Study on the Processing Method of Two Types of Molybdenum Alloy Parts (Threads)

May 02, 2021

Molybdenum alloys have high strength and hardness, poor toughness, brittle fracture, and poor machinability. However, due to their high melting point, low density, and low coefficient of thermal expansion, they are commonly used in the manufacture of various high-temperature parts for aviation and aerospace. . The two molybdenum alloy parts are respectively molybdenum screws and molybdenum bolts, as shown in the drawings. Raw material is hot-rolled Ф16mm molybdenum alloy steel rod. The density of the material is low during machining and the hardness of the skin is very high. It should be the residual hardness after annealing. Hardness is still high inside the surface layer, poor toughness, slow heat conduction. It is very difficult to process using ordinary high speed steel or carbide tools. Firstly, the front and back surfaces of the tool wear rapidly and the cutting tip is filled with cutting fluid. Many tests with different cutting amounts under normal conditions cannot achieve satisfactory results. Generally, only 4 external cylindrical inserts need to be replaced; The part has a dross defect when the tool wears slightly, and the dregs phenomenon occurs when the flat end surface, the cutting, and the thread are threaded. The outer circle machining and the end face machining can basically meet the requirement of the drawing under the condition that the fine turning tool is kept sharp, but the dregs can not be avoided in the thread machining.

In the actual processing, the M6 ​​threading system has a poor effect, and the thread segment breaks from the ending position often occurs in the processing and cannot be avoided. Only dies are used. M12 uses CNC high-speed double-knife thread, the effect is very good.

Because of the long two kinds of parts, in the actual processing, generally the thread step is placed on the outer circle of the car → cut off → the total length of the flat is finished. Because the M6 ​​screw and M12 bolt are not allowed to leave the center hole in the process file, the center hole must be flattened out during the horizontal lathe machining. The method of subsection processing is used in the processing of several cars, and a center hole is not drilled. Sectional processing Ф 12mm outer circle (M12 big end diameter) → cutting; section processing Ф 6mm outer circle (M6 big end diameter) → cone and → cut. The procedure is relatively simple and is omitted here. It should be noted that when the Ф6mm outer diameter is machined to Ф5.85-Ф5.93mm, it is obviously beneficial for threading. If the outer circle is larger than Ф5.93mm, the allowance is large, and thread incomplete slag, missing threads, etc. may occur. If the outer circle is less than Ф5.85mm, the amount of extrusion deformation during threading is limited. If the size of the machine tool fluctuates, it is easy to make the M6 ​​thread large end size close to the limit or beyond tolerance. When cutting off, the total length should be more than 1.5mm. In order to prevent the cutting blade from becoming blunt or squeezing out, the part's root cone surface (screw) is dross-removed, and it cannot be repaired after the end surface is flattened. In other words, there are still slag dregs after the end face is leveled.

When machining the outer circle and total length of these two types of parts, the material grades of ordinary carbide tools use YG6 or YG8. The grinding angle can be milled by referring to the angle of stainless steel semi-finishing tools. If possible, the machine tool can be used for processing and the coating can be used. The tool is better. Coated tools are generally classified into physical vapor deposition (PVD) and chemical vapor deposition (CVD). Compared with the traditional carbide tool, the coated tool has greatly improved strength, hardness and wear resistance. High-speed cutting can be performed on materials with a hardness of 55 HRC or less, and the use of (CVD) blades is slightly better when the two parts are processed in a round shape (PVD) or (CVD).

Cutting amount: Speed ​​600～900r/min. Back to eat knife 0.3 ~ 0.6mm. The feed rate is 0.2-0.3mm/r. Due to the large cutting heat generated, a cooling-based water-soluble cutting fluid is used during processing.

After processing the shape and the overall length, the thread is to be processed. When machining the M6-6h thread, the centering of the chuck must be accurate. Try not to use a die rack and use a threader to process it better because the material is hard and brittle. The center line of the die holder clamping dies is slightly offset from the center line of the workpiece, and the parts are easily broken or dregs off. The threader can properly compensate for the deviation of the centerline between the two, and the oil used for the cutting oil industry, which is based mainly on lubrication, is used for the threading.

