Sheet metal fabrication basics: 7 strategies for punching success

The punch press is one of the most mature technologies in metal fabrication, and yet it’s still one of the most misunderstood. Modern machines and tooling have turned the punch press into the Swiss army knife of the fab shop. It punches holes, just a few here and there or in a dense perforation. It cuts large panels. It forms louvers, embosses, and other complex shapes, and in some cases bends flanges several inches high.

Still, because the punch press can do so much, processing variables abound, and if they’re not accounted for, part quality and throughput can suffer. Understanding a few punching fundamentals can go a long way in ensuring that the multitude of processing variables remain well under control.

1. Communicate Forming Variables
Walk into a fab shop and you’ll likely see technicians working with CAD. They’re transforming a drawing into a part that can actually be made out of sheet metal. A big part of this is taking into account bend radii.

Just as bends made on the press brake require radii, so do forms on the punch press. These realities, while obvious and virtually second nature to most fabricators, may not be so readily apparent to part designers, especially those who don’t work regularly with sheet metal.

Consider the embossed hat section with a 0.500-in. flat section, a 0.200-in. height, and a 45-degree emboss angle, as shown on the left in Figure 1. It’s simple to draw on a computer, but it’s impossible to manufacture out of a single piece of sheet metal. Sheet metal forms to a radius, and something has to give. So we usually start with defining the part’s design intent and the critical dimensions required for that intent.

Yet another variable is material thickness. If, say, a part designer specifies 16 gauge, that’s fine, but depending on the source, a single material gauge can vary 0.005 in. or more. Many designers specify the “top of sheet to top of form” as a critical dimension. But if a fabricator needs to hold the overall dimension, from the bottom of the sheet to the top of the form, problems arise as material thickness varies ever so slightly from one lot to another. The same thing applies to tensile strengths, which also can vary slightly.

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