Question

What are the methods by which the effect of punched holes can be considered in
the calculation?

Answer 1

Metal punching processes are widely used in mass production because of their simplicity, high levels of productivity, and low costs. The quality of the product made by punching greatly depends on various processing parameters such as the tool geometries, sheet material properties, punching conditions, punch and die material properties, and tooling wears. In this study, the effects of die clearance angles and die opening sizes were compared on the accuracy and quality of punched holes produced on 1.4-mm thick S275 mild steel metal sheet. A total of nine punching dies were made with die clearance angles of 0.25°, 0.50°, and 0.75° for each die opening sizes of ø20.225 mm, ø13.225 mm, and ø12.225 mm. The burr formations and die weight losses were monitored for 10th, 50th, 100th, 500th, and 1000th strokes. The results show that the die weight loss, indicative of wear, increases as the number of strokes increase, with the most losses at the initial stages of the punching process (10th–50th strokes). The lowest weight loss was obtained for dies with 0.75° angular clearance, corresponding to the lowest wear occurrence. The low wear indicates that the shearing edge remains relatively sharp and thus able to produce good quality holes. Thus, for producing good quality holes and increasing tool life, it is recommended that an angular clearance of 0.75° is considered in the design of punching dies.

The punch force required to punch a piece of sheet metal can be estimated from the following equation:^[5]

{\displaystyle F=0.7tL(UTS)}

Where t is the sheet metal thickness, L is the total length sheared (perimeter of the shape), and UTS is the ultimate tensile strength of the material.

Die and punch shapes affect the force during the punching process. The punch force increases during the process as the entire thickness of the material is sheared at once. A beveled punch helps in the shearing of thicker materials by reducing the force at the beginning of the stroke. However, beveling a punch will disort the shape because of lateral forces that develop. Compound dies allow multiple shaping to occur. Using compound dies will generally slow down the process and are typically more expensive than other dies. Progressive dies may be used in high production operations. Different punching operations and dies may be used at different stages of the operation on the same machine.