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Processing Of Threads

Nov 27, 2019

1 thread cutting


Generally refers to the method of processing threads on the workpiece with forming tools or abrasive tools, which mainly include turning, milling, tapping and grinding, grinding, and cyclone cutting. When turning, milling, and grinding threads, each turn of the workpiece, the machine's drive chain ensures that the turning tool, milling cutter or grinding wheel moves accurately and evenly along the workpiece axis by a lead. When tapping or threading, the tool (tap or die) makes relative rotational movement with the workpiece, and the previously formed thread groove guides the tool (or workpiece) for axial movement.


2-thread turning


Turning threads on a lathe can be done with a profile turning tool or a thread comb (see Threading Tools). Turning threads with forming turning tools is a common method for single and small batch production of threaded workpieces due to the simple structure of the tool. Turning threads with a thread comb has high production efficiency, but the tool structure is complex and only suitable for medium and large-scale production Short-threaded workpiece with fine teeth turned. The pitch accuracy of turning a trapezoidal thread on an ordinary lathe can generally only reach the level of 8-9 (JB2886-81, the same applies hereinafter); when processing a thread on a specialized thread lathe, the productivity or accuracy can be significantly improved.


3 thread milling


Milling with a disc or comb cutter on a thread milling machine. Disc milling cutters are mainly used for milling trapezoidal external threads on workpieces such as screws and worms. Comb milling cutter is used for milling internal and external common threads and taper threads. Because it is milled with a multi-blade milling cutter and the length of its working part is longer than the length of the thread being processed, the workpiece can be processed only by rotating 1.25 to 1.5 Completed, high productivity. The pitch accuracy of thread milling can generally reach 8 to 9 grades, and the surface roughness is R5 to 0.63 microns. This method is suitable for batch production of general-precision threaded workpieces or roughing before grinding.


4 thread grinding


It is mainly used for processing precision threads of hardened workpieces on a thread grinder. There are two types of single-line and multi-line grinding according to the cross-sectional shape of the grinding wheel. The single-line grinding wheel can achieve a pitch accuracy of 5 to 6, and a surface roughness of R1.25 to 0.08 microns, which makes the dressing of the wheel more convenient. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and shovel grinding precision hobs. Multi-line grinding is divided into two types: longitudinal grinding and plunge grinding. The width of the grinding wheel in the longitudinal grinding method is smaller than the length of the thread to be ground, and the thread can be ground to the final size by one or several strokes of the wheel in the longitudinal direction. The width of the grinding wheel in the cut-in grinding method is greater than the length of the thread being ground. The grinding wheel is cut into the surface of the workpiece in a radial direction. The workpiece can be ground after about 1.25 revolutions. The productivity is higher, but the accuracy is slightly lower. The dressing of the grinding wheel is more complicated. The plunge-cut method is suitable for grinding larger taps and grinding certain fastening threads.


5Thread grinding


A nut-type or screw-type thread grinding tool is made of a softer material such as cast iron, and the parts of the processed thread on the workpiece are subjected to forward and reverse rotation grinding to improve the accuracy of the pitch. Hardened internal threads are usually ground to eliminate distortion and improve accuracy.


6 tapping and threading


Threading is to use a die to cut the external thread on the bar (or pipe) workpiece. The accuracy of tapping or threading depends on the accuracy of the tap or die. Although there are many methods for processing internal and external threads, small diameter internal threads can only be processed by taps. Tapping and threading can be performed manually, as well as lathes, drilling machines, tapping machines and threading machines.


7 thread rolling

Machining method for plastically deforming a workpiece with a forming rolling die to obtain a thread. Thread rolling is generally performed on a thread rolling machine or on an automatic lathe with an automatic opening and closing thread rolling head, which is suitable for mass production standards. External threads of fasteners and other threaded connections. The outer diameter of the rolled thread is generally not more than 25 mm, the length is not more than 100 mm, and the thread accuracy is up to level 2 (GB197-63). The diameter of the blank used is approximately equal to the middle diameter of the thread being processed. Rolling cannot generally process internal threads, but for softer workpieces, slotless extrusion taps can be used to cold-extrude internal threads (the maximum diameter can reach about 30 mm). The working principle is similar to tapping. The torque required for cold extrusion of internal threads is about twice as large as tapping, and the machining accuracy and surface quality are slightly higher than tapping.