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How to determine the appropriate amount of fillet for sheet metal parts?

Nowadays, only deburring the surface of sheet metal parts is often not enough. More and more users need to fillet the edges of sheet metal parts. But do you know the size of rounding? How to determine the appropriate fillet amount?

The answer depends on the purpose of the fillet. The agent processing enterprise must process the sheet metal parts according to the drawing specifications. Generally speaking, these specifications must be accurate and unchangeable. If it is indicated in the production drawing that the fillet must be rounded, the specification of the chamfer will have a considerable impact on the cost of the workpiece. Therefore, whether for the manufacturer itself or for the processing enterprise, it is very important to correctly select the appropriate chamfer amount.


Meet the needs as the premise, as little as possible

In order to ensure the safe and efficient use of downstream processing equipment, sheet metal parts must be smooth and free of burrs. These downstream devices can be gate presses, bending machines or leveling machines. Before bending or leveling, if the material is burr free, there is no need for rounding, because the leveling roller is usually damaged only by raised burr. The same is true for other tools such as turning tools and milling cutters. Therefore, parts without burrs are more suitable for subsequent processing.

In order to avoid workers being scratched, experience has shown that only 0.1mm chamfer is needed to ensure safety. Even latex gloves, pneumatic hoses or cables that often contact the edges of sheet metal parts will not be damaged by sheet metal parts with a rounding range of less than 0.1mm.

When the coating is considered, the influencing factors become complex and diverse. The first is the type of coating (KTL, powder, wet paint), the type and quality of coating, and then the equipment parameters, such as pretreatment, drying time or drying temperature, until the final placement of the product. These factors, together with the edge radius, play a decisive role in the duration of rust protection. Therefore, the appropriate chamfer amount must be determined through test to prolong the anti-corrosion protection period. Of course, all other influencing factors must remain unchanged in the test.


A minimum chamfer of 0.5 mm provides a reliable coating

According to DIN EN ISO 9227:2017, “neutral salt spray test NSS” is usually used as a method to test corrosion protection. The corrosion process is considered over time. In addition, the cross section can be made to show the thickness change of the coating, so as to measure the rounding amount of the sample (supplementary measurement can be made if necessary). These samples can also clearly show that the shape of the radius has an effect on the coating thickness. In the case of asymmetric fillet, the coating thickness decreases at the maximum curvature. Therefore, it can be concluded that an ideal edge rounding effect can be obtained with a uniform radius.

The standard we can get from this (for example, for steel structures or offshore buildings) requires that the edge rounding radius of some sheet metal parts is 2.0mm or greater. However, most practical applications show that good coating adhesion can be obtained as long as 0.5mm rounding is done on sheet metal parts. Because the chamfer amount and the resulting tool cost will increase proportionally with the increase of edge radius, it is necessary for users to find the correct rounding amount to ensure reliable and economical machining. That is: meet the needs, as little as possible.