# Understanding how to force the direction of a curve (or any action) for vaulted ceiling

Hello,

I’m working on a pretty basic problem, I’m trying to model a square room with 4 arches and a circular domed ceiling. Attached is a really bad picture but it should give you a sense of what I’m trying to do.

My approach is to make 4 arcs along the 4 walls

Then I want to make an arc across the diagonals. I’ll then rotate that arc to create the ceiling and cut it with surfaces from my 4 archs.

I think this all makes sense except for the life of me I can’t get an arc to go from one corner to the next. I am using the perspective view to chose my center point and start of arch, but then the curve gets created along the X,Y axis not the Z. If I use my Front view, I can’t select the correct points at all.

Below I use the Perspective and I create an arc from the correct point A to point B, but it’s on the wrong axis.

Below I use the Front view and I can’t even select the correct point A.

So my question is, if using the perspective view, how do I tell rhino that I want a curve (or any action) to occur along the Z axis?

Many thanks & apologies for such a simple question.

Many actions are aligned to the CPlane (Construction Plane). The construction plane is the gray plane with the red/green axes and the measurement grid. You will probably need to re-define your CPlane with the CPlane command, such that the plane in which your arc lies coincides with the CPlane.

Yeah, you can do that.

Also draw a vertical line up from the centre of the line going between the corners and you should be bale to snap along that as a guide. Should work with the near snap tuned on.

or reorient the c plane with plane by three points will work well too.

this is probably going to be more confusing to type/read than to do but here it is anyway… (there are other ways using the prebuilt arc macros but for this case, just use the standard ‘Arc’ command)

• run the Arc command
• choose the ‘Start Point’ option
• click on one of the corners (the command will now switch to ‘End Point’)
• click on the corner diagonal to the start point corner
• choose the ‘Direction’ option
• place the cursor on the vertical edge of the starting point
• press the Tab key (this will lock the direction vertically)
• move the cursor up
• mouse click when vertical half circle is previewed

it’s not as complicated as it sounds
good luck

there are some other ways as well which don’t require cplane modifications… try this one for now though.

If you just want to create the thing without all the geometry. This is how I would do it. A sphere whose diameter is the diagonal of the box. Trim away its lower half. Extrude the sides of the box up to trim the sphere. Use surface from planar curves to fill the gaps.domed roof.3dm (170.0 KB)

Draw a square with vertices outside the sphere. trim the sfere like in the attached picture
Ciao Vittorio

sorry for the slow response everyone, apartment got flooded

anyway the sphere method seemed the easiest, and it worked for me. I still need to read up on how cplane affects commands tho, cuz I’m having a really hard time controlling where my points land when using my mouse. For simple geometries I keep just doing the math in my head and using point coordinates, but it’s starting to get confusing.

thanks all

math? what’s that?

you should barely have to use arithmetic style maths when using 3d drawing apps.
geometry skills combined with inferencing/snaps & a few key functions eliminate most of the need for a calculator…

i don’t know… mostly just keep drawing/practicing… it will get easier or less confusing.

sorry about the apartment though… that happened to me last year and it sucks… bad.

thanks Jeff.
yes I’ve been working more with guide lines and snap onto those lines, def useful.

apartment is ok now but I’m doing all this on my little laptop. desktop seems to work tho, just nowhere to put it. what can you do?