Drawing punch and die clearance
The punch-die clearance, defined as half the difference between the punch’s outer diameter and the die’s inner diameter, is a crucial parameter in drawing die design, directly impacting the quality of the drawn part, the die’s lifespan, and the smoothness of the drawing process. A reasonable clearance ensures smooth material flow, reduces friction and wear, and lowers drawing forces. However, excessive or insufficient clearance can lead to reduced part dimensional accuracy, deteriorating surface quality, and even defects such as wrinkling and cracking. Therefore, scientifically determining the punch-die clearance is crucial to ensuring a successful drawing process.
The gap between the punch and die during drawing must be determined based on the material thickness, number of draws, and part precision requirements. For the first draw, due to significant material deformation and significant thickness variation (especially at the transition between the wall and the bottom), the gap is typically set at 1.1 to 1.3 times the material thickness (double-sided gap). For subsequent draws, as material deformation decreases and thickness variation stabilizes, the gap can be reduced to 1.05 to 1.2 times the material thickness. For example, for a mild steel part with a material thickness of 2mm, the double-sided gap Z = 2 × 1.2 = 2.4mm for the first draw, and Z = 2 × 1.1 = 2.2mm for the second draw. For high-precision parts, this gap must be strictly controlled within a narrow range, with a deviation of no more than ±0.01mm. For standard precision parts, the gap deviation can be relaxed to ±0.03mm.
The impact of clearance on the quality of drawn parts is primarily reflected in three aspects: dimensional accuracy, surface quality, and shape accuracy. When the clearance is too large, the radial dimensional accuracy of the drawn part decreases, the mouth tends to develop a bell-mouth shape, and the barrel wall develops a large taper. Furthermore, due to the material oscillating in the clearance, the surface is prone to scratches and indentations. For example, when the clearance exceeds 1.5 times the material thickness, the mouth diameter of a low-carbon steel drawn part will be 0.5-1mm larger than the design value, and the surface roughness Ra value will increase from 1.6μm to over 3.2μm. When the clearance is too small, the friction between the material and the punch and die increases, causing severe scratches on the part surface. Excessive friction also increases the drawing force, increasing the thinning of the barrel wall and even causing rupture.
The impact of the gap on the life of the mold is also significant. A reasonable gap can reduce wear and thermal damage to the mold and extend its service life. If the gap is too small, the material between the punch and the die is strongly squeezed, causing the die edge to bear large lateral forces and friction, exacerbating wear. At the same time, the heat generated by friction increases the temperature of the mold, making thermal fatigue cracks more likely to occur. For example, when the gap is 0.8 times the material thickness, the mold wears 2 to 3 times faster than when the gap is reasonable, and the service life is reduced from 100,000 times to 30,000 to 40,000 times. When the gap is too large, although friction is reduced, the material is prone to shaking during the drawing process, causing the punch and die to bear uneven impact loads. Especially during high-speed drawing, this impact will cause vibration fatigue in the mold and shorten its service life.
Different materials have varying sensitivities to gaps during deep drawing, requiring targeted adjustments. For materials with good plasticity (such as mild steel and aluminum alloys), the gap can be appropriately larger, as these materials experience significant thickness changes during the drawing process. A larger gap can accommodate this increase in thickness and reduce the risk of wrinkling. For example, the drawing gap for aluminum alloys can be 1.2 to 1.4 times the material thickness, slightly larger than for mild steel. For materials with less plasticity, such as high-strength steel, the gap needs to be smaller to ensure sufficient restraint and prevent fracture caused by excessive material flow. Typically, the gap is 1.0 to 1.1 times the material thickness . For highly viscous materials, such as stainless steel, the gap needs to be 10% to 20% larger than for ordinary steel, and good lubrication should be used to reduce friction and adhesion.
Gap uniformity is more important than a single gap value, especially for large or complex drawn parts. Uneven gaps can lead to different material flow rates. Thinner parts experience greater friction and slower flow due to the smaller gap, while thicker parts experience less friction and faster flow due to the larger gap. This can ultimately cause part distortion, skewness, or partial cracking. To ensure uniform gaps, precision guides (such as rolling guide pins and guide bushings) must be used during mold assembly, with guidance accuracy controlled to within 0.01mm. Furthermore, the punch and die must be machined to IT6-IT7 precision, with flatness and perpendicularity errors within 0.005mm/100mm. For multi-station drawing dies, the gaps between each station must be consistent, with a deviation of no more than 0.02mm, to ensure continuous and stable material deformation. Properly determining and maintaining gap uniformity can significantly improve drawn part quality and mold life.
