Tapping internal threads is called tapping for short. It has been more than 100 years since Rael invented the rotary nut tapping machine in 1872. Most of the tapped internal threads are used in nuts, and there are also internal threads on special parts or the main body. The types of internal threads are: ordinary thread (metric thread), American thread (UN and UNR), inch thread (BS), cylindrical pipe thread, tapered pipe thread, trapezoidal thread, and internal thread for installing wire thread inserts. Among them, the commonly used internal threads of fasteners are ordinary taps for tapping internal threads, extrusion taps for extruding internal threads, and grinding internal threads. Among them, the processing method of tap tapping is the most widely used.
1. Equipment for internal thread processing
Due to different product requirements, material properties, internal thread precision grade, batch size, and production form are different, the equipment and tools used when processing internal threads are also different. Commonly used internal thread processing machine tools are: ordinary machine tools, thread lathes, thread grinders, lathes and tapping machines. The tapping machine has horizontal multi-axis tapping machine, vertical multi-axis tapping machine, automatic variable-axis tapping multi-axis tapping machine, vertical multi-axis tapping machine, automatic variable-axis tapping machine, spiral roller guide rail High-speed tapping machine and special tapping machine for blind hole internal thread. The most commonly used standard fastener production plants are automatic nut tapping machines and vertical multi-axis tapping machines.
1.1 Vertical multi-axis tapping machine
Vertical multi-axis tapping machine has low speed, low production efficiency, simple mechanism, low equipment cost, easy to change nut specifications, and even one machine tool can add several threads of different specifications at the same time. The manual operation of the semi-automatic silk machine has high labor intensity and low overall cost. This tapping machine is an irreplaceable thread processing equipment for processing special forms or large specifications of nuts, as well as multiple varieties and small batch production. The machine tool can use short shank taps or long shank taps. The principle of tapping is shown in Figure 1.
1.2 Automatic tapping machine The tapping principle of the automatic tapping machine is shown in Figure 2. The machine tool uses a 90° curved shank nut tap with a bending arc. The curved shank is installed in the curved groove of the main shaft of the machine tool, and is driven by the main shaft to rotate. The nut blank is automatically fed into the slideway through the feed gate. A cone rod continuously feeds the nut blank one by one to the tap to cut it in. Through the rotation of the tap to achieve the same pitch feeding, and cutting to complete the processing of the internal thread. Another feeding method is to use 3 spiral rollers to rotate the feeding (nut blank) according to the same pitch to complete the tapping. After tapping, the nuts gather on the shank of the tap. Keep the tap in the center position. The nuts are threaded into the tapping from the tip of the tap one by one, and pushed against each other under the support of the nut, and the nuts are thrown out and dropped into the product barrel (box) under the action of centrifugal force.
The machine tool has a high degree of automation, and one operator can take care of more than one machine tool at the same time. Its main responsibility is to deliver the nut blank to the feed gate and inspect whether the machine tool is working properly. The main production efficiency of the machine tool is high, and the labor is low. It is suitable for large-volume specialized production plants, but it cannot be used for nuts with special shapes and large-size nuts. Also, due to the long auxiliary hours for adjusting the machine tool, it is not economical to use it in a variety of varieties and small batches. In addition, if you do not pay attention to machine maintenance, it will affect the cone life and thread accuracy.
The types of taps are: hand taps, machine taps, short shank nut taps, long shank nut taps, curved shank taps, extrusion taps, spiral flute taps and jumping thread taps, etc., as shown in Figure 3, according to different thread requirements and The production conditions also choose the most suitable tap.
The commonly used number of grooves for taps is three grooves or four grooves. Large-size taps have five grooves and six grooves. Because odd-numbered grooves are asymmetrical, with good cutting performance, and high precision of threaded threads, the taps with odd-numbered grooves are generally used. However, it is difficult to measure the pitch diameter of the tap and requires special measuring tools. ,
Hand taps and machine taps are used in groups of two taps and two taps or three taps to produce internal threads. Generally on the main body or parts, the internal thread part of the blind hole can be tapped to the maximum depth, and the thread end length of the internal thread can be minimized.
Short shank nut taps are used to tap through-hole internal threads. Generally on a machine tool or a vertical multi-axis tapping machine, the internal thread processing is completed by a tap. Due to the use of single tap tapping, the cutting load is relatively large. Therefore, the length of the cutting part of the tap is longer than that of hand and machine taps.
