Thread cutting methods and depth - PAY for CNC components
Thread cutting methods and depth
 The thread is mainly divided into connecting thread and driving thread

For connecting threads, the main processing methods are: tapping, threading, threading, thread rolling, thread rolling, etc.

For transmission threads, the main processing methods are: rough and fine turning---grinding, whirlwind milling---coarse and fine turning, etc.

The following describes various processing methods:

1. Thread cutting

Generally refers to the method of machining threads on a workpiece with a forming tool or an abrasive tool, mainly including turning, milling, tapping, threading, grinding, grinding and whirlwind cutting. When turning, milling and grinding threads, the transmission chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves exactly and evenly along the axis of the workpiece by one lead for each revolution of the workpiece. When tapping or threading, the tool (tap or die) rotates relative to the workpiece, and the first formed thread groove guides the tool (or workpiece) to move axially.

Thread turning. Turning threads on a lathe can use a forming turning tool or a thread comb (see thread processing tools). Turning threads with a forming turning tool is a common method for single-piece and small batch production of threaded workpieces due to the simple tool structure. Turning threads with a threading tool has high production efficiency, but the tool structure is complex and is only suitable for turning in medium and mass production. Short threaded workpieces with fine pitch. The pitch accuracy of ordinary lathes for turning trapezoidal threads can generally only reach 8-9 (JB2886-81, the same below); when threading on specialized threading lathes, productivity or accuracy can be significantly improved.

2. Thread milling

Milling is performed on a thread milling machine with a disk milling cutter or a comb milling cutter. Disc-shaped milling cutters are mainly used for milling trapezoidal external threads on screw, worm and other workpieces. The comb milling cutter is used for milling internal and external common threads and taper threads. Because it is milled with a multi-edge milling cutter and the length of its working part is greater than the length of the thread being machined, the workpiece can be machined only by rotating 1.25 to 1.5 revolutions Finished, productivity is high. The pitch accuracy of thread milling can generally reach 8-9, and the surface roughness is R 5 to 0.63 microns. This method is suitable for mass production of threaded workpieces with general accuracy or rough machining before grinding.

3. Thread grinding

Mainly used for machining precision threads of hardened workpieces on thread grinders. According to the different cross-sectional shape of the grinding wheel, there are two types of single-line grinding wheel and multi-line grinding wheel grinding. The pitch accuracy that can be achieved by single-line grinding wheel grinding is 5 to 6, and the surface roughness is R 1.25 to 0.08 microns. The grinding wheel dressing is more convenient. This method is suitable for grinding precision lead screws, thread gauges, worms, small batches of threaded workpieces and relieving precision hobs. Multi-line grinding wheel grinding is divided into two types: longitudinal grinding method and plunge grinding method. The width of the grinding wheel of the longitudinal grinding method is smaller than the length of the thread to be ground. The thread can be ground to the final size by moving the grinding wheel one or several strokes longitudinally. The width of the grinding wheel of the plunge grinding method is greater than the length of the thread to be ground. The grinding wheel cuts into the surface of the workpiece radially. The workpiece can be ground after about 1.25 revolutions. The productivity is higher, but the accuracy is slightly lower, and the grinding wheel dressing is more complicated. The plunge grinding method is suitable for relieving larger batches of taps and grinding certain fastening threads.

4. Thread grinding

Use softer materials such as cast iron to make nut-type or screw-type thread grinds, and perform forward and reverse rotation grinding on the parts of the machined threads with pitch errors on the workpiece to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate deformation and improve accuracy.

5. Tapping and Threading

Tapping is to use a certain torque to screw the tap into the pre-drilled bottom hole of the workpiece to process the internal thread. Threading is to use a die to cut an external thread on a bar (or pipe) workpiece. The processing 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 operated manually, and can also be used with lathes, drilling machines, tapping machines and threading machines.

Selection principle of cutting amount for thread turning

Since the thread pitch (or lead) is specified by the pattern, the key to selecting the cutting amount when turning the thread is to determine the spindle speed n and the cutting depth ap.

1) Choice of spindle speed

According to the mechanism that the spindle rotates 1 revolution during thread turning and the tool feeds 1 lead, the feed speed of the CNC lathe when turning the thread is determined by the selected spindle speed. The thread lead commanded in the thread processing block (single-start thread is the pitch) is equivalent to the feed rate expressed by the feed amount f (mm/r) vf  vf = n f (1)

It can be seen from formula 1 that the feed rate vf is proportional to the feed amount f. If the spindle speed of the machine tool is selected too high, the converted feed rate must greatly exceed the rated feed rate of the machine tool. Therefore, when selecting the spindle speed for thread turning, the parameter setting of the feed system and the electrical configuration of the machine tool should be considered to avoid the occurrence of "message" of the thread or the thread pitch near the start/end point that does not meet the requirements.

In addition, it is worth noting that once the thread processing is started, the spindle speed value generally cannot be changed, and the spindle speed including finishing must be the value selected during the first feed processing. Otherwise, the numerical control system will cause the screw thread "tooth chaos" due to the "overshoot" of the pulse encoder reference pulse signal.

2) Selection of cutting depth

Because thread turning is forming turning, the tool strength is poor, and the cutting feed is large, and the cutting force on the tool is also large. Therefore, it is generally required to process with fractional feeds and choose a relatively reasonable cutting depth according to the decreasing trend. Table 1 lists the feed times and cutting depth reference values ​​for common metric thread cutting for readers' reference.

Table 1 The number of feeds and depth of cut for common metric thread cutting

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