November 21, 2002
Over time negative tonnage can cause significant press and die damage. Understanding the factors that influence the amount of negative tonnage can help you control it.
Negative tonnage, often referred to as reverse or snap-through tonnage, is the undesirable result of cutting operations. Basically, when a punch breaks through the metal during a cutting or piercing operation, a negative force results that pulls down both the cutting punch and the top portion of the press assembly.
Here is a good analogy to describe negative tonnage, one that I use when teaching seminars. Imagine trying to push a Magic Marker® through the surface of a tautly held piece of paper. Now imagine trying to stop the marker's penetration so that it does not travel beyond flush with the bottom surface of the paper. It's nearly impossible, right? That's because for whatever positive force it took to make the marker penetrate the paper, there is an equal and opposite force needed to stop it from going too far. This opposite force is called negative, reverse, or snap-through tonnage.
Over time severe negative tonnage most likely will result in severe press and die damage. This article discusses the factors how to control reverse tonnage by examining ways to reduce it, utilize it, or adsorb it.
Several factors influence the amount of reverse tonnage generated:
The shear strength of a metal basically can be described as how much force it takes to fracture or break the metal when utilizing normal cutting clearance. Two main factors determine the shear strength of the metal: hardness and toughness. Higher shear strengths create higher reverse tonnages. For example, the shear strength of mild carbon steel, 1010 for example, is 22.5 tons, while the shear strength of 1070 (high-carbon steel) is 40 tons.
The metal's thickness also has a great effect on the force needed. Very simply, the thicker the metal, the deeper the punch has to penetrate before fracture occurs. To calculate roughly the cutting force needed, use this formula:
Thickness of the material x Length of cut x Shear strength of the material
For example, to cut a .100-inch thick, 10-in.-dia. blank made of 1010 plain carbon steel takes 70.67 tons, or 141,345 pounds, of pressure.
Tighter clearances require a higher force to fracture the metal. When the amount of clearance between the cutting punch and the die is small, the metal must be sheared a greater amount of its thickness before fracture begins.
Special operations, such as fineblanking and Grip Flow®, require high cutting tonnages to retain a fully sheared edge. Figure 1shows parts made using a specialized Grip Flow® process. Although this special process requires a great deal of tonnage, the reverse tonnage produced is not as great proportionally because the process uses slower speeds than conventional blanking or piercing. Always try to use the optimum cutting clearance for your particular cutting operation. Shoot for the old rule of thumb: One-third shear, two-thirds break.
As the cutting edges of tool steel begin to erode, the force requirements for cutting rise. Try to keep your tools sharp. Doing so decreases tonnage and reduces the cutting burr height.
Cutting shear is defined as an angle or series of angles ground or machined on either the punch or the die that function to reduce the necessary cutting tonnage. Grinding cutting shear is done to create a scissorlike action between the punch and the die. Figure 2shows punches with grinding shear. One metal thickness of shear angle can reduce the cutting tonnage by as much as half of the normal tonnage. Shear angles not only reduce the force necessary by the scissor action, but also reduce the tonnage by reducing the surface contact with the metal. Because shear has a tendency to distort the metal, it is recommended that it be ground on the scrap side of the die. Keep in mind that creating excessive shear may cause the punches to deflect or have a steep wall angle that's prone to excessive burring. Excessive shear most likely will result in short punch and die life.
The amount of shear is usually calculated with respect to the metal's thickness as well as the length of cutting perimeter. Avoid putting shear angles on small-diameter pierce punches such as quill-style punches. Having a sufficient amount of cutting shear is undoubtedly the most influential factor controlling reverse tonnage. Whenever possible, avoid cutting and blanking parts using dies without shear.
As the strokes per minute increase, so does the press ram velocity. As the velocity increases, so does the force needed to stop the ram at the bottom and return it to the top of the stroke. The faster you run a press, the higher the reverse tonnage. Because most stampers want to maximize output, slowing the press down usually is not an option for reducing reverse tonnage.
One of the best ways to check for excessive negative tonnage is to purchase a tonnage monitor with signature capabilities. The signature shows how much positive or negative tonnage is being used to perform the work. Every die has a certain "signature" that shows different tonnages needed at different points during the press stroke. Figure 3shows such a signature.
Most press manufacturers recommend not exceeding a reverse tonnage that is more than 10 to 15percent of the total rated tonnage for standard multipurpose presses and no more than 40 to 50 percent of the rated tonnage for blanking and lamination presses. For example, a 150-ton, standard, multipurpose straight-side press should not exceed 22.5 tons of reverse tonnage.
You can do a few things to help adsorb the negative tonnage created during cutting. First, stagger your cutting punch heights so that they are approximately one shear length or one-third of your metal's thickness. Various materials and thicknesses may shear differently depending on the selected cutting clearance, resulting in differing amounts of shear and fracture. The key is to stagger the punch lengths in such a fashion that you can utilize the reverse tonnage of one piercing or blanking operation to cut secondary holes.
Figure 4shows punch staggering. With punch staggering you reduce the punch contact area, as well as adsorb the reverse tonnage of one cutting punch that you can use to create an additional hole. In a sense, a free hole![image6]
Adding nitrogen cylinders and neoprene blocks to the die or press also will help adsorb negative tonnage. However, be careful not to load the press with more tonnage than it has available at any point during the stroke.
The causes of negative tonnage are numerous. Identifying the cause is the first step to resolving your problem. First, do everything possible to reduce the needed tonnage, and if necessary, find a means of utilizing or adsorbing it.
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