Sweep 1 or 2 rail help

hello I’m quite new to Rhino, getting used to it quickly though. Am on Rhino7 (7.2.21012.11001, 12/1/21). Im trying to recreate a dome shape for a bus model i’m making. Not sure if im using the right method but ive tried using sweep 1 rail, and sweep 2 rail. I can get close to how i want the result but cant perfect it.Screenshot 2021-01-23 144529 Screenshot 2021-01-23 144606 Screenshot 2021-01-23 144651

In the first/second pic you can see the sides of the bus taper inwards. The top red profile is the end of the main roof where the dome begins.Pink is the centre line, bottom red rail is where the dome ends. All three of these are the right shape and cant be altered. (there are two other red rails on the sides -see later on).
First attempt was to use sweep 1 rail, using the pink centre rail, then the lower profile, then the upper profile. This works great, except at the sides of the dome where it meets the window, there is a gap (see pics 3,4). I can over come this by adding the red side rails mentioned earlier. But using sweep 2 rail, i can only use one of those side rails plus the centre rail. When i do that its virtually perfect, but i can only do that on one side, as i understand it you can only use max. 2 rails for that operation? I should add I’m aware that the squared off corners above the last windows have yet to be clipped off to shape. In real life the fibreglass dome rivets over the metal panel the windows are mounted into.)
Is there a way round this, I’m sure there is, its just with my limited knowledge i cant find it yet! thanks to anyone who might be able to help. Mark

maybe you can try making several surfaces instead of solving many curvature situations with a single one.
something like this

the blue and red surfaces canbe done by Loft or sweep2 or sweep 1 and the green surface could be a blendSrf between them

Can you upload a .3dm file with just the curves. Include all the curves you want to match. You can create the file using Export and selecting the curves to be exported. Upload the file by either dragging it to where you type your post, or by clicking on the vertical arrow icon above where you type your post.

How accurately does the surface have to match the input curves?

thanks Diego, a bit beyond my capabilities at the moment, ive had some success with experimenting with the profiles.

i experimented with the profiles. I moved the one at the start of the dome further back slightly, and made it so it was only half the width of the bus.Likewise made the lower profile only half width. I adjusted the curve on the side next to the window so it was a closer fit. Now i could use that curve plus the main centre line curve as the two rails and use sweep 2 rail. Worked a treat, then just mirrored the one half to make the other. A workaround maybe but it’ll probably be good enough on such a small sized model .Just need to trim the sharp corners off projecting through the dome from the body, they would have to go anyway. David Cockey i’ll still upload the information as I’d be interested to see if there might be another solution.
Screenshot 2021-01-23 172403 Screenshot 2021-01-23 172436

D11 Rear Dome profiles.3dm (325.7 KB)
For David Cockey, thank you

His suggestion is intended to make it easier to solve the problem. There is the rule of thumb that you split surfaces where the surface shift significantly in curvature or other features. The reason for doing so is that it is harder, sometimes impossible, to make a single surface which involves so many “multiple features” as your surface does. Try the simpler way (Diego’s suggestion) because what you are trying is not simple, as you have noticed.

// Rolf

Was the rear of the original bus metal or fiberglass? Any photos? Did you create the curves? If so based on what?

I frequently find studying the original shape and trying to understand how it was designed and built can be a major aid in modeling.

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David Cockey i had a 2D photocopied plan which was originally hand drawn.I scanned it into Rhino, scaled/adjusted it to the correct size for the model, then traced over the drawing to get the curves/profiles. Ive got all four sides and an interior top view in the drawing , all lined up as near as possible so i can ge the positioning of items somewhere near; i can then switch the plans layer off and fine tune the drawing. Not very conventional i expect but it works ok for me. The rear dome was made of fibreglass, but all the edges/adjoing panels were metal.


Thanks Rolf, as i get more experience with Rhino and learn how to do new tasks, I’m sure it will become apparent that there are easier ways to do things. For the time being making just one half of the dome seems to have worked, then just mirroring it across and joining the two halves.

another way that could help in the future is to think in macro shapes or basic volumes and add features later

Modeling one half is conventional practice in boat design and auto design. Unless there is a crease along the center plane the surface is normal to the center plane. If a curve or surface is normal to the center plane then a mirrored curve or surface will have both tangential continuity and curvature continuity across the center plane with the original curve or surface… The curvature continuity is an automatic bonus, no additional work required.

How accurate are the line drawings known to be? How closely does the model need to match the line drawings?

the plan is supposedly based on the bodybuilders drawing but isnt very well drawn ( i suppose there are limitations with drawing in such a small (4mm to 1ft.) scale. Really i’m just using it as a guide and using my own measurements and photos to get things reasonably accurate. It’s not critical at the end of the day, the aim is to make a 3d printed bodyshell (also the interiors) ,and use those to make rubber moulds from which resin copies can be cast. This is my first attempt so learning all the way and im grateful for the help

thank you Diego for that, it hadnt occured to me to do it that way, but i can see you end up with the same result so that will definitely be useful.