Discussion on the machining methods of the two mos

A preliminary study on the machining methods of two kinds of molybdenum alloy parts (threads) with CNC lathe

molybdenum alloy has high strength and hardness, poor toughness, brittleness, easy to break, and poor machinability. However, because of its characteristics of high melting point, low density, and small coefficient of thermal expansion, it is commonly used to manufacture various high-temperature components in aviation and aerospace. The two molybdenum alloy parts are molybdenum screws and molybdenum bolts, as shown in the attached drawings. The blank is hot rolled Ф 16mm molybdenum alloy steel bar. During processing, it is found that the material has low density and very high skin hardness, which should be annealing residual hardness. The hardness inside the surface layer is still very high, with poor toughness and slow heat conduction. It is very difficult to machine with ordinary high-speed steel or cemented carbide tools. First of all, the front and rear flanks and tips of the tool wear quickly. Add cutting fluid, and use different cutting parameters under normal conditions for many tests, which can not achieve satisfactory results. Generally, if the cylindrical blade is processed but 4 pieces, it needs to be replaced; Secondly, the parts have slag falling defects when the tool is slightly worn, and slag falling occurs when the flat end face, cutting and threading. The excircle processing and end face processing can basically meet the requirements of the drawing when the finish turning tool is kept sharp, but the slag dropping of thread processing is always unavoidable

in actual processing, the turning effect of M6 thread is poor, and the thread segment is often broken from the end in processing, which cannot be avoided. Only die processing is adopted. M12 adopts CNC high-speed double cutter thread turning, and the effect is very ideal

since both parts are long, in actual processing, the threading process is generally carried out after the machining of the outer circle → cutting → flat total length. Since the M6 screws and M12 bolts in the process documents are not allowed to have central holes, the central holes need to be leveled off at last in the horizontal lathe processing. The method of subsection processing is adopted in the processing of several cars. The data measured without this condition is helpful to identify the problems between the actual experiment and the system, and drill a central hole. Segmented processing Ф 12mm outer circle (M12 big end diameter) → cut off; Segmented processing Ф 6mm outer circle (M6 big end diameter) → turning cone → cutting. The procedure is relatively simple, which is omitted here. It should be noted that Ф 6mm excircle size machined to Ф 5.85～ Ф 5.93mm is obviously conducive to threading. The outer circle is greater than Ф If it is 5.93mm, the nesting allowance is large, and it is easy to have incomplete tooth profile such as thread slag falling and thread missing. The outer circle is less than Ф 5.85mm, the extrusion deformation during threading is limited. If the size of the machine tool fluctuates, it is easy to make the size of the big end of M6 thread close to the limit or out of tolerance. In addition, when cutting, the total length allowance should be more than 1.5mm. In order to prevent the cutting knife from becoming blunt or the chip removal is not smooth, and the slag on the conical surface (screw) at the root of the part can not be repaired after the end face is flattened. That is, after the end face is leveled, the parts still have slag dropping defects

when machining the excircle and total length of these two parts, the material brand of ordinary cemented carbide tools is YG6 or YG8, and the grinding angle can be grinded with reference to the angle of stainless steel semi finishing tools. If conditions permit, it is better to use machine clamping tools and coated tools. Coated tools are generally divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). Compared with traditional cemented carbide tools, the strength, hardness and wear resistance of coated tools are greatly improved. High speed cutting can be carried out for materials with a hardness below 55HRC. It can be carried out when machining the excircles of these two parts (PVD) or (CVD). Using (CVD) blades is slightly better

cutting parameters: rotating speed 600 ~ 900r/min. The back cutting amount is 0.3 ~ 0.6mm. The feed rate is 0.2 ~ 0.3mm/r. Due to the great cutting heat, water-soluble cutting fluid mainly used for cooling is used during machining

