Thread machining of the hottest material 0

2022-08-03
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Threading of hard materials

for metalworking materials, the hardness value hrc36 is usually regarded as the boundary between soft materials and hard materials. Hrc36 is taken as the starting point of hard materials because when the hardness is greater than this value, the elongation of the material is usually less than 10%. Generally speaking, this kind of hardness material is not suitable for processing threads with cold extrusion forming tap or rolling forming tap. Therefore, the screw thread processing of hard materials is widely used by the new material enterprises represented by polyurethane, carbon fiber, aramid fiber and fluorosilicone materials in Shandong Province recently. They often need to use cutting taps or thread milling cutters to seize opportunities

however, this general limitation is not absolutely insurmountable. Some cold extrusion taps can also process threads on materials with hardness up to hrc44. Cutting taps can be used to machine threads on both soft and hard materials. However, when tapping hard materials, some changes need to be made to the structural design of the tap

application field

manufacturers need to process threads on various hard material workpieces, including aviation parts and medical parts. For example, American CIM company needs to process the threads of medical parts with hardness of hrc32-39 (such as bone saw guide) and materials with hardness of hrc48-52 (such as stainless steel laser devices used in optical industry). CIM was originally a turbine manufacturer. Turbine parts are usually made of nickel base alloy (such as stellite31), and the hardness after heat treatment reaches hrc48-52

mold manufacturers also need to process threads on hard materials. The die is usually made of tool steel (such as H-13, P-20, etc.). The hardness of H-13 is hrc48-52, while the hardness of P-13 can reach hrc50-52. Hard materials for dies also include heat treatable alloys and cobalt based alloys

although Hss-e high speed steel (e refers to "super grade", indicating cobalt rich high speed steel) tap can be used to tap materials with hardness up to HRC55, the manufacturer should use cemented carbide tap with higher hardness. Cemented carbide taps can be used to process martensitic stainless steels with high hardness (the hardness of 400 series is hrc50-60). The main alloying element of these stainless steels is 11.5% - 18% chromium

hard material tapping can also be applied to the machining of some process control parts. For example, nickel base alloys can be used to make parts for measuring the flow rate of chemicals (including acid). This material must have good resistance to corrosive chemicals

tap design

compared with processing soft materials, taps processing hard materials need to use harder coatings (such as tin and TiCN coatings). These taps also include other structural features

one feature is the number of taps. In order to process hard materials, more slots are required. For example, two slot taps are suitable for processing aluminum, while taps used for processing hard materials may need to use 5-6 slots. More slots can produce more cutting edges for the surface roughness of the parts of the impact broaching machine, thus dispersing and reducing the cutting force and tool wear

another feature is the relief grinding amount of the cutting tool. The relief grinding amount of the tap designed for processing soft materials is small, because reducing the relief grinding amount can improve the stability of processing these materials by the test method of mechanical properties of ceramics. For taps made of hard materials, a large amount of shovel grinding is required. The purpose of increasing the grinding amount is to form a negative (radial) cutting surface, while the tap for processing soft materials will form a positive cutting surface, which is somewhat similar to the sharp eagle claw. This eagle claw shaped cutting surface may damage the harder workpiece materials. In addition, a large amount of relief grinding can also be obtained by chamfering relief grinding, eccentric relief grinding (which will affect the pitch and small diameter of thread tapping) and tooth relief grinding (which will affect the large diameter, pitch and small diameter of thread tapping). Increasing the relief grinding amount can reduce the cutting resistance of the tap, which is conducive to the processing of hard materials. Because the elasticity of hard materials is small, if the cutting resistance is too large, it may cause the tap to break. In addition, reducing the cutting resistance can improve the free cutting ability of the tap and reduce the torque required for tapping. However, torque reduction should not be confused with very small torque. Tapping on hard materials needs to consume a lot of power, and the cutting edge of the tap also has to bear a lot of cutting force

in addition to increasing the amount of relieving and grinding, the tap for machining hard materials also needs to use a large front angle. The influence of tap rake angle on cutting edge stability is greater than that of tap rake angle. Reducing the rake angle can improve the stability of the cutting edge movement and usually produce a more favorable chip shape. However, reducing the rake angle, which is also necessary for machining hard materials, increases the cutting force and torque

in addition, when machining hard materials, manufacturers should expect that even if the tap structure is specially improved, its service life is still relatively short. The service life of the special tap depends on the depth of the threaded hole and the hardness of the workpiece material. Manufacturers should also expect that taps for machining hard materials are more expensive than taps for machining soft materials. For example, in order to process workpiece materials with particularly high hardness, it is usually necessary to use carbide taps with higher cost

