Determination of basic mold step distance and step distance accuracy
Basic die stepover and stepover accuracy are core parameters in multi-station progressive die design. Basic stepover refers to the distance between two adjacent stations and directly determines the strip feed length. Stepover accuracy is the deviation between the actual stepover and the designed stepover, affecting part dimensional consistency and die life. Determining both requires comprehensive consideration of factors such as part size, process flow, material properties, and production equipment. A combination of theoretical calculation and empirical verification ensures stamping process stability and product quality.
The basic stepover is determined based on the maximum part size and process distribution. For symmetrical parts, the basic stepover is typically equal to the maximum dimension of the part along the feed direction plus the overlap width on both sides. For example, if a rectangular part is 50mm long and has a 3mm overlap on each side, the basic stepover is 50 + 3 × 2 = 56mm. For irregularly shaped parts or parts arranged in multiple rows, the basic stepover must meet the layout requirements of all processes to ensure that mold parts at each workstation do not interfere with each other. For example, in multi-row arrangements, the stepover must be an integer multiple of the lateral spacing between the parts to ensure synchronous forming of all rows of parts. The basic stepover must also match the nominal pressure of the press and the size of the worktable. Excessive stepover (over 1000mm) increases mold length, requiring a larger press and increasing production costs. Excessive stepover (less than 20mm) can lead to crowded workstations, affecting operation and nesting.
The determination of step accuracy depends on the part’s precision requirements and production batch size, and is generally categorized into standard accuracy (±0.05mm), precision (±0.02mm), and ultra-precision (±0.01mm). Standard accuracy is suitable for daily necessities and low-precision structural parts, where side blades or stop pins can be used for positioning. Precision accuracy is suitable for automotive parts and electronic connectors, requiring a combination of guide pins and side blades for positioning. Ultra-precision accuracy is suitable for medical devices and aerospace parts, requiring servo feeding and multiple sets of guide pins for positioning, coupled with online detection devices for real-time error compensation. Step accuracy is also related to material thickness. Thin materials (t < 0.5mm) require higher step accuracy because they are more prone to deviation during feeding. Thick materials (t > 3mm) can have a more relaxed step accuracy, but reliable positioning during the forming process must be ensured.
Factors that affect step accuracy include feeding mechanism, positioning device, mold rigidity and material properties, and targeted measures must be taken during design. The accuracy of the feeding mechanism is the basis. The step error of mechanical feeding is ±0.03-±0.05mm, and the step error of servo feeding can be controlled within ±0.01-±0.02mm. Therefore, servo feeding is preferred for precision parts. The number and arrangement of positioning devices have a significant impact on step accuracy. Multi-station progressive dies require an initial positioning device (side blade or stop pin) at the 1st or 2nd station, followed by a guide pin every 2-3 stations to form a continuous positioning correction system. The clearance between the guide pin and the positioning hole must be controlled within 0.01-0.03mm. Insufficient mold rigidity will cause elastic deformation during stamping, affecting the step accuracy. Therefore, the die base thickness of large progressive dies needs to be increased by 20%-30%, and reinforcing ribs need to be provided. The elastic deformation of the material will also affect the step accuracy. For high-strength steel or materials with low elastic modulus (such as aluminum alloy), a compensation amount of 0.02-0.05mm must be reserved in the layout to offset the rebound after feeding.
The adjustment and compensation mechanism of the step distance is the key to ensuring long-term production accuracy, and the mold design needs to reserve adjustment space. In the mold with side blade positioning, the length of the side blade can be adjusted by shims. For every 0.1mm thick shim, the step distance increases by 0.1mm accordingly; in the mold with servo feeding, the feeding step distance parameters can be modified through the PLC control system to achieve stepless adjustment. The adjustment range is generally ±1mm. For forming processes such as continuous drawing, the material will change in length due to plastic deformation, and a step distance compensation station needs to be set. For example, an empty station is added after drawing, and the actual length is measured by the detection device, and the feedback is fed back to the servo system to adjust the subsequent step distance. During the mold trial stage, the step distance error needs to be measured through multiple stampings, and the error curve is drawn. According to the curve trend, the position of the positioning device or the feeding parameters are adjusted until the error is controlled within the design range.
Pitch accuracy inspection and maintenance must be maintained throughout the mold’s lifecycle. New molds must be tested for positional accuracy at each workstation using a coordinate measuring machine to ensure that the pitch error meets design requirements. During trial punching, 50-100 parts must be produced continuously to measure the dimensional deviations of the parts and calculate the cumulative pitch error. If the cumulative error exceeds 0.1mm, the positioning system must be readjusted. During production, positioning components must be regularly inspected for wear, and guide pins must be replaced promptly if wear exceeds 0.02mm. Surface wear or slippage on feed rollers can also affect pitch accuracy, requiring regular degreasing and replacement of severely worn rollers. By scientifically determining the basic pitch and pitch accuracy, combined with effective control and maintenance measures, multi-station progressive dies can consistently produce high-quality parts.
