One of the piston rods, during normal operation, withstands alternating loads. At the φ50mm × 770mm location, there is a sealing device that reciprocates and rubs against its surface. Therefore, this area requires high hardness and wear resistance.
The φ50mm×770mm section of the 38CrMoALA material has been heat-treated and surface-nitrided. The core hardness is 28~32HRC, with a surface nitriding layer depth of 0.2~0.3mm and a surface hardness of 62~65HRC, providing it with both toughness and excellent wear resistance.
The structure is relatively simple, with a large length-to-diameter ratio, categorizing it as a slender shaft component with poor rigidity. To ensure machining accuracy, rough and finish machining must be separated during turning. Additionally, both rough and finish machining should utilize a tool holder to minimize deformation of the workpiece during processing. A support stand should be used when machining threads at both ends.
In selecting the positioning datum, to ensure the concentricity tolerance of the parts and the accuracy of their mutual positioning, all machining processes use two center holes for positioning, adhering to the principle of unified datum.
Four, When grinding the outer cylindrical surface, the workpiece tends to produce knife wobble and elastic deformation, which affects the precision of the piston rod. Therefore, during the machining process, it is necessary to refine the holes and keep them clean. The tightness between the hole and the top should be appropriate, and good lubrication is required. The general choice of abrasive is white corundum (WA) with a grit size of 60#, medium-soft hardness, and a ceramic bond. Additionally, the width of the abrasive wheel should be narrower to reduce radial grinding force. Pay attention to the selection of grinding quantities during machining, and ensure the grinding depth is minimal.
In the process of grinding the φ50mm×770mm external diameter and 1:20 taper, the two operations must be conducted separately. When grinding the 1:20 taper, the test piece must be ground first, and the test piece must pass inspection before the actual workpiece can be ground. The inspection of the 1:20 conical surface is done using a standard 1:20 ring gauge with color marking, with the contact area being no less than 80%.
To ensure the stability of piston rod machining precision, manual straightening is prohibited throughout the entire processing process.
During nitriding treatment, protective devices should be used to safeguard threaded parts.
