processing method, which does not have the problems of chip removal and chip scratching the workpiece, but it has high requirements for the plastic deformation ability of materials, and the range of applicable materials is relatively small, which is most used in aluminum alloy processing.
thread mill
Thread milling is a processing method in which the milling cutter is lowered to the bottom of the screw hole first, and then the thread interpolation method is used to close to the workpiece. Every rotation cycle, the thread pitch rises along the axial direction of the screw hole. The advantage of this method is that the torque is small during processing, and the safety of the process is easy to be guaranteed; In addition, it also has good applicability. As long as the pitch is the same, you can use a tool to process internal threads with various thread diameters or tolerance ranges for different materials.
However, the disadvantages of this processing method are also obvious. It requires the use of three-axis CNC machine tools, which has low processing efficiency and relatively high cost. It is mainly used in processing small batches of large-diameter threaded holes.