When processing M12-6h thread, because the torque during threading is large, it is not easy to clamp, which causes the workpiece to spin. What's more, due to the special nature of the material, the thread will have serious slag dregs and incomplete teeth. The use of die can not process qualified products. Screwdriver thread on horizontal lathes, high-speed steel (W18Cr4V) cutting tools, use low-speed (50r/min) machining, quickly reached the tool wear limit, the cutting effect is also very unsatisfactory. The use of cemented carbide YG8, YG6, YW1 and other welding tools and coating tools PVD, CVD using 1200r/min the following speed test, the effect is also not ideal.

Based on experimental results and material properties, it was decided to use the new international processing theory—high speed, fast walking, and snack knife cutting test. In view of the material properties and tool wear, the thread of the car adopts a difficult high-speed (2100r/min), double-edged roughing and precision turning. After the comparison of the thread machining instruction format, the G34 (variable pitch thread command) and G32 (single thread cutting command, advance and retract tool must be specified separately, too cumbersome) were eliminated first, and it was decided to use the G92 and G76 programming tests respectively. The processing procedure is as follows:

During processing, the rough-cut No. 1 thread cutter uses a water-soluble cutting fluid for cooling purposes; the fine-machining No. 2 thread cutter uses industrial-purpose soybean oil for lubrication purposes and is wiped with a brush.

Programming instructions:

(1) The above two programs O0001 and O0002 are based on the FANUC 0-T system. Even if the O0001 program is omitted for a long period of time, it can be seen that programming with G92 is far more cumbersome than programming with G76.

(2) Two thread cutting cycle command format features: G92 thread cutting cycle adopts straight feed method, because the two sides of the tool cutting the workpiece at the same time, cutting force is large, and chip removal is difficult, so when cutting, two cutting The blade is easily worn. When cutting a thread with a large pitch, the cutting edge wears quickly due to a large depth of cut, resulting in an error in the diameter of the thread. However, due to the high precision of the tooth shape, it is generally used for the processing of small pitch and high-precision threads. Because its tool movement cutting is done by programming, the machining program is longer. As the cutting edge wears during processing, it is often measured during processing.

The G76 thread cutting cycle adopts the oblique feed method. Because the single-side cutting edge of the workpiece, the cutting edge is easy to damage and wear, so that the thread surface is not straight, the tool angle changes, resulting in poor tooth shape accuracy. However, due to its single-edge operation, the tool load is small, the chip removal is easy, and the depth of cut is a degressive type. Therefore, this processing method is generally applicable to the processing of large-pitch, low-precision threads. This processing method is easy to remove chips, the cutting edge working condition is better, and the machining method is simpler and more convenient under the condition that the precision of threading is not high. If you need to machine high-precision, large-pitch thread, you can use G92, G76 mix method, that is, the first use of G76 thread roughing, and then G92 for finishing. It should be noted that the starting point for rough finishing must be the same to prevent the occurrence of thread breakage.

(3) O0001 programming is complicated and error-prone. However, on the basis of the test-cutting process of the test piece, after careful programming, debugging, and setting procedures, it is entirely possible to process qualified parts. For the disadvantages of easy wear of the tool and easy slag dropping of the parts, the rough machining program changes the force condition of the tool and the part by changing the position of the tool point of the thread knife, so as to improve the working environment of both, thereby prolonging the tool life and reducing the tool life. Dregs effect. In addition, the use of the second thread cutter can obtain a stable thread diameter size, ensure the thread accuracy, and effectively compensate for the G92 command format defects. In the finishing process, changing the position of the cutter point of the thread cutter is a “caught up knife” performed on both sides of the thread, and the actual effect of the thread is very satisfactory.

(4) The O0002 program is simple and easy to edit. In practice, qualified parts can also be machined. However, in high-volume production, the pass rate of finished products is lower than that of the O0001 program, which is mainly due to slight dregs and slight surface roughness on both sides of the thread. The reason for the analysis is determined by the format of the G76 command format. Although the two tools are also used for coarse and fine turning to avoid the effect of tool wear on the thread size accuracy, the G76 format cannot perform the repetitive tool size machining, that is, our turning process. Frequently referred to as the "round" processing referred to as "round knife", and in the processing of the parts, it must be carried out "round knife", otherwise the dregs phenomenon is more serious. Although the “inclined” feed method of the G76 command format can improve the stress condition of the workpiece and the tool and reduce the drossing phenomenon of the workpiece, it cannot completely eliminate the dross. In order to improve the surface quality of the screw thread, it is necessary to perform the finishing process. The G76 format is also very difficult to carry out. Therefore, the O0002 program is abandoned.

