Design of the Drawing Die for Tapered Parts
The design of the taper drawing die is a special technology for forming taper parts (taper angle 15°-120°) such as frustum and funnel. It is suitable for materials such as low carbon steel and aluminum alloy with a thickness of 0.5-6mm. It can achieve uniform forming of the taper surface, control the wall thickness difference within 8%, the mouth diameter accuracy of ±0.1mm, and the taper error of <0.5°. Unlike cylindrical drawing, the material needs to flow in both radial and axial directions during the drawing of the taper, and the deformation is complex and prone to wrinkles or cracks. The core of the design is to gradually transition to the target taper through segmented drawing. The first drawing usually uses a punch with a transition fillet (R=5-15mm), and the subsequent process gradually reduces the fillet to the target value. For example, a part with a taper angle of 60° and a height of 100mm requires 3-4 drawing, the first drawing to a taper angle of 30°, and then gradually increases to 60°.
The mold structure adopts a multi-station progressive layout, including pre-drawing, secondary drawing, shaping and trimming stations. The pre-drawing punch is made of Cr12MoV steel (HRC58-62), the working surface is polished to Ra0.4μm, and the straightness error of the cone generatrix is less than 0.02mm/m; the die is made of the same material, and the matching clearance with the punch is 1.2-1.5 times the material thickness (2mm thick material takes 2.4-3mm), and the radius of the die entrance fillet is 5-8 times the material thickness to guide the material flow. The punch of the shaping station is designed according to the final size of the part, and the cone surface is set with a rebound compensation angle of 0.5°-1° to ensure that the taper requirements are met after forming. The trimming station uses a carbide insert (YG15), and the edge sharpness is less than Ra0.1μm to ensure that the mouth is flat and burr-free.
The drawing process parameters need to accurately match the taper change. The drawing speed is controlled at 80-150mm/min. When the taper angle is greater than 90°, the low speed end is selected to avoid excessive stretching of the material. The blank holder force is adjusted according to the material thickness and taper angle. The blank holder force of thick materials or parts with large taper angles is 30%-40% of the drawing force (about 20-50kN). A partitioned blank holder ring is used to apply greater pressure at the bottom of the cone (15% higher than the mouth) to prevent wrinkling at the bottom. Extreme pressure emulsion (concentration 6%-10%) is used for lubrication to form a continuous oil film (thickness 3-8μm) on the surface of the die. For aluminum alloy materials, aluminum-specific corrosion inhibitors need to be added to prevent surface oxidation and discoloration.
The positioning and guiding system ensures the conical surface coaxiality. Each station is equipped with 3 sets of guide pins (φ3-6mm, H7/g6 matching), which match the process holes on the blank, and the cumulative positioning error is less than 0.05mm. Large conical parts (diameter>500mm) need to be equipped with anti-deformation supports during the drawing process, and a conformal support block is installed in the middle of the mold, with a fit degree of more than 90% with the conical surface to suppress lateral deviation during drawing (offset <0.1mm). The parts after drawing need to be tempered at low temperature (120-180℃, 1 hour) to eliminate residual stress (stress value <80MPa) and avoid deformation in subsequent processing.
During commissioning and maintenance, a key focus is on the quality of the taper surface. During mold trials, the taper angle and wall thickness are measured after each workstation. If localized thinning exceeds tolerance (>10%), the die clearance in that area should be reduced (by 0.05mm at a time). If the taper surface exhibits ripples, the blank holder force in that area should be increased (by 5% at a time). After every 800 pieces, the punch taper surface should be inspected for wear (reground if the profile error is >0.05mm), and metal debris should be removed from the die. During long-term storage, a 0.5mm-thick wooden board should be placed between the punch and die, and the surface should be coated with anti-rust oil. Regular (monthly) mold accuracy checks should be conducted to prevent deformation during storage that could affect forming quality.
