Peter Ulintz
PMA Technical Consultant

Punching and Cutting Guidelines

Punching holes in a flat blank generally avoids the need for expensive cams and increased die complexity. But should a hole be required in a vertical flange, a process engineer may choose to punch the hole after completion of all forming operations. Though punching the hole in the flat may be less expensive, maintaining an accurate position in the final stamping with such an approach might prove difficult or impossible.

Stampers should punch holes and openings in deep-drawn parts after forming. Extensive material flow in deep drawing makes developing final hole sizes and their locations very difficult. Holes located in regions of high compression or tension will distort easily.

Punch holes located close to the edge of a blank prior to blanking. The minimum distance of the punched hole to the blank edge should be no less than two times material thickness. Slot features greater than 10 times material thickness in length require a minimum distance to the blank edge of four times material thickness (Fig. 1). 

When the distance of a hole to a flange is specified as less than 2.5 times material thickness plus the bending radius, 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 to prevent the slot edge from distorting (Fig. 2). If additional holes are required, punch the holes first, form the flange and then punch the slots.

When large and small holes are located close to each other, punch the larger holes first. This prevents distortion of the smaller holes caused by the cutting action of the larger punch.

Cut-Edge Quality

The cut-edge quality of a punched hole proves critical when forming stretch flanges and flanged holes, with sharp tools and proper die clearances required to produce quality edges that will stretch uniformly and without fracturing prematurely. 

Hole flanges and extrusions start with a punched hole in a flat surface of the sheet metal, with the hole subsequently expanded to the required ID, usually in one hit. The wall height results from the punched-hole diameter, the quality of the punched-hole edge, the diameter of the flanged hole and the amount of allowed wall thinning. The best solution for maximum hole-flange and -extrusion heights: Create the hole in three singular steps, punching, shaving and coining the bottom side prior to forming the extrusion. 

For most materials, the amount of material to be removed by shaving usually amounts to 10% of material thickness. This provides good surface finish and leaves enough material in the scrap ring for slug retention, important as slug retention generally is considered to be the most common problem associated with shaving operations.