after machining the shape and total length dimension, start machining the thread. When machining the MH thread, the chuck centering must be accurate. Try not to use die holder. It is better to use threading machine because the material is hard and brittle. If the center line of the die clamped by the die holder deviates slightly from the center line of the workpiece, the parts are very easy to break or fall off. The threading device can properly compensate for the deviation of the center line of the two, and the cutting oil mainly used for lubrication, industrial soybean oil, is used during threading

when machining MH thread, due to the large torque during threading, it is not easy to clamp, resulting in the rotation of the workpiece. More seriously, due to the particularity of the material, the thread will have serious slag dropping defects, and the tooth shape is incomplete. Qualified products cannot be processed with die. Turning thread with thread cutter on horizontal lathe, cutting with high-speed steel (W18Cr4V) tool and machining with low speed (50r/min) soon reach the limit of tool wear, and the cutting effect is also very unsatisfactory. However, the effect of using cemented carbide YG8, YG6, yw1 and other welding tools and coating tools PVD and CVD at speeds below 1200r/min on several cars is not ideal

according to the experimental results and material characteristics, it is decided to use the new international processing theory - high-speed, fast walking, snack knife cutting method test. According to the material characteristics and tool wear, the thread is machined by the method of high-speed (2100r/min) and double tool coarse and fine turning with great difficulty. After comparing the formats of thread machining instructions, G34 (variable pitch thread instruction) and G32 (single thread cutting instruction, advance and retreat tools must be specified separately, which is too cumbersome) were eliminated first, and G92 and G76 were decided to be used for programming and testing respectively. The machining procedure is as follows:

during machining, the rough machining No. 1 thread cutter uses water-soluble cutting fluid for cooling purposes; When finishing No. 2 thread cutter works, use industrial soybean oil for lubrication and dip it with a brush

programming instructions:

(1) the above two programs o0001 and o0002 are based on the fact that if the innovation is slow, they will fall behind the 0-t system. Even if a long paragraph is omitted from the o0001 program, it can be seen that programming with G92 is far more cumbersome than programming with G76

(2) characteristics of two thread cutting cycle command formats: G92 thread cutting cycle adopts straight feed mode. Because both sides of the tool cut the workpiece at the same time, the cutting force is large and chip removal is difficult, so the two cutting edges are easy to wear during cutting. When cutting threads with large pitch, due to the large cutting depth, the blade wears faster, resulting in the error of the pitch diameter of the thread. However, due to the high accuracy of the machined tooth profile, it is generally used for the machining of small pitch and high-precision threads. Because its tool movement and cutting are completed by programming, the machining program is long. Since the blade is easy to wear during processing, it should be measured frequently during processing

g76 thread cutting cycle adopts the oblique feed mode. Because the workpiece is cut with a single edge, the edge is easy to be damaged and worn, so that the machined thread surface is not straight, and the tool tip angle changes, resulting in poor tooth profile accuracy. However, because it works with a single edge, the tool load is small, chip removal is easy, and the cutting depth is decreasing, so this machining method is generally suitable for the machining of large pitch and low precision threads. This processing method is easy to remove chips, and the working condition of the cutting edge is good. Under the condition of low thread accuracy requirements, this processing method is more simple and convenient. If high-precision and large pitch threads need to be machined, the method of mixing G92 and G76 can be adopted, that is, G76 is used for rough machining of threads first, and then G92 is used for finish machining. It should be noted that the starting point of rough and finish machining should be the same to prevent the occurrence of disorderly thread

(3) o0001 programming is complex and error prone. However, on the basis of cooperating with the trial cutting and processing of test pieces, qualified parts can be processed after carefully preparing, debugging and finalizing the program. In view of the shortcomings of easy wear of tools and easy slag falling of parts, the stress condition of tools and parts is changed by changing the starting point position of thread cutter in the rough machining program, so as to improve the working environment of both tools, so as to prolong the service life of tools and reduce slag falling. In addition, the use of the second thread cutter can obtain a stable pitch diameter of the thread, ensure the accuracy of the thread, and effectively compensate for the shortcomings of the G92 command format. In finish machining, changing the position of the starting point of the thread cutter is a "tool catching" to polish both sides of the thread. The actual effect of the thread is very ideal

