Machining Requirements and Process Specifications for Cold-Drawn Cylinder Steel Tubes

Cold-drawn cylinder steel tubes are made of 38CrMoALA material. After quenching and tempering as well as surface nitriding, the φ50 mm×770 mm part has a core hardness of 28–32 HRC, a surface nitriding layer depth of 0.2–0.3 mm, and a surface hardness of 62–65 HRC. In this way, the cold-drawn cylindrical steel tube not only has a certain degree of toughness but also excellent wear resistance.

(2) The structure of cold-drawn cylinder steel tubes is relatively simple, but their aspect ratio is very large. They belong to slender shaft parts with poor rigidity. A tool rest is always used to reduce workpiece deformation during machining, and a stable tool rest is used when machining the threads at both ends. However, despite the good design of these seals, in this case, their performance is only as good as that of the components they work with the cold-drawn cylindrical steel tubes. On long shafts, the over-lubricated outer surface cannot maintain the lubricating oil film, and the seals will overheat and fail; an excessively rough surface will cause the seals to wear quickly.

(3) When selecting positioning datums, to ensure the coaxiality tolerance of the part and the mutual positional accuracy of each component, two center holes are used for positioning in all machining processes, which conforms to the principle of a unified datum. Traditionally, seal manufacturers rely on three factors to measure surface finish and define seal requirements: average roughness (Ra), maximum peak-to-valley roughness (Rmax), and average peak-to-valley roughness (Rz). Although these measurements are simple and based on industry standards, these traditional factors are not sufficient to accurately evaluate surface quality.

(4) When grinding the outer cylindrical surface, the workpiece is prone to yield and elastic deformation, which affects the accuracy of the cold-drawn cylindrical steel tube. Therefore, the center holes should be repaired during machining, and the center holes should be kept clean. The tightness between the center holes and the center should be appropriate, and good lubrication should be ensured. General selection of grinding wheels: abrasive (white corundum, WA), grain size (60#), hardness (medium or medium-hard), bond (vitrified bond); the width of the grinding wheel should be narrow to reduce radial grinding force. Attention should be paid to the selection of grinding allowance during machining, especially the grinding depth, which should be small.

(5) When grinding the φ50 mm×770 mm outer circle and the 1:20 taper, these two processes must be carried out separately. When grinding the 1:20 taper, a test piece must be ground first; only after the test piece is qualified can the formal grinding of the workpiece be carried out. The inspection of the 1:20 conical surface is conducted by color inspection using a standard 1:20 ring gauge, and the contact area should not be less than 80%.