Project Cad Technote: Sheet Metal Bending in CAD.
Sheet Metal Work is an interesting subject to which I could no doubt devote an entire blog to. Fortunately for us we don’t have to as this topic is covered in detail by the many professionals working in this industry.
However understanding some of the key principles is imperative to ensure that our CAD models created from the aviation manufacturers drawings are correct as the dimensions given do not always suit the CAD development process.
One particular aspect relates to something the Sheet Metal guys refer to as the Outside Setback. The Outside Setback is the distance from the apex of the outside mold lines to the tangent point of the outside radius. When the sheet metal is bent the inside radius pulls the edge of the material away from the apex of the bend.
Typically on many occasions we will have a developed profile for the part which is to be bent to the required profile with only a few dimensions noted to achieve this including a bend coincidence point and angle.
The image on the left is indicative of many situations that arise when working with the manufacturers drawings. It is not unusual for a dimension to be given to the projected point at “A” which understandably is important to ensure the part mates properly with another.
However in Inventor; for example; we only have selections at 1,2 & 3 for “folding” a part from a development sketch and no option to define the stated “Dim” to the point of coincidence; which therefore may not provide the desired result. We may of course have the angle, material thickness and usually the inside radius.
In Inventor this is the middle option from the sheet metal fold dialogue. In this case we have specified the complimentary angle (97 degrees).
In order for this to work we need to calculate the dimension OSSB. The smart guys in the sheet metal industry have this stuff all worked out and have an easy equation that we can use to ensure consistent accurate results.
B> denotes the complimentary angle which must be less than 170 degrees.
IR is the Inside radius and MT is the material thickness. (the dot in the middle by the way is multiplication).
In this example the dimension from the manufacturers drawing is stated from the hole center, which has been adjusted to locate point 3 by deducting the value OSSB.
It works perfectly and we now have a folded bracket from a development plan that complies with the stated drawing dimensions.
I should note that some CAD products take this into account and provide the necessary options for developing this folded model but where we have limitations a touch of maths goes a long way to achieving the desired result.
In this example the hole is very close to the bend causing a slight deformation. This could initially be drilled to a smaller diameter and reamed after bending or we could simply use a smaller bend radius; if permissible!