I must acknowledge the help of my colleague Graham Stephenson with this tip.
I was in a Sheetmetal training class with a customer who uses lofted bends a lot in his sheetmetal products - chutes and hoppers for the food industry. We created a series of lofted sheetmetal parts but the option to add the bend lines and control the deviation was not always available. This is a limitation and affects parts that have loft sections that are not exactly parallel and have dissimilar shapes - e.g. arcs with dissimilar included angles.
However, being able to add the bend line positions is invaluable when you come to fold the part - otherwise how do you know where to fold it? The workaround for this is dead easy, once you are in the know! You simply edit the loft sketches and use the 'Fit Spline' tool to run an open spline through the section. The spline will convert the underlying geometry to construction geometry so that when you select the sketch it picks up the spline. Hey presto! We now can add as many bend lines as we need and see the deviation.
Below is an image of a standard Design Library lofted sheetmetal part. I have modified the profiles and you can see I now have control over the bend lines.
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