Long shank nut taps and bent shank nut taps are generally used in vertical multi-axis tapping machines or automatic tapping machines. The tap shank diameter is smaller than the small diameter of the internal thread, and it is an extended shank, so it is better to store more nuts in the tap shank. The curved pomelo has a circular arc shape and is only used for automatic tapping machines. The blade material of the tap is made of high-speed steel alloy tool steel, and the handle can be welded with 45 steel or alloy steel.
Extrusion taps are also dance groove taps, and the entire length of the extrusion taper should be a tapered thread with a complete profile. The horizontal part of the working calibration part is a waiting side triangular prism. The tap is suitable for processing materials with good plasticity such as aluminum and brass. The internal thread strength and surface roughness are better, and the tap has a long service life. It can be used for processing through-hole or blind-hole internal threads.
Spiral flute taps are suitable for processing deep internal threads without through holes or grooves and notches. They have smooth cutting, good chip removal performance and large chip holding capacity.
Jumping taps are actually taps that remove one tooth every other tooth, and are suitable for materials with high strength and toughness, such as stainless steel, heat-resistant alloys and titanium alloys. When using general taps to process this type of material, the chip has a very high viscosity to the tap edge. During tapping, the tap’s blades will produce a large extrusion deformation on the workpiece and increase the friction, resulting in a very rough surface of the processed internal thread. Poor and easy to break taps. Using skipping taps can overcome such defects.
The tap materials are commonly used high-speed steel W18Cr4V, alloy steel 9CrSi and high-carbon steel T12A. There are taps for grinding threads after heat treatment, and taps for cold rolling threads and machining, and finally heat treatment. The accuracy grades of standard taps are: H1, H2, H3 and H4 in total 4 types.
3. Process elements of internal thread processing
3.1 Aperture before internal thread tapping
According to the thread standard, the aperture before tapping should be within the limit size range of the minor diameter of the internal thread. If the aperture before tapping is small and equal to or greater than the minimum size of the minor diameter of the internal thread, it will be very unfavorable to the cutting work of the tap. The small diameter of the tap may also participate in the cutting, increasing the working load of the tap, increasing the cutting force, and easily breaking the tap. For this reason, it is necessary to choose an unused aperture before tapping according to the material of the internal thread of Zhonggong. The aperture must be larger than the minimum size of the minor diameter of the internal thread. The formula given in Table 1 is recommended.
3.2 Selection of tolerance zone code for tap accuracy grade
The tolerance zone codes of the wire cone accuracy grade are divided into 4 types: H1, H2, H3 and H4. Among them, H1 is suitable for processing 4H and 5H internal threads; H2 is suitable for processing 5G and 6H internal threads; H3 is suitable for processing 6G and 7G internal threads; H4 is suitable for processing 7H internal threads. However, due to the material properties of the internal thread being processed, machine tool conditions, the cutting speed of the tap clamping method, cooling and lubrication conditions, etc., directly affect the size of the internal thread. Among them, the expansion of the screw hole is different, such as machining magnesium alloy, Stainless steel and heat-resistant steel materials sometimes shrink the screw holes. Therefore, Table 2 only selects the taps for reference. When in use, you can select the most suitable tap according to the processing conditions, your own production conditions and experience, or through tests, within the range listed in the table. It should also be pointed out: due to poor processing conditions, a large amount of expansion may be caused. The so-called expansion, that is, the pitch diameter of the internal thread is greater than the pitch diameter of the tap. It is not correct to reduce the size of the tap to ensure the accuracy of the internal thread. Instead, the processing equipment should be controlled and improved.
3.3 Cutting speed and cooling lubrication for machining internal threads
The cutting speed of internal thread tapping is related to the material production, hardness, thread tolerance grade and tap material (such as high-speed steel or alloy steel or high-carbon steel) of the processed internal thread parts, and the cold lubricating oil used. If the cutting speed is too fast, it will promote the early wear and breakage of the tap, which will cause the internal thread flanks to be rough and expand the size of the internal thread; if the cutting speed is too slow, although the service life of the tap can be prolonged, the productivity will be reduced. The material of the internal thread is medium For carbon steel, use vegetable oil (rapeseed oil, soybean oil) as much as possible for lubrication. Table 3 shows the cutting speed when machining internal threaded parts of various materials with high-speed steel wire tapers.