If a hole is drilled or CNC punched too close to a bend in a sheet metal component it can take on a tear drop or “pulled” form. In Figure 1 below you can see a hole that has been pulled by the action of the bend that is too near to it. This may not be a problem if the hole is a simply for clearance in the sheet metal such as for a screw or bolt but would certainly be no good if the hole had to be tapped accurately with a thread. The hole position would either have to be moved away from the bend in the sheet metal or drilled out after the bend has taken place which is always a more costly and time consuming option.
There are 2 formulas that can be used to define whether a a pre-pierced or drilled hole will fall into the bending deformation zone when the sheet metal component is being bent.
When the hole is less than 25mm in diameter: d = 2t + r
When the hole is greater than 25mm in diameter: d = 2.5t + r
where
d = minimum distance
t = material thickness
r = radius of the bend
Figure 2 shows a hole correctly placed outside the bending deformation zone.
Form height compared to sheet metal thickness, an approximate ratio.
To establish the height of the bend with regard to any sheet metal material thickness, as shown in Figure 3 use the formula:
d = 2.5t + r
again where
d = minimum distance
t = sheet metal thickness
r = radius of the bend
We can produce a smaller distance (d) by secondary post bend operations such as machining the sheet metal down to a given size but this will be a costly process. Some tooling can produce tighter bend rads which can reduce the bend height. To be sure of your design options please take a look at our CNC bending tooling tables, if you are still not sure please contact us and we talk through your specific needs.