engineers revealed the internal thread processing method of the lathe

There are actually many ways to process internal threads. Today we look at a master who does the internal thread of a tool. It is really amazing. Corresponding to the introduction below, you can easily understand what kind of processing method this is.

Keywork ： Tapping Cutting process， Tap Cutting， tap Extrusion ，tapping Turning thread，CNC Machining Maunfacturer China ，Low-Vloume Manufacturing

Let's share the five main machining methods of internal threads: tapping, extrusion, milling, turning and grinding, and compare their advantages and disadvantages.

1.Tap processing of internal thread

For many thread processing, tapping is an effective and commonly used 

processing method, which usually has the lowest initial cost, but overall it is not necessarily the best economy.
Spiral groove tap adopts variable lead chip flute with excellent chip control performance.

Tapping as a continuous cutting process, the workpiece material is sequentially cut by the cutting edges arranged in sequence. The final thread size can be obtained by one pass. Taps are specially produced according to the large, small and medium diameters of the threads. Because the taps must be roughed and finished at the same time as the tool. Therefore, a large amount of chips must be effectively discharged, and excessive pressure may be generated. This can lead to problems with thread quality or damage to the tap.

When tapping, chip control is a big problem that cannot be ignored, especially when processing workpiece materials with low hardness, high viscosity, and easy to produce long chips. These strip-shaped chips may form bird nest-shaped chip clusters around the taps or accumulate in the chip flutes, causing the taps to break in the holes. Aluminum, carbon steel, and 300 series stainless steels are often the most challenging workpiece materials for chip control.

Taps can process almost any workpiece material with a hardness lower than HRC50. Some tool manufacturers provide taps that can even process workpiece materials with hardness up to HRC65.

Pore size is another factor to consider. Most end users can only tap for screw holes with a diameter of less than 16mm. If the hole diameter exceeds 16mm, they will face the question of whether the machine has enough power to rotate the tap. When the diameter of the screw hole is less than 6.35mm, due to the limited chip space and low strength of the small diameter tap, the tapping process is also easy to cause problems.

In addition, the length of the internal thread that a tap can process can usually reach more than three times its diameter. For deep hole threads, taps are often processed faster than single-tooth thread milling cutters. As long as the chips can be successfully discharged out of the hole, tapping can be performed on the screw holes with a depth within the allowable range of the tap design.

Because the diameter and pitch are fixed, a tap cannot process screw holes of different specifications. In addition, since the tap has a large contact area with the hole wall during tapping, a large cutting force is generated. Therefore, the tap may be broken and stuck in the hole, which may cause the workpiece to be scrapped. In order to effectively complete processing, tapping also places high requirements on lubricants.

2.Extrusion molding of internal thread

By transferring (rather than cutting) workpiece material, extrusion taps can machine internal threads up to 4 times the diameter. Since no chips are generated, there is no need to worry about the formation of bird nest-like chips. However, the extruded thread requires that the workpiece hardness be limited to about HRC40 or less. In addition, because the material needs to be transferred, the workpiece material must have good ductility.

The diameter of the extruded tap is usually less than 19mm and can be as small as 0.5mm. The larger the diameter of the tap, the greater the friction generated during machining, and the higher the power requirements of the machine tool.

　

Compared with cutting taps, extrusion taps have better rigidity and are less likely to break. The pressure acting on the cutting tap is a tangential force through its polygonal surface. The pressure acting on the extrusion tap is a radial force toward the center of the tap, so it is much greater than the tangential force.

Taps can be used for thread extrusion of cast aluminum parts

Compared to cut threads, extruded threads are stronger because the extrusion taps form threads by compressing (rather than cutting) the grain structure of the workpiece material.

Compared with cutting and tapping, extrusion tapping requires a machine tool with greater torque and power, and requires higher workpiece clamping stability. The force required to transfer the workpiece material is greater than that of cutting workpiece material. Extrusion tapping The drilling accuracy requirements for screw holes are also higher.

Extrusion threads are not accepted in some industries, including the medical and aerospace industries. There is a defect in the thread diameter formed by extrusion tapping, and the aerospace industry does not allow sharp points (U-shaped teeth) at the thread diameter. However, this defect does not affect the tensile strength of the thread, so for general-purpose parts, it will not be a reason for rejection.

