It's Time to Abandon 10-Percent Rules
February 1, 2017Comments
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.”
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