The preparation of rough machining programs with G76 and the preparation of finishing programs with G92 are not effective. Due to the drossing phenomenon that occurs during rough machining, it cannot be repaired during finishing. Incomplete defects will be produced and parts will be scrapped. Therefore, the O0001 program is used in actual processing.

Program Description:

(1) The X dimension of the part is connected. In the roughing process, the tool has been machined to X11.1, and the finish machining has been started from X11.3. This is a margin that is reserved in consideration of tool setting errors, rough turning tool wear, etc. When the knife is not worn and the cutting tool is accurate, the fine turning tool is not cut at the beginning and can be used to prepare the workpiece brush for applying the soy oil.

(2) The starting point is set at Z15. Because the machine tool speed and the tool carriage rapid traverse speed are high, the encoder must be given enough reaction time so that the machine tool can only start when the rotation speed and the spiral trajectory reach the fixed value. Thread interpolation. When starting trial production, the thread cutter starts at Z5. However, during the self-test, it was found that the diameter of the thread in the thread has reached the lower limit of the tolerance, and in the case where the thread has a large diameter and a medium diameter with no taper over the entire thread length, when the ring gauge rotates to the third or fourth thread, there is a noticeable suddenness. Astringent' phenomenon, and after passing this point, it is screwed in very loosely. After analyzing the reasons, the phenomenon disappeared after changing the knife value to Z15. Even if the tool wears, the diameter of the thread can be easily adjusted when the diameter is 0.05mm.

(3) Improve efficiency and reduce work time per piece. After the tool U20 is retracted, the spindle is stopped in advance and M5 is set. When the tool holder is retracted away from the workpiece, the spindle has basically stopped and you can start testing and loading and unloading the workpiece. When testing, it is mainly based on ring gauges. Each thread detects the overall size of the thread, and is guaranteed by the ring gauge and the end. Every 5 pieces of thread diameter micrometer to measure the difference between the diameter before and after, to master the tool wear. When machining a 20mm thread, the length of chuck extended when the workpiece is clamped is 26mm. Z0 is determined by the end point of the workpiece as the point of origin of the program. When the other end is 14mm, when the thread is changed, the length of the workpiece is changed to 20mm to start. Processing, there is no need to re-scratch, thus avoiding the possibility of reprogramming and countermeasures. If there are conditions to use two blades and blades that are produced in the same factory and in the same batch, after the initial set of blades, the production of cattle can be quickly restored every time the worn blades are replaced without re-setting the blade.

Tool setting and tool making: The tool is set on the basis of the flatness on the left side of the standard insert and Z0 is determined on the end face of the workpiece. Because the left side facet of the blade of the same batch of the same factory and the distance of the tool tip are the same, the helixes machined by the two blade tips are coincident, which meets the requirements of the threading of the double tool. If conditional rough turning blade selects 14 teeth/in, that is, the angle of the cutting edge is 55°, and the pitch is 1.815mm, if the fine turning blade adopts the cutting angle of 60° and the pitch of 1.75mm, the surface roughness of the thread is better. . With a coated blade PVD or CVD, CVD can be slightly better. If there are no standard tools and inserts, YG8, YG6, YW1 and other welding tools can be used to cut from a wire cutting machine. A small plane is machined at about 1.5mm to the left of the tool tip and used for tool setting. The tolerances of the rough and fine turning tool tip and facet are controlled within ±0.01mm, and the sharp and back angles of the thread cutter are cut out at the same time. Hand-grinding out the front horn of the thread cutter (large arc front angle, the ball of swarf, life is slightly better), and then use the whetstone to grind the front and rear corners before use, can also meet the processing requirements, but the tool life is poor. As for the tool setting method of the double-threaded cutter thread, interested parties can contact, communicate and discuss with the author. Because the material is indeed difficult to process, despite the adoption of various measures, tool wear is still high, but the product qualification rate can be basically guaranteed, can reach 95% or more.

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