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A general rule of thumb in sheet metal is to keep the edge of holes at least three times the thickness away from the bend line. Following this rule will ensure that the hole do not deform during bending and also allows for more consistent bending.

The reasoning behind this rule has to do with the geometry and limits of the tooling that is actually performing the bending. To keep pressing tonnage within reasonable limits, the general rule for bending is to select a die with an opening 6-8 times larger than the thickness of the material (the latter for thicker plate or higher tensile materials). Since half of the bend happens on each side of the bend line, having the edges of holes at least 3x the material thickness away from the bend line will ensure that the hole is properly supported during bending.

There are several options for when a hole absolutely has to be closer to the bend line. One option is to machine the hole or slot after bending, or use special (and expensive) tooling that can handle higher tonnage and supports the entire bending region. For more information on this type of tooling click here.

Another issue to consider that falls along the same principle is the minimum flange length that can be bent with the needed die width.