The basics of form cutting with punching tools

March 2011 - Form cutting using punching tools combines the fundamental elements of cutting with wedges and shear cutting, the first two topics covered in this three-part cutting series. 

Power punching tools, power nibblers and power shears follow the same principles of applied physics, displacement of material, compressive force, cutting force and movement of the position of the blades (punches or nibblers) in separating material.

Any desired shape can be cut out of sheet metal by using cutting tools consisting of punches (top die) and cutting plates (bottom dies, also known as negative dies). This kind of process is suited to production, because the cutting tool must be specific for each desired workpiece shape.

How does it work?

To begin, the material is indented slightly because of its cold formability when force starts to push the punch down onto the material. A slightly bulging edge appears around the punch, and the underside of the sheet buckles slightly, too. When the punch penetrates deeper, the cutting force exceeds the shearing strength of the material. 

In the very beginning, the structure of the material separates with a smooth cutting face. Before the edges of the punch and the cutting plate cross each other, the remainder of the sheet cross section breaks, which results in further rupture down the cross section of the sheet. (See Figure 1)

A guide plate can make the cutting process more precise because it pulls the material off of the punch after cutting.

The sequential punching tool combines several punches in succession in one tool. With every successive punch stroke, the cut outlines of the workpiece change until it is cut off or cut out from the continuous strip of material. The feed between each cut must remain exactly equal.

Combination cutting tool

A multipurpose tool can be used to cut workpieces very precisely in large numbers. The main punch cuts the external form of the workpiece and functions at the same time as the cutting plate for the inside shape. This cutting system ensures the correct relative position between the outer and inner shapes.

The main cutting punch for the outer form is pinned to the base of the cutting tool. The cutting plate and the required piercing die are mounted in the upper part of the machine. This arrangement ensures the cut-off portion can fall down through the main punch.

Also, the main punch pushes the workpiece upward into the negative die, from which the ejector pushes the workpiece out after it has been finished. The ejector also has the function of guiding the piercing die until it penetrates the workpiece.

The combination cutting tool works with column guidance and eliminates inaccuracies caused by the feed in sequential punching. The exact position of the perforations, relative to the external outline, depends only upon the accuracy of the tool. (See Figure 2)

Always use caution and apply safety measures when working with any cutting, shearing or punching equipment. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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