Design of the Forming Die for the Locking Boss
The design of the lock boss forming die is a local forming technology developed for the boss structures in various locks (such as buckles and lock tongue positioning platforms). It can press out bosses with a height of 2-10mm and a diameter of 5-20mm on the plate. The boss verticality error is less than 0.1mm/m, and the surface roughness Ra is less than 3.2μm. It is suitable for materials such as brass (H62) and stainless steel (304) with a thickness of 0.8-3mm. Boss forming is a local tensile deformation. The mold design needs to control the material flow in the deformation zone to avoid necking (thinning rate <15%) or cracking. The boss forming force needs to be calculated in the early stage of the design using the formula: F=K×A×σs (K is the forming coefficient 1.2-1.5, A is the boss projection area, and σs is the yield strength). For example, the forming force for a φ10mm boss made of 3mm thick 304 stainless steel is about 15-20kN.
The mold structure consists of an upper die seat, a boss punch, a die, a press plate, a material ejection device and a guide mechanism. The boss punch is made of W6Mo5Cr4V2 high-speed steel, with a quenching hardness of HRC62-65. The radius of the head fillet is 1-2 times the material thickness (3mm thick takes 3-6mm) to avoid cracks during forming. The working surface is polished (Ra0.1μm) to reduce friction. The die is made of Cr12MoV steel, with a quenching hardness of HRC58-62. The diameter of the die hole is 0.1-0.2mm larger than the boss diameter (φ10mm boss takes φ10.1-10.2mm), and the bottom radius of the hole is 2-5mm to guide the material to flow to the bottom of the boss. The press plate is made of 45 steel quenched and tempered (HRC28-32), with a clearance of 0.05-0.1mm with the die. The spring provides a press force (20%-30% of the forming force) to prevent the sheet from sliding during forming.
The process parameters of boss forming need to accurately match the material properties. For materials with good plasticity such as brass, the forming speed is 50-80mm/s, and for high-strength materials such as stainless steel, the forming speed is 30-50mm/s. In terms of lubrication, brass uses special drawing oil (viscosity 20-30cSt), and stainless steel uses extreme pressure grease (dropping point > 180℃), forming a uniform oil film (thickness 5-10μm) on the surface of the die. For bosses with a height of > 5mm, step-by-step forming is required, with the first forming to 70% of the height and the second forming to the final size. Low-temperature annealing (brass 200-250℃, 1 hour) can be arranged in the middle to eliminate work hardening.
The positioning system ensures the accurate position of the boss. It uses a combination of positioning pins and baffles. The clearance between the positioning pins and the prefabricated holes is 0.02-0.05mm, and the baffles control the longitudinal dimensions (accuracy ±0.1mm). For parts without prefabricated holes, a V-shaped positioning block is set to fit the edge of the part, and the positioning error is <0.15mm. The guide mechanism uses ball guide pins and guide sleeves. The diameter of the guide pins is 16-25mm, and the clearance is 0.005-0.01mm, ensuring that the coaxiality error of the punch and the die is <0.03mm, avoiding eccentricity of the boss (deviation <0.1mm).
During debugging and quality control, attention should be paid to the integrity of the boss. During the mold test, the height and diameter of the boss should be measured. Ten pieces should be sampled for each batch. When the height deviation is greater than 0.2mm, the punch stroke should be adjusted (±0.1mm). If necking occurs, the radius of the die should be increased or the forming speed should be reduced. After every 1,000 pieces, check the wear of the punch head (grind when the radius increases by more than 0.1mm). When scratches appear on the surface of the die hole, polish it with fine sandpaper (grit size 400#). During mold maintenance, clean the waste around the boss every day, and add lithium-based grease to the guide mechanism every week to ensure flexible movement.
