Design of floating core punching die
The floating core punching die design is a precision forming technology for punching holes in thick-walled cylindrical parts or high-strength material pipes. It is widely used in petroleum machinery, hydraulic systems and other fields, and is especially suitable for processing parts that require high verticality and no collapse of the hole wall after punching. Compared with floating coreless punching dies, its core advantage is that it provides rigid support through the internal mandrel to avoid part deformation caused by excessive radial force when punching thick-walled materials. The punching size accuracy can reach IT7 level, and the hole wall roughness is controlled below Ra1.6μm. At the beginning of the design, the mandrel diameter needs to be determined based on the outer diameter of the part (such as 50 – 200mm), wall thickness (3 – 10mm) and punching diameter (8 – 50mm). The clearance between the mandrel and the inner wall of the part is 0.05 – 0.1mm to ensure that it can be smoothly installed and provide effective support. For example, for a 45 steel pipe fitting with a wall thickness of 8 mm and an inner diameter of 100 mm, the mandrel diameter is set to 99.9 mm and the fit clearance is 0.1 mm, which can significantly reduce the radial deformation during punching.
The mold structure consists of a floating punching punch, a rigid mandrel, a die, a synchronous drive mechanism and a positioning device. The punching punch is made of W6Mo5Cr4V2 high-speed steel, and its hardness reaches HRC62-65 after overall quenching. The head adopts a stepped design, the guide section length is 1.5 times the punching diameter, and the working section surface is titanium-plated (thickness 0.005-0.01mm) to reduce the friction coefficient to below 0.08. The mandrel is made of 40CrNiMoA alloy structural steel, and its hardness after quenching and tempering is HRC30-35, which not only ensures the support rigidity, but also has a certain toughness to avoid impact fracture. The synchronous drive mechanism is driven by gear racks to ensure the synchronization of the downward movement of the punch and the feeding of the mandrel. The phase difference is controlled within ±0.5° to prevent interference between the mandrel and the punch. The die adopts a split structure, consisting of a die sleeve and a carbide insert (WC-Co). The insert hardness is HRA90-92, and the radius of the punching edge is less than 0.03mm to ensure a smooth cut surface.
The coordinated design of the floating and support systems is the key to the technology. The floating mechanism of the punch is driven by a nitrogen spring, with a floating amount of ±1mm and a preload of 5-15kN. It can be dynamically adjusted according to the hardness of the material. For high-strength alloy steel (such as 35CrMo), the preload needs to be increased to 12-15kN to ensure that the punch fits tightly with the end face of the mandrel. An axial thrust bearing is set at the support end of the mandrel to withstand the axial reaction force during punching. The rated dynamic load of the bearing needs to be greater than 1.5 times the punching force. A spiral chip groove (groove width 3-5mm, depth 1-2mm) is opened on the surface of the mandrel to facilitate the discharge of punching waste and avoid the mandrel stuck due to accumulation. In addition, the coaxiality error between the mandrel and the die needs to be controlled within 0.02mm, which is guaranteed by a precision guide sleeve. The guide sleeve is made of bronze ZCuSn10Pb1 and has excellent self-lubricating properties.
The positioning and clamping device needs to adapt to the characteristics of thick-walled pipe fittings. The three-jaw centering chuck and the end face locating ring are used for positioning. The three-jaw chuck is driven by a servo motor. The clamping force can be monitored in real time by a torque sensor (range 5 – 30kN). For high-strength materials, it needs to be increased to 20 – 30kN to prevent the part from rotating during punching. The contact area between the end face locating ring and the end face of the part is not less than 60% of the end area of the part to ensure the axial positioning accuracy of ±0.1mm. For long pipe fittings (length > 500mm), an intermediate support bracket needs to be added. The bracket spacing is 200 – 300mm. The bracket height can be adjusted by fine-tuning bolts to ensure that the axis of the pipe fitting coincides with the center of the mold.
During debugging and maintenance, attention should be paid to synchronization accuracy. During mold testing, a laser interferometer is used to detect the synchronization of the movement of the punch and the mandrel. When the synchronization error exceeds 0.05mm, the gear rack clearance needs to be adjusted. The punching section is observed through a metallographic microscope. The width of the bright band must reach more than 70% of the wall thickness. If tearing occurs, the radius of the punch edge needs to be increased (adjustment amount 0.02 – 0.05mm). During daily maintenance, the mandrel needs to be subjected to magnetic particle inspection every 500 times of operation to check for surface cracks; the synchronous gear is lubricated once a month with extreme pressure gear oil (viscosity grade ISO VG220); the jaws of the three-jaw chuck are ground once every 1,000 punchings to ensure centering accuracy.
