A reader called and inquired as to acceptable burr standards for metal stampings. The reason he needed the information was that his firm was preparing for ISO 9000 registration and with that, everything requires documentation.
I've had calls previously from people that purchased stamping dies that wanted to know what an acceptable burr was. Evidently, the parts produced from the new tools had perceived excessive burrs but since no known standards existed, there wasn't documentation to support the argument. To my knowledge, there are no acceptable burr standards. If someone has information to the contrary, please advise and we'll disclose it in a future article.
Perhaps the reason that there are no (known to me) acceptable burr standards is that multiple factors and combinations produce varying burrs with varying acceptance criterion due to those factors. For example, the process of fineblanking produces a burr that would not be acceptable with ordinary stamping. Large fineblanking burrs are acceptable because the process itself produces those large burrs. (See Fig. 1)
There's a tradeoff. If you want a smooth-edge blank, you must accept
the burr that results. It's not uncommon for fineblanked parts to be "timesaved"
(belt sanded) to remove the protruding burr prior to tumbling to remove the peripheral
remaining burrs.
Draw dies often blank the remaining flange from the drawn shell by a process known as "pinch trimming." Pinch trimming is done by cutting the metal with one sharp cornered cutting member and the other having a radius. In this case, if both cutting members were sharp, it wouldn't work satisfactorily. (See Fig. 2)
A variable that results in different size burrs is how sharp or dull the
cutting edges are. Usually, the sharper the tooling, the less the burr. I say usually
because certain materials and their characteristics produce an excessive burr initially.
As the tools dull, the burr diminishes. Those are rare circumstances.
A dull die will inevitably produce a large burr. That's commonly the sign that the die needs sharpening.
Configuration of the cut also determines the amount of burr. If a round hole is pierced, for example, the burr will be uniform and controlled. If a square hole with sharp corners is pierced, the corners will show the highest burr. That's due to the early breakdown of the sharp corner on the punch resulting in excessive clearance between punch and die that produces a large burr.
Clearance between the punch and die plays a critical role with resultant burrs. If the clearance is excessive, it doesn't always mean the burr will be high. Most often the inverse is true. A lot of it has to do with the configuration of the cut and how firmly the material being cut is restricted from lateral movement.
Material variation is the biggest contributor to burr fluctuations. Soft materials will obviously produce a different burr than hard materials under the same circumstances. Material characteristics, other than hardness, also will produce varying burrs.
The combination of materials and clearances will produce the biggest
fluctuation of burrs. People often use burrs to determine if the clearance between the
punch and die is correct. The cut edge will provide much more information than the burr as
to proper clearance between punch and die. Usually 1/3 shear and 2/3 break is optimum but
not with all materials.
Perhaps the reason that there are no published standards for acceptable burrs is the multitudes of possible variations that would result in different size burrs. I've heard that 10 percent of material thickness is the acceptable limit of burr height but don't have much credence in it. Fineblanking and pinch trimming would routinely exceed that limit and for conventional stampings, generally speaking, 10 percent would be excessive.
If you've been in this business long enough, you know whether a burr is acceptable or not without measuring it. Perhaps educating your customer beforehand with comparable samples will prevent misunderstandings.
Larry Crainich, President, Design Standards Corporation
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