Molybdenum alloy has high strength and hardness, poor toughness, brittleness, easy to break, and poor machinability. However, because of its high melting point, low density, and low coefficient of thermal expansion, it is often used to manufacture various high-temperature parts in aviation and aerospace. The two molybdenum parts are the molybdenum screw and molybdenum bolt, as shown in the attached figure. The blank is hot rolled Ф 16mm molybdenum alloy steel bar. During processing, it is found that the material has a low density and very high skin hardness, which should be annealing residual hardness. Within the surface layer, the hardness is still very high, the toughness is poor, and the heat conduction is slow.
It is very difficult to process 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 obtain satisfactory results. Generally, if there are only 4 pieces of cylindrical blades, they need to be replaced; Secondly, when the tool is slightly worn, the parts have slag falling defects, and slag falling occurs when the flat end face, cutting and threading. The outer circle machining and end face machining can basically meet the drawing requirements when the finish turning tool remains sharp, but the slag dropping of thread machining can not be avoided.
In the actual processing, the turning effect of M6 molybdenum screws is poor, and the thread section is often broken from the end in the processing, which can not be avoided. Only screw die machining is used.M12 adopts CNC high-speed double cutter thread turning, and the effect is very ideal. Due to the long length of both parts, in the actual processing, the thread turning step is generally carried out after the overall length of the cutting plane of the outer circle is processed. Since the M6 molybdenum screw and M12 bolt in the process document are not allowed to have a central hole, which needs to be leveled off at last during horizontal lathe machining.
The method of subsection processing is adopted in several vehicles, and a central hole is not drilled. Segmented processing Ф 12mm outer circle (M12 large end diameter) cut off; Segmented processing Ф 6mm outer circle (M6 large end diameter) is cut by turning cone. The procedure is relatively simple and is omitted here. It should be noted that Ф 6mm outer circle size machined to Ф 5.85～ Ф 5.93mm is obviously conducive to threading. Outer circle greater than Ф 5.93mm has a large nesting allowance, which is prone to incomplete tooth profile such as thread slag dropping and lack of thread. Outer circle 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 large end size of the M6 thread close to the limit or out of tolerance. In addition, when cutting, the total length allowance shall be more than 1.5mm. To prevent the cutting knife from becoming blunt or the chip removal is not smooth, 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 flattened, the parts still have slag-dropping defects.