"pecking" tapping

in addition to tap design, manufacturers should also understand some technical points of hard material tapping

since the tap is easier to break when machining hard materials, special attention must be paid to the operation method. The operator can use the "pecking" method to reduce the risk of tap breakage. This method is to process the required thread through several incremental feedings. Since only a small amount of cutting is completed per feed, the cutting load acting on the tap can be reduced. For example, when manufacturing flanges for the semiconductor industry, highvac needs to use high-efficiency cutting taps to tap nickel base alloys (such as Hastelloy, Inconel625 and Inconel718 with hardness of HRC45-50 before heat treatment). The company adopts the "pecking" method, and only a part of the thread depth is processed each time. If the depth of the entire screw hole is 12.7mm, the cutting amount of the cutting tap is only 2.54mm for each feed. The entire thread can be machined through five feeds. However, to adopt this tapping method, the manufacturer must have a machine tool with "pecking" processing function. In addition, the processing speed of this method is slower than that of conventional tapping. However, this machining method is useful in order to reduce the possibility of scrapping the near finished workpiece due to the broken tap

comparison with thread milling

threaded holes on hard materials can also be machined with thread milling cutters. Generally speaking, there is no big problem in machining high hardness materials with carbide thread milling cutter. Thread milling can better control the feed rate of the tool. In addition, thread milling is usually the safest way to process thread holes on hard materials. In particular, once the thread milling cutter is broken in the screw holes, it is easier to take rescue measures. Once the cutting tap is broken in the screw hole, it is much more difficult, sometimes even impossible, to take out the broken tap from the hole and save the usually valuable workpiece. Therefore, in the mold manufacturing industry and aviation industry, thread milling is usually the preferred way to process threaded holes on hard materials

in addition, during thread milling, the manufacturer can more easily control the thread size. The thread milling cutter Milles the thread through the screw interpolation movement (including the simultaneous movement of the X, y and Z axes of the CNC machine tool). If no special knowledge is used, it is easy to be confused. When the thread milling cutter is worn, the screw interpolation of the cutter can be compensated through the machine tool programming, so as to ensure that the processed thread size always meets the requirements. However, compared with thread tapping, the processing cost of thread milling may be higher, which must be considered when selecting the processing technology

link: cold extrusion forming tapping of soft materials

compared with cutting taps, cold extrusion forming taps have many advantages in processing threaded holes of soft materials (≤ HRC35). Since the cold extrusion forming tapping is to extrude rather than cut the material, no chips will be generated. Compared with the cut thread, the cold extrusion thread has higher surface tensile strength, yield strength and shear strength. The higher strength comes from the thread forming mechanism, which is formed by permanent plastic deformation of the workpiece material

the service life of cold extrusion Taps is usually longer than that of cutting taps, because cold extrusion Taps do not have sharp cutting edges that are easy to be worn quickly. Cold extrusion taps can also process threads with a larger aspect ratio than cutting taps. In addition, compared with the cutting tap, the thread size processed with the cold extrusion tap usually has better consistency

however, there is a cost to achieving this dimensional consistency. In cold extrusion forming tapping, it is necessary to machine a more precise center hole first. The success of cold extrusion tapping depends on the diameter of the central hole, and its dimensional tolerance is usually smaller than that of tapping with a cutting tap. A deviation of one unit of hole size may cause a deviation of four units of thread size. Therefore, the central hole may need to be reamed before the cold extrusion forming tapping

if the central hole size is too large, there will not be enough material for the cold extrusion forming tap to extrude the thread of the correct size. If the size of the center hole is too small, the cold extrusion tap will have to squeeze more materials to form threads, resulting in rapid wear of the tap. In addition, when machining coarse thread, in order to form each thread, the cold extrusion forming tap must squeeze more materials, and the more extruded materials, the greater the possibility of cold work hardening. Stainless steel, titanium alloy and nickel base alloy are prone to cold work hardening. Therefore, these materials may shorten the tap life or cause the tap to break

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