Concrete stair, turning.

Hi.

I am about make a free standing almost half circle concrete stair. At the moment I am making the form, needed to cut a concrete block, to shape the stair. At the same time this form in Dietrichs could be used to make the form in the real life. Steps and sides are not so difficult, but the lower side I cannot figure out. I think I am in the right direction - but I don't get it.

I was thinking this method: I copy the steps down and move them to form a flat surface. Then angle all of this in the angle of the stair. But this gives wrong result. Then I copied the steps so there are the same distance (not moving them on flat surface) and the angled each step this stair angle. But that looks wrong also.

How to calculate / measure how the steps should be copied and angled to form the underside of the stair?? What line should the be rotated around? Anybody knows? I have a feeling that it is not enough only to rotate them around a edge of the step or along the middle of the step, but on the other hand the stair should have the same thickness under all steps, so I don't understand why.

I have a slight idea how I may be able to to do it. Copy the first step down a certain distance - maybe the step height. Then copy that step polar - but with maybe 20 x more copies, or more, than I used to make the top steps. Then move each step, up like I did with the top steps, but this time 20 x less than the step height. Then I am not sure, but maybe it is possible to camfer the edge of the steps, so the surface seems smooth. Maybe it can be done before copying the steps polar, I am not sure.

I have figured it out, kind of. More or less acc. do my idea above. Just the heights teased me a lot so it took quite some time to get to a result that I am very satisfied with. I.e. a "form" to "cast" the concrete in - and this model of the form, can actually with a few modifications be used to make a actual form for the stair.

Then I was thinking to put in a big block of concrete (panel) in the wall design and then in D-cam with intersecting volumes, with the spf as male and the concrete as female, cut the stair out of the concrete. I don't know why but it doesn't work as intended. The cutted concrete does not form the stair. I thought I would get two concrete parts - one that was the stair inside the form and one for the rest of the big concrete block outside the form. But no. I don't know if it is my version of Dietrichs that cannot do it or if I have done something wrong. But the shape of the lower side of the stair is (almost) perfect.

Well, I have come quite much closer to the end. But I cannot get the steps cut out. I cannot understand why the underside of the stair cannot be cut-out into the concrete block. I can maybe understand the step side, because the should go through the concrete block. So I need to make them wider.

Finally I got the result I wanted: A form to cast the stair in and the finished concrete stair. And what got me to the result? Well, I think two tips could be good to remember another time:
1) The things you make to cut the result, is good to make over dimensioned, ro they totally cover the volume that you cut into. So the only thing that is left is the result volume.
2) To set the precision value to a higher value than you expect. In this case 0,2 and 0,02, was precisions that did what I wanted.

Thank you very much. The job took a bit long time - but that is because I needed actually to do 3 things: A form that could be produced as a form to cast the stair, a "form" only to use in Dietrichs to cut the concrete stair with intersecting volumes - and the concrete stair it self. Each of these jobs required a bit different methods and that I was not aware of from the beginning. It required almost all my patience, but I am happy with the result.