Thread cutting
Generally, it refers to the method of processing threads on workpieces with formed cutting tools or grinding tools, mainly including turning, milling, tapping, threading, grinding, lapping and whirlwind cutting, etc.
When turning, milling or grinding threads, the machine tool’s transmission chain ensures that the turning tool, milling cutter or grinding wheel moves accurately and uniformly along the workpiece’s axial direction by one lead for each revolution of the workpiece.
During tapping or threading, the tool (tap or die) and the workpiece perform relative rotational motion, and the previously formed thread grooves guide the tool (or workpiece) to move axially.
On a lathe, thread cutting can be done using a form tool or a thread comb tool (see thread processing tools). Thread cutting with a form tool is a common method for single-piece and small-batch production of threaded parts due to its simple tool structure; thread cutting with a thread comb tool has high production efficiency, but its tool structure is complex and is only suitable for medium and large-batch production of short fine-threaded parts.
The pitch accuracy of trapezoidal threads cut on a general lathe is generally only up to grade 8 to 9 (JB2886-81, the same below); when processing threads on specialized thread lathes, productivity or accuracy can be significantly improved.
Thread milling
Milling is carried out on a thread milling machine using disc cutters or comb cutters. Disc cutters are mainly used for milling trapezoidal external threads on workpieces such as lead screws and worm gears. Comb cutters are used for milling internal and external common threads and conical threads.
As multi-edge cutters are used and the length of the working part is greater than the length of the thread to be machined, the workpiece only needs to rotate 1.25 to 1.5 turns to complete the processing, resulting in a high production rate.
The pitch accuracy of thread milling can generally reach grade 8 to 9, and the surface roughness is R5 to 0.63 microns. This method is suitable for batch production of general-precision threads or rough machining before grinding.
Thread grinding
It is mainly used for machining precision threads on hardened workpieces on a thread grinding machine.
Thread grinding is divided into single-line grinding and multi-line grinding according to the cross-sectional shape of the grinding wheel. Single-line grinding can achieve a pitch accuracy of 5 to 6 grades and a surface roughness of R1.25 to 0.08 microns, and the grinding wheel dressing is relatively convenient.
This method is suitable for grinding precision lead screws, thread gauges, worms, small-batch threaded parts and grinding and dressing precision hobs. Multi-thread grinding wheels grinding can be divided into longitudinal grinding method and plunge grinding method.
In the longitudinal grinding method, the width of the grinding wheel is less than the length of the thread to be ground, and the grinding wheel moves longitudinally once or several times to grind the thread to the final size.
In the plunge grinding method, the width of the grinding wheel is greater than the length of the thread to be ground, and the grinding wheel plunges into the surface of the workpiece radially. The workpiece can be ground well after about 1.25 rotations, with higher productivity but slightly lower accuracy.
The dressing of the grinding wheel is more complex. The plunge grinding method is suitable for grinding large-batch taps and grinding certain fastening threads.
Thread grinding
Thread grinding tools made of relatively soft materials such as cast iron in the form of nuts or screws are used to grind the parts with pitch errors on the already machined threads of the workpiece in both forward and reverse directions to improve the pitch accuracy. Hardened internal threads are usually also ground to eliminate variations and improve accuracy.
Tapping and threading
Tapping is the process of using a certain torque to screw a tap into a pre-drilled bottom hole in a workpiece to create an internal thread.
Threading is the process of cutting external threads on a bar (or tube) workpiece with a die. The machining accuracy of tapping or threading depends on the accuracy of the tap or die.
Although there are many methods for machining internal and external threads, small-diameter internal threads can only be machined with taps. Tapping and threading can be done manually or with lathes, drilling machines, tapping machines and threading machines.
Thread rolling
A processing method that uses a formed rolling die to cause plastic deformation of the workpiece to obtain a thread. Thread rolling is generally carried out on a thread rolling machine, a thread forming machine, or an automatic lathe equipped with an automatic opening and closing thread rolling head, and is suitable for the mass production of external threads of standard fasteners and other threaded connection parts.
The outer diameter of the rolled thread is generally no more than 25 millimeters, and the length is no greater than 100 millimeters. The thread accuracy can reach grade 2 (GB197-63). The diameter of all blanks is approximately equal to the pitch diameter of the thread to be processed.
Rolling generally cannot process internal threads, but for workpieces with relatively soft materials, internal threads can be cold extruded with a slotless extrusion tap (the maximum diameter can reach about 30 millimeters). The working principle is similar to tapping.
The torque required for cold extruding internal threads is about twice that of tapping, and the processing accuracy and surface quality are slightly higher than those of tapping.
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