I've got the attached image. It's obviously a screenshot from some paid 3D CAD program. The person who gave it said "this is how the box corners should be rounded. It's symmetric, it fits inside the original rectangular box and the rounded zone meets all 3 planes tangentially, i.e. the rounded area has no sharp edges"
He did not know how to make it. Being full of confidence based on total ignorance I said "A piece of cake, simply sweep one of the edge arcs simultaneously along the 2 others, use the generated surface to cut the box body."
But It did not work. Any attempts to make a sweep along the arcs or to make a blend or fill between the arcs resulted an error or at least one of the edges was still non-tangential, sometimes even bulged beyond the original box area. Trying to apply Pull to round a remaining sharp edge gives totally different result which was not accepted.
Maybe I do not understand what options should be selected and which selections result something impossible. So, how to make that symmetric tangential rounding in DesignSpark Mechanical?
Add 10 days later: I have got a STP file which is said to be a step- export of a rounded box in a 3dm-model. It at least looks the same, but as a STP-file, it's read-only i.e. quite useless in unexpanded DSM. People who know more or have file conversion tools may use it or see something useful from it. The file is an attachment.