3.Milling of internal thread

Thread milling cutters use helical interpolation to cut internal and external threads. Most CNC machines produced in the past 10-15 years have thread milling capabilities.

Thread milling can use solid carbide thread milling cutters or indexable insert thread milling cutters (using steel tool holders and carbide inserts). A multi-tooth thread milling cutter can cut a full-depth thread by rotating around a screw hole. A single-tooth thread milling cutter only has a cutting edge on one processing surface, so only one thread can be cut at a time. However, most thread milling cutters have multiple teeth.

Thread milling is suitable for machining workpiece materials with hardness below HRC65, and has excellent versatility. Usually one or two thread milling cutters with different coatings can process a variety of different workpiece materials.

Chip control for thread milling is usually not difficult. Thread milling is an interrupted cut, which means that no matter what the chip characteristics of the workpiece material, broken short chips can be formed.

The thread milling cutter covers a wide range of machining sizes, from threads as small as 0-80 (cutting diameter 1.524mm) to threads with the largest diameter. In general, the optimal hole depth for thread milling cutters should be controlled within about 2.5 times the hole diameter. The cutting force of thread milling is not balanced. If the milling length is too large, the large radial cutting force will cause great lateral pressure, which may cause problems such as deflection of the milling cutter, cutting edge chipping, etc. Broken small cutter.

However, single-tooth thread milling cutters can process deeper screw holes, even screw holes with a depth of up to 20 times the hole diameter. Since all cutting is performed at the end of the milling cutter, there is no problem of tool deflection. There are many users of oilfield equipment or large energy components that require long-handled thread milling cutters. For them, milling multiple threads with a single-tooth milling cutter is slower, but still more cost effective than investing $ 1,000 to buy a 250mm long tap.

Flat-bottomed thread milling cutter for complete threads at the bottom of blind holes

Thread milling has many advantages. A single milling cutter can process a series of screw holes with the same pitch and different apertures, while a single-tooth milling cutter can process screw holes with multiple pitches and multiple apertures. In addition, a thread milling cutter can be used for both blind and through holes, and both right-handed and left-handed threads can be processed. Because the thread milling cutter has a flat bottom structure, a complete thread can be machined near the bottom of the blind hole. Even if the cutter breaks, it is unlikely that the part will be scrapped. Finally, thread milling cutters can be combined with other hole-machining tools into compound tools (such as drilling, chamfering, and thread milling compound tools).

However, compared to tapping, the machining cycle for milling threads is usually longer. Because milling threads requires a special machining program, some users may be reluctant to use this machining method. However, this program is not complicated and can be prepared with many NC programming software.

Some companies still prefer tapping because they don't want operators to intervene in the machining process. Thread milling requires operators to make some compensation adjustments to the machine. The diameter of the milling cutter will gradually decrease due to normal wear. In order to maintain a suitable machining size, the operator must compensate for the amount of tool wear by adjusting. Need to measure the thread tolerance first, and then adjust the processing parameters based on the measured wear amount. The operator can only use a gauge to periodically check the thread. If the test result is unsatisfactory, the tap needs to be replaced.

4. Turning of internal threads

Another way to machine internal threads is to turn the threads with indexable inserts or integral small boring tools on multi-axis machines or lathes. This process can use both single-tooth and multi-tooth blades. Multi-tooth inserts have multiple teeth on each cutting edge, and each subsequent tooth has a deeper cutting depth than the previous one. The use of multi-tooth inserts reduces the number of passes required to complete the thread. However, multi-tooth inserts are more expensive, so they are more advantageous for mass production, but they have no advantage in small batch processing.

Thread turning inserts can process both internal and external threads

Internal threads can also be turned with integral boring tools. When turning a thread with a single-tooth tool, the user can use a full-profile or partial-profile insert (multi-tooth inserts have only full-profile). Footpath). With this type of blade, a separate blade is required for each pitch.

Compared with part-shaped inserts, full-shaped inserts can produce threads with higher strength and accuracy with fewer passes, because the insert can simultaneously process the large, small and medium diameters of the thread.

