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It's Time to Abandon 10-Percent Rules

Wednesday, February 1, 2017
 

At PMA’s Metal Stamping Technology and Tool and Die Conference, held last December in Chicago, several attendees discussed acceptable burr-height limits for stamped parts. Surprisingly, many agreed that the maximum acceptable burr height is equal to 10 percent of sheet thickness.

While this old rule of thumb may be true if all stampings were produced from low-carbon steel less than 0.050-in. thick, burrs become noticeable as they approach 0.003 in. high. And, in some instances, burr heights exceeding 0.005 in. can become dangerous, having the potential to cut or harm assembly workers or customers. Thus, for materials greater than 0.050 in. thick, the 10-percent rule becomes unreasonable.

Furthermore, die-cut edges subject to high tensile stresses—such as stretch-flanging—can be very sensitive to edge quality and burr height. The quality of the cut edge, including burr height, can be the difference between producing a good stretch flange or not.

Another rule that seems to garner equal consensus: Punch-to-die cutting clearance should be eight to 10 percent of material thickness per side. This and so many other so-called rules of thumb have been repeated by so many, and for so long, that we’ve come to accept them as fact without regard for their origins. The 10-percent rule for burr height is no exception. It can be found in publications as early as the 19th century; a book titled “Dies and Die Making (Journal of Commerce,” 1897) by J.L. Lucas is one example.

The techniques and processes for metalforming and tool-and-die construction have changed dramatically since the 1800s, so it should come as no surprise that many rules of thumb no longer apply. Case in point: Lucas also claims, “In the re-punching of brass and copper, the use of butter-milk as a lubricant will give better results than any oil or soap water that we have yet found.”

Today, metalformers routinely stamp a variety of material types, with a wide variety of mechanical properties and in a range of thicknesses, a far cry from the limited metallurgy and slab-rolling capabilities of 19th-century steel mills. As a result, one must discard archaic rules of thumb and apply an engineering approach to specifying burr-height limits.

In this case, an engineering approach would recognize that cutting and punching processes impart shear stresses into the workpiece; these stresses relate to the ultimate tensile strength of the workpiece material. Sheet materials with high yield-to-tensile-strength ratios (Inconel, for example) require tight punch-to-die cutting clearances, typically 5 to 8 percent of material thickness per side. These highly formable materials want to deform or extrude the hole feature rather than shear or cut, making burr-height control difficult to manage.

Materials with low yield-to-tensile-strength ratios (many high-strength steels, for example) require greater punch-to-die clearance to provide the mechanical leverage required to break the slug cleanly with a minimum burr. Here we use engineered clearances—punch-to-die cutting clearance beyond the traditional 5 to 8 percent of sheet thickness per side, the result of extensive research conducted over the past 60 years.

Recent research conducted on advanced high-strength steels indicate that as much as a 21-percent-per-side clearance may be required for some materials. Engineered clearances as great as 40 to 50 percent per side may be necessary for very thin materials, depending on hole size and material type.

Today, metalformers should use standards—not rules of thumb—to establish burr-height limits on stamped parts. Two such standards: DIN 9830 (German) and NF E81-010 (French). Both classify acceptable burr heights relative to the workpiece-material thickness and its tensile strength.

The illustration includes an excerpt from each standard for a precision-punched hole in 0.063-in.- and 0.157-in.-thick material. Notice that the maximum burr allowance for 0.063-in. sheet with a tensile strength less than 36.3 ksi is 0.0067 in. per side. This follows the 10-percent rule of thumb. However, as the tensile strength for the same material thickness increases to 90 ksi, the maximum allowable burr drops to 0.0023 to 0.0028 in.

When material thickness is increased to 0.157 in., the allowable burr height for the 36-ksi and 90-ksi materials are, respectively, 0.0142 in. and 0.0063 to 0.0079 in. (less than 10 percent of material thickness). Certainly, most stamping-shop customers would not accept 0.016-in. burrs (10 percent of 0.157-in.-thick material) on any of their products. MF

 

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