Bolt Thread Processing Expert Tips, Read This Article Is Enough!

Thread is mainly divided into connecting thread and driving thread.

For connecting thread, the main processing methods are tapping, threading, turning, rolling, rubbing, etc. For the transmission thread, the main processing methods are rough and fine turning — grinding, whirlwind milling — rough and fine turning, etc.

The following are various processing methods:

1. Thread cutting

Generally, it refers to the method of machining thread on a workpiece with a forming cutter or abrasive tool, mainly including turning, milling, tapping, threading, grinding, and whirlwind cutting, etc. When turning, milling and grinding thread, the transmission chain of the machine tool ensures that the turning tool, milling cutter, or grinding wheel can move one lead accurately and evenly along the axial direction of the workpiece. During tapping or threading, the tool (tap or die) rotates relative to the workpiece, and the tool (or workpiece) moves axially guided by the first formed thread groove.

Thread turning on a lathe can be done with form turning tool or thread comb tool (see thread processing tool). Turning thread with form turning tool is a common method for single piece and small batch production because of its simple structure; turning thread with thread cutter has high production efficiency, but the tool structure is complex, so it is only suitable for medium and large-scale production of short thread workpiece with small thread. In general, the pitch accuracy of turning trapezoidal thread by ordinary lathe can only reach 8-9 grade (JB2886-81, the same below); the productivity or accuracy can be significantly improved when machining thread on specialized thread lathe.

2. Thread milling

Milling is carried out on a thread milling machine with a disc cutter or a comb cutter. The disk milling cutter is mainly used for milling trapezoidal external threads on screw rod, worm, and other workpieces. The comb milling cutter is used to mill internal and external common threads and taper threads. Because it is milled by a multi-edge milling cutter and the length of its working part is larger than the length of the thread to be processed, the workpiece can be processed only by 1.25 ~ 1.5 rotation, and the productivity is very high. The pitch accuracy of thread milling can reach 8-9 grade, and the surface roughness is R 5-0.63 μm. This method is suitable for mass production of thread workpieces with general precision or rough machining before grinding.

3. Thread grinding

It is mainly used for machining precision thread of hardened workpiece on thread grinder. According to the different cross-section shapes of the grinding wheel, it can be divided into a single line grinding wheel and a multi-line grinding wheel. The results show that the pitch accuracy of the single line grinding wheel is 5-6 grade, and the surface roughness is R 1.25-0.08 μm, It is convenient for grinding wheel dressing. This method is suitable for grinding precision lead screw, thread gauge, worm, a small batch of the threaded workpiece, and relief grinding precision hob. Multi-line grinding wheel grinding is divided into a longitudinal grinding method and cut in the grinding method. 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 moving the wheel longitudinally once or several times. The grinding wheel width of the cut in the grinding method is larger than the length of the thread to be ground. The grinding wheel cuts into the surface of the workpiece radially, the workpiece can be completed after about 1.25 revolutions. The productivity is higher, but the precision is slightly lower, and the dressing of the grinding wheel is more complicated. The cut in the grinding method is suitable for relieving grinding taps with large batch and grinding some fastening threads.
4. Thread grinding

The nut type or screw type thread lapping tool made of soft materials such as cast iron is used to grind the parts of the machined thread with pitch error in forward and reverse rotation to improve the pitch accuracy. The deformation of the hardened internal thread is usually eliminated by grinding to improve accuracy.
5. Tapping and jacking

Tapping is to use a certain amount of twist to screw the tap into the pre-drilled bottom hole of the workpiece to process the internal threads. The sleeve is to use die to cut outward threads on the rod (or pipe) workpiece. The machining accuracy of tapping or sleeve depends on the precision of the tap or die. Although there are many ways to process internal and external threads, the internal threads of small diameter can only rely on tap processing. Tapping and threading can be done by hand, as can lathes, drill presses, tapping, and threading machines.

Selection principle of cutting parameters for thread lathe

Because the drawing specifies the pitch (or lead) of thread , the key to select the cutting parameters is to determine the spindle speed “n” and cutting depth “ap”.

1) Selection of spindle speed

According to the mechanism that the spindle rotates, one turn and the tool feeds one lead when turning thread, the selected spindle speed determines the feed speed of CNC lathe. The thread lead (pitch in case of the single thread) in the thread processing program section is equivalent to the feed speed “vf” expressed by feed rate “f (mm/r)”.
vf = n f (1)

It can be seen from the formula that the feed speed “vf” is directly proportional to the feed rate “f”. If the spindle speed of the machine tool is selected to be too high, the converted feed speed must be greatly higher than the rated feed speed of the machine tool. Therefore, the parameter setting of the feed system and the electrical configuration of the machine tool should be considered when selecting the spindle speed when turning thread, to avoid the occurrence of “disordered thread” or the pitch near the start/endpoint that does not meet the requirements.

Besides, it should be noted that once the thread processing is started, the spindle speed value can not be changed generally, and the spindle speed including finish machining must use the selected value during the first feed. Otherwise, the CNC system will cause the “disordered thread” because of the “overshoot” of the reference pulse signal of the pulse encoder.

2) Selection of cutting depth

Due to the poor tool strength, large cutting feed rate, and large cutting feed from thread turning to form turning, it is generally required to perform fractional feed machining and select a relatively reasonable cutting depth according to the decreasing trend. Table 1 lists the reference values of feed times and cutting depth for common metric screw thread cutting.