Tooling by Design
Cutting and Punching Sequence
• Punch holes in the flat blank whenever possible to avoid expensive cams and increased die complexity.
• Punch holes after forming or drawing whenever tight tolerances are specified. Depending on product geometry and tolerances, this may require tipping of the part in the die.• Punch holes located close to the edge of a blank prior to blanking. This prevents the holes from distorting during the blanking process. The minimum distance from the edge of a punched hole to the edge of the blank should be no less than two times material thickness. For slot features greater than 10 times material thickness in length, the minimum distance to the blank edge should be no less than four times material thickness (Fig. 1). When these distances are less, the chances of bulging and distortions in soft materials and cracking in harder materials increase significantly.• When the distance of a hole to a flange is specified less than 2.5 times material thickness plus the bending radius (2.5t +R), punch the hole after forming to avoid distortion. Slot-to-form spacing for long slots should be at least four times material thickness plus the bending radius (4t + R) to prevent the slot edge from distorting, as shown in Fig. 2. When additional holes are required, punch the holes first, form the flange and then punch the slot.
• The edge quality of the punched hole is critical when forming flanged or extruded holes, also known as hole expansions. Sharp tools and proper die clearances are required to produce quality hole expansions without fractures. For maximum extrusion length, form the hole from the direction opposite of the punching operation. When extruding and punching in the same direction —oftentimes an unavoidable necessity —coin the bottom side of the punched hole first or consider shaving the hole before extruding.
• When cutting stretch carriers in progressive-die strips for cup drawing, cut the inside diameter first. Cut only half through the material and use a pressure pad in the die cavity that bottoms out. The bottoming pad assures that the cut will only penetrate half through the material thickness. This eliminates having to push the blank back into the punched opening, which usually leads to slivers in the die. Cut the outer diameter next, using the same technique.
• Punch holes and openings in drawn parts after forming. Extensive material flow in deep-drawing operations make it very difficult to develop final hole locations. Holes located in regions of high compression and/or tension are distorted easily.
• Punch pilot holes in progressive-die strips undersize if the final diameter is critical. The hole may be punched larger or shaved in the final operation to establish its final size.
These punching sequence guidelines will help improve product quality and process reliability for most stamping operations, at very little additional cost. MF
Related Enterprise Zones: Tool & Die
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