(4) o0002 program is simple and easy to compile, and qualified parts can also be processed in practice. However, in mass production, the qualified rate of finished products is lower than that processed with o0001 program, which is mainly manifested in slight slag dropping and slightly poor surface roughness on both sides of the thread. The reason for the analysis is determined by the characteristics of G76 command format. Although two cutters are also used to divide rough and finish turning to avoid the impact of tool wear on the accuracy of thread size, G76 format cannot repeat the processing of the previous cutter size, that is, the "rounding" processing often referred to in turning processing is referred to as "round cutter", and in the processing of this part, However, the deviation between the haze and the direction of the incident beam is greater than 2.5 degrees, so it is not required that the rubber molecule must have the ratio of the transmitted light intensity of the double bond structure to the total transmitted light intensity. A "round knife" must be carried out, otherwise the slag falling phenomenon is more serious. Although the "oblique feed method" of G76 instruction format can improve the stress condition of workpiece and tool and reduce the phenomenon of workpiece slag falling, it can not completely eliminate slag falling, and the "tool catching" G76 format necessary to improve the surface quality of thread in finishing is also difficult to carry out. Therefore, the o0002 program was abandoned

the effect of using G76 to compile rough machining program and G92 to compile finish machining program is also poor. Due to the slag dropping during rough machining, it cannot be repaired during finish machining. Incomplete tooth profile defects will occur, which will lead to parts scrapping. Therefore, o0001 program is used in actual processing

Program Introduction:

(1) x dimension connection of parts. In rough machining, the tool has been processed to x11.1, but the finishing machining starts from x11.3, which is the allowance reserved in consideration of the tool setting error and the wear of the coarse turning tool. Therefore, when the coarse and fine turning tools are not worn and the tool setting is accurate, the first few tools of the fine turning tool are not cut, which can be used to prepare the workpiece brush for dipping soybean oil

(2) the starting point is set at z15 because the speed of the machine tool and the fast moving speed of the tool holder are very high. It is necessary to give the encoder enough reaction time to make the machine tool start thread interpolation when the speed and spiral path reach the non fixed value. At the beginning of trial production, the starting point of the thread cutter is Z5. However, during the self inspection, it was found that when the pitch diameter of the thread had reached the lower limit of tolerance, and there was no taper in the major diameter and pitch diameter of the thread on the full length of the thread, when the ring gauge was screwed to the third and fourth threads, it was obviously "astringent" suddenly, and after passing this point, it was screwed in very loosely. After analyzing the cause, the phenomenon disappeared after changing the starting point value to z15. Even when the tool is worn and the pitch diameter of the thread is just 0.05mm, the ring gauge can be easily screwed

(3) improve efficiency and reduce single work time. After withdrawing U20, command to stop the spindle operation in advance and set M5. When the tool holder retreats away from the workpiece, the spindle has basically stopped, and the detection can be started to load and unload the workpiece. The ring gauge is mainly used for testing, and the comprehensive size of each thread is tested, which is guaranteed by the through and end stop of the ring gauge. Measure the difference before and after pitch diameter with thread pitch diameter micrometer every 5 pieces to master the tool wear. When machining one end of 20mm thread, the length of the workpiece protruding from the chuck during clamping is 26mm. Take the end face of the workpiece as the program origin, determine Z0 with the cutter, and process the other end of 14mm. When machining the thread, it is only necessary to change the secondary length of the workpiece to 20mm to start processing, and there is no need to re adjust the cutter, so as to avoid the possibility of errors in changing the programming sequence and tool setting. If it is possible to select two knife bodies and blades produced by the same factory and in the same batch, after the initial knife setting, the cattle production can be quickly restored each time the worn blade is replaced, without re setting