Piston Rod Machining Process
① Raw Material Selection: Commonly used materials for piston rod machining include 45# steel and 2Cr13 stainless steel. The appropriate material is selected based on the operating environment and performance requirements. For example, a hydraulic piston rod manufacturer, to meet corrosion resistance requirements, chooses 2Cr13 stainless steel for piston rods used in marine equipment. During procurement, the material’s quality certificate must be checked to ensure that the chemical composition and mechanical properties meet standards. For instance, the carbon content of 2Cr13 needs to be controlled between 0.16% and 0.25%.
② Blanking and Cutting: The piston rod is cut according to its design dimensions. Common cutting equipment includes saws and lathes. For example, to machine a piston rod with a diameter of 50 mm and a length of 1000 mm, a metal circular saw is used to cut 45# steel bars. Before cutting, the perpendicularity of the saw must be calibrated. After cutting, the flatness error of the bar end face should not exceed 0.5 mm to avoid affecting subsequent processing.
③ Rough Turning: The blanked bar stock is clamped on a lathe for rough turning. The purpose of rough turning is to remove most of the rough material allowance and achieve preliminary shaping. The lathe speed is set at 500-800 rpm, and the feed rate is 0.3-0.5 mm/rpm. A finishing allowance of 2-3 mm should be left during turning. For example, when rough turning a piston rod with a diameter of 50 mm, the outer diameter is first turned to 53 mm, ensuring that the cylindricity error does not exceed 0.1 mm.
④ Quenching and Tempering: The piston rod after rough turning needs to undergo quenching and tempering to improve the overall mechanical properties of the material. The piston rod is heated in a box-type resistance furnace. The heating temperature for 45 steel is 840-860℃. The holding time is determined according to the diameter; for a diameter of 50 mm, the holding time is 2 hours. Then, it is cooled with water or oil, followed by tempering at 550-600℃ for 2 hours, and then air-cooled. After tempering, the piston rod should have a hardness of 28-32 HRC.
⑤ Semi-finish turning: After tempering, semi-finish turning is performed to further improve the dimensional accuracy and surface roughness of the piston rod. The lathe speed is increased to 800-1000 rpm, the feed rate is 0.1-0.2 mm/rpm, and a grinding allowance of 0.5-1 mm is left on the outer diameter after turning. For example, during semi-finish turning, the outer diameter of 53 mm is turned to 51 mm, and the surface roughness Ra is controlled within 3.2 μm.
⑥ Keyway milling: If the piston rod has a keyway, it needs to be milled on a milling machine. First, mark the keyway position line on the piston rod, then clamp it with a vise or indexing head, and select a suitable keyway milling cutter for machining. For example, when machining a keyway 10 mm wide and 5 mm deep, the milling cutter speed is 1200 rpm and the feed rate is 80 mm/min. During milling, the symmetry error of the keyway must be ensured to be no more than 0.05 mm.
⑦ Surface Hardening: To improve the surface hardness and wear resistance of the piston rod, surface hardening is required. A common hardening method is high-frequency induction hardening. The piston rod is placed in an induction coil, and a high-frequency current is passed through it, rapidly heating the surface to the hardening temperature. For 45 steel, the surface hardening temperature is 860-880℃, followed immediately by water cooling. After hardening, the surface hardness should reach 48-55 HRC, and the hardened layer depth should be 0.8-1.2 mm.
⑧ Precision Grinding of the Outer Diameter: After surface hardening, the outer diameter is precision ground to achieve the designed dimensional accuracy and surface roughness requirements. Using an external cylindrical grinder, the grinding wheel speed should be 3000-3500 rpm, and the workpiece speed 100-150 rpm. The feed rate should be small, generally 0.01-0.02 mm/pass. For example, when precision grinding a 50 mm diameter piston rod, the dimensional tolerance should be controlled within ±0.01 mm, and the surface roughness Ra should be below 0.8 μm.
⑨ Chrome plating: To further improve the wear resistance and corrosion resistance of the piston rod, chrome plating is usually applied. Before chrome plating, the piston rod surface should be polished to remove burrs and oxide scale. Then, it should be placed in a chrome plating bath. The temperature of the chrome plating solution should be controlled at 50-60℃, the current density at 40-60 A/dm², and the chrome plating layer thickness generally 0.02-0.05 mm. After chrome plating, the surface should be bright, free of bubbles and cracks.
⑩ Grinding and Polishing: The surface of the chrome-plated piston rod may be somewhat rough and requires grinding and polishing. Use a grinding machine or hand grinding with an abrasive of 1000-2000 mesh alumina or silicon carbide. Add the abrasive continuously during grinding to ensure uniform grinding. After polishing, the surface roughness Ra should reach below 0.4μm to make the piston rod surface smoother.