Some threaded inserts have no thread tips (it cannot cut the small diameter of the thread), and some threaded inserts have only one tooth. Therefore, threads with different pitches can be processed with different cutting depths. This kind of thread has a very sharp arc of the crest, so it will reduce the strength of the coarse thread, and it will take longer to process.

The range of machining sizes for turning threads with indexable tools is wide, from the largest diameter to as small as 6mm. Screw holes with a diameter of less than 6mm need to be processed with solid carbide tools, and the smallest hole diameter that can be processed can reach about 1.25mm. For large-diameter holes, Vargus once machined large screw holes with a diameter of 0.9m on a vertical turret lathe with a service life of about 100 years. There is no other way to machine such large-hole threads except turning. . This older machine does not have helical interpolation.

A thread turning tool with a steel shank is suitable for machining screw holes with a depth of no more than 3 times the hole diameter, while a thread turning tool with a carbide shank can process screw holes with a depth of 4-5 times the diameter.

Vargus's V6 thread turning inserts have a total of 6 cutting angles

Thread turning can also process a variety of workpiece materials, turning threads on workpieces up to HRC50 or high-temperature alloys such as Hastelloy and Inconel alloys. However, due to the high hardness and abrasiveness of these materials, tool life will be shortened.
　　

Chip control is critical in internal thread turning, especially when turning blind hole threads. The user can use the geometry of the blade to control the chip, and use the cross-cut feed method (including radial cross-cut feed, flank cross-cut feed, flank modified cross-cut feed or flank alternate cross-feed) The reverse helix method (the thread formation direction is away from the main axis instead of the main axis) helps chip removal.

Which cross-cut feed method to use depends on the processing conditions, but in most cases, a radial cross-cut feed with modified flanks is beneficial and harmless, so this can be your default preference. However, on almost all machine tools, if a certain parameter in the machining program is not changed, machining will be performed in a radial crosscut feed mode.

5.Grinding of internal threads

Thread grinding is a high-precision machining method, and it is an effective choice for precision internal threads with strict tolerance requirements. Various internal threads, grooves, bearing raceways and other related part features can be machined on the grinder. Typical parts that can be machined with internal thread grinders include threaded ring gauges, roller nuts, ball screws, and more.

Internal thread grinding usually requires a dedicated grinder. Generally speaking, in order to grind a thread with a precise tooth profile, the grinding wheel installation position of the machine tool must be inclined to change according to the spiral angle of the thread, which requires a rotating shaft, which is not available in most general-purpose grinding machines. Sometimes, the A-axis parallel grinding method can also be used, and the multi-toothed grinding wheel that has been modified (corrected its spiral profile) can be directly inserted into the workpiece to grind the external thread. Single tooth grinding wheel.

The internal diameter of thread grinding with better processing economy is usually 10-525mm. The rule of thumb for grinding deep hole internal threads is: the ratio of the length of the grinding wheel shaft to the diameter does not exceed 7: 1. The main challenge in grinding deep hole internal threads is the mutual restriction of the helix angle and the hole diameter. With the increase of the thread length and the decrease of the hole diameter, it is very difficult to grind the workpiece with a large helix angle because the grinding axis is more likely to collide with the workpiece.

Chip control for internal thread grinding involves flushing the grinding zone with coolant. Similarly, because the space of the inner hole is limited, it is quite difficult to make the coolant reach the grinding area in the direction of the rotation of the grinding wheel without preventing the grinding wheel and the grinding shaft from entering the small hole.

The machining accuracy of internal thread grinding is very high, and the grinding wheel can be accurately modified. After the grinding wheel is formed, it can be quickly reshaped as needed. In addition, internal thread grinding can increase productivity. Grinding wheels can be reshaped to process threads of different shapes without having to replace other grinding wheels.

An internal thread grinder with excellent processing performance must have several characteristics: good rigidity and thermal stability, high shafting motion accuracy, precise closed-loop position feedback, and a temperature-controlled precision spindle.
　

Thread ring gauge for internal thread grinder

How does a part manufacturer determine which internal thread processing method should be used? Each method has its own advantages and disadvantages. If one method does not achieve satisfactory results, it is necessary to try other methods. When determining the internal threading process, it is important to consider what kind of machine tool you have and carefully evaluate tool costs, machining cycles, and tool life.

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