What causes burrs on metal?

What causes burrs on metal?
 
During metalworking, burrs are a common and frustrating problem. Burrs not only affect the appearance of metal products but can also negatively impact their performance and subsequent processing. So, what exactly causes burrs on metal?
 
(I) Process Factors
 
◦ Cutting: During turning, milling, drilling, and other cutting processes, if the tool's cutting edge wears or the cutting parameters are improperly selected (such as excessive cutting speed or feed rate), the metal material can easily tear during cutting, resulting in burrs. For example, when the tool's cutting edge becomes blunt, the cutting force increases, preventing the metal from being removed smoothly, and irregular burrs form on the workpiece surface.
 
◦ Stamping: During stamping, improper die clearance is a major cause of burrs. If the die clearance is too large, the material cannot be completely cut, resulting in large burrs on the cut surface. If the clearance is too small, the blanking force increases, causing excessive compression of the material, which also results in burrs. In addition, insufficient precision in the stamping equipment can cause the die to wobble during operation, which can also affect stamping quality and produce burrs.
 
◦ Casting Processing: During the casting process, if the molten metal has poor fluidity and cannot completely fill all areas of the mold cavity, defects may remain on the casting surface. These defects may form burrs during subsequent processing or cleaning. Furthermore, high surface roughness of the mold can also reduce the surface quality of the casting and increase the likelihood of burrs.
 
(II) Material Property Factors
◦ Metal Hardness: Metals of different hardnesses have different propensities to produce burrs during processing. Generally speaking, metals with lower hardness, such as aluminum alloys and copper alloys, are more susceptible to plastic deformation during processing, making burrs more likely to form. In contrast, metals with higher hardness, such as alloy steels, are more difficult to process, but under appropriate processing conditions, are less likely to produce burrs.
◦ Metal Microstructure: The metal's microstructure also affects burr formation. For example, metals with coarse grains are more susceptible to tearing at grain boundaries during processing, resulting in burrs. Metals that have undergone grain refinement have better machining performance and are less likely to produce burrs.
(III) Processing Equipment Factors

◦ Equipment Accuracy: The accuracy of processing equipment directly impacts machining quality. If the equipment's guide rails are worn or the lead screw backlash is excessive, the tool or workpiece's motion accuracy during machining will decrease, resulting in an uneven surface and burrs. For example, an old milling machine's worktable guide rails may have lost accuracy due to long-term use. This makes it difficult to maintain a flat milled surface when milling metal, making burrs more likely to form.
◦ Equipment Stability: The stability of the equipment during operation is also critical. If the equipment vibrates during machining, the relative position between the tool and workpiece will shift, leading to unstable cutting forces and, in turn, burrs. For example, if the flywheel of a punch press is unbalanced, it will cause vibration during the stamping process, affecting the quality of the stamped part and causing burrs.
Understanding the causes of burrs in metal helps us take targeted measures during metal processing to reduce burrs and improve the quality of metal products.

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