Design of floating coreless punching die
The floating coreless punching die design is an innovative technology for punching holes in thin-walled cylindrical and tubular parts, or parts with deep cavities. It eliminates the need for a traditional internal support mandrel. A floating mechanism adapts to part deformation, achieving high-precision punching without a core. It is widely used in automotive exhaust pipes, hydraulic fittings, medical devices, and other fields. Its core advantage lies in eliminating the interference between the mandrel and the part’s inner wall, which can hinder part removal in traditional cored punching. It is particularly suitable for parts with hole diameters of 5-30mm and wall thicknesses of 1-5mm, and can maintain a punching position tolerance within ±0.1mm. Initially, the punching force must be calculated based on the part’s outer diameter, wall thickness, punching position (e.g., 20-100mm from the end), and material hardness (e.g., HB150-180 for 20 steel). The press tonnage is determined using the formula F = L × t × σb (L is the hole circumference, t is the wall thickness, and σb is the tensile strength). A 30% safety factor is typically added.
The die structure consists of a floating punching punch, a die, a positioning device, an elastic floating mechanism and a discharge device. The punching punch is made of W18Cr4V high-speed steel, with a quenching hardness of HRC62-65. The head is ground with a 60°-90° taper angle to reduce the radial thrust during punching. The punch handle and the floating seat are clearance-matched (0.01-0.02mm), allowing a floating amount of ±0.5mm. The die adopts a split structure, consisting of a fixed die sleeve and a replaceable die. The replaceable die is made of cemented carbide (YG20) and is connected to the die sleeve by threads, which is easy to replace. The clearance between the die hole and the punch is 8%-12% of the material thickness. For example, for a 1mm thick steel pipe, the clearance is set to 0.08-0.12mm.
The floating mechanism is the core of the technology. It is driven by a spring or nitrogen spring and provides a preload of 1-5kN, so that the punch is in a floating state before contacting the part. After contact, the angle is adaptively adjusted according to the surface shape of the part (within the range of ±3°) to ensure that the punching direction is perpendicular to the inner wall of the part. The floating amount is determined according to the roundness error of the part, usually 1.5-2 times the outer diameter tolerance of the part. For example, for a part with an outer diameter tolerance of ±0.2mm, the floating amount is set to ±0.3-0.4mm. A guide key is set between the floating seat and the die frame to limit the rotational freedom and ensure that the punch axis and the die hole axis remain parallel within the floating range.
The positioning and unloading system must adapt to the coreless nature of the punch. The positioning device utilizes a combination of V-blocks and end stops, with the V-blocks angled at 90° to 120°. A pneumatic cylinder actuates the clamping force, which is 20% to 30% of the punching force, preventing rotation or axial movement during punching. The end stops are adjustable via bolts to accommodate varying punching distances (adjustment range ±10mm). The unloading device utilizes a spring-driven elastic unloading ring, which fits over the punch. After punching, the unloading ring scrapes the waste material away from the punch. The unloading force is 5% to 10% of the punching force, ensuring smooth discharge of the waste material.
The lubrication and cooling systems need to adapt to high-speed punching. A micro-spraying device is set up in the contact area between the punch and the die, and 0.01-0.02ml of extreme pressure cutting oil (viscosity 10-20cSt) is sprayed each time a hole is punched to form an oil film to reduce friction. For continuous punching (>30 times/min), a cooling water channel (diameter 5-8mm) is opened inside the die sleeve, and circulating water at 30-40℃ is passed through to control the die temperature below 60℃ to prevent the burrs from increasing due to material softening. The lubrication system is equipped with an oil metering pump to accurately control the amount of oil sprayed; the cooling system is equipped with a flow sensor to ensure stable water flow.
Debugging and maintenance require attention to precision control. When testing the mold, first use scrap parts for testing, and use a three-dimensional coordinate measuring instrument to detect the punching position and aperture accuracy. If the position deviation exceeds the tolerance, adjust the preload of the floating mechanism (increase or decrease the spring gasket); if the aperture tapers, grind the matching clearance between the punch and the die. In daily maintenance, check the wear of the punch every 1,000 times (replace when the diameter wear is greater than 0.03mm); clean the guide key of the floating mechanism once a week and apply high-temperature grease; polish the clamping surface of the positioning V-block every 500 times to remove scratches and maintain positioning accuracy. For high-hardness materials (such as 45 steel), it is recommended to perform low-temperature aging treatment on the punch (keep warm at 120℃ for 2 hours) after every 500 punchings to eliminate internal stress and extend service life.
