Clamshell Niche Help

Howdy! I hope this is the right place to ask this question, but if not, just let me know.

I’m looking to create a Greek Wall Niche with a Clamshell ceiling, like this one.

I have the basic outline drawn up, but I can’t figure out how to shrink the profile of the arches as they travel back to the center. My profiles just stay large as they travel back, and trying to intersect the faces doesn’t work.

I’ve looked high and low, and I’ve only found one model similar to what I want (Here, credit to Benji_02), but after taking it into my system and moving the groups around, I still can’t figure out how it was done. I’ve attached the file, has anyone got any ideas? Thanks for the help! Greek Arch.skp (2.0 MB)

Try this. It needs the Fredo6 Plugin and the LibFredo library plugin to work. Download both from the SketchUcation plugin store (NOT the Extension Warehouse) and install them. The plugins are free, but you need to sign up for a free account there.

Draw a quarter circle centred at the origin, centred on the Y axis (use the Arc tool, not the two point arc tool)

Rotate/Copy the arc about the blue axis, by (say) 15° depending on how many segments you want in your niche.

Use the two point arc tool (default shortcut a to make a bulge on the red/green plane between the two large arcs.

Select all the edges, then run the Curviloft/Skin contours option.

Click anywhere in the model to finish (the big green tick should be over a blank area of the screen). Reverse faces if necessary to get the front (white) faces inside.

Rotate/Copy the segment you’ve just created 11x round the blue axis.

Done

PS. Your upload of the model you found hasn’t worked - upload it again.

PPS. I found the clamshell model on the 3D warehouse. Looks as though I drew the initial quarter circles on the wrong axis. Start with a quarter circle on the red green plane, then rotate that about the green axis, select all edges, then run Curviloft to get this.


Then rotate/copy 11x as before, but about green not blue…

3 Likes

Thanks for the information! I had tried Curviloft after doing some research, but I couldn’t get it to work on my end. Comparing your work and mine, I think that it’s because I have the center circle drawn in, and the plugin can’t make sense of the extra vectors I have drawn in. I will try out your steps next, but after that, I guess my next step is to figure out how to actually draw in that center circle. I’m thinking that after I’ve done what you layed out, I can hopefully move that circle into position and intersect the faces to create the center that I want.

I did reupload the file, if you wanted to take a look at it.

Again, thanks for all your help, I really appreciate it.

Just done that, and had a look. I couldn’t understand at first why the centre was black, until I looked more closely. You have FAR too many segments in your curves there.
image

It’s only when I zoom in that I can see the individual lines. And there are a lot of holes, because some edges are too short for SU to recognise.

Let me try drawing it again…

I draw the ‘clamshell’ first, as I showed above, but using your dimensions - 26" outside radius, and 20" inside (your inside points are 19 65/64" which makes me think you intended 20")

Make life easier for yourself and centre things at the origin.

[Pause] SU is misbehaving - the Rotate tool isn’t working as it should. Will have to restart my machine.

Another pause - for thought.You have drawn 13 sections in your clamshell, which gets an opening in the centre at the top. The model you referred to in the 3D Warehouse has a point in the centre, with a much easier-to-use 15° rotation between grooves, rather than the awkward ~13.845…° needed for 13 sections.

I’ll draw it with 15° rotation. It makes alignment with the centre boss much easier.

Drew the shell segments as before, 12 of them, with 12 segment large (20" R) arcs, and 8-segment semicircle ends. Run Curviloft/Skin countours to create the skin of the segment

Centre boss - draw a circle with 48 sides, push pull to the right thickness - protruding from the clamshell by 1" copying your model.

Line up the profile, and rotate it so it aligns with the MIDDLE of an edge of the circle. That way, it starts at right angles to the follow me path, and produces a clean round centre, not the U shape you had.

Run FollowMe to create a full circular boss, here shown with hidden lines showing.

To cut the boss in half, draw a rectangle through its horizontal diameter within the context of the component.


Intersect that face with with context, check that the front outside face splits in half (mine didn’t so I drew to lines to get it to split).

Then change to Camera/Parallel projection and View/Standard Views/Front.
Draw a R-L selection box to select the lower half, then delete it. Tidy up any stray edges, including the front of the rectangle.

Exit the editing context, and R-click Change axes to place the origin of the component at the centre of the boss.
image

Turn off View/Hidden geometry and you get the basics of the top of the niche.

Add the front rim. Draw a 26" R circle, select it, then Intersect Faces/With model to get this:


Erase all the unselected faces in this image, reverse the face shown selected, and push pull it (say) 2" to create the rim.

Sorry, forgot to upload the SKP file.
Here:
Greek Arch SU v2017.skp (2.3 MB)

1 Like

Many thanks, that was a good exercise. Cheers, Steve StewartFlute for Shell Alcove 2.skp (417.5 KB)

Many thanks John. I have been wanting to draw one of those. Years ago I worked on a Victorian house that had a shell carved under the balcony. It was about 12 feet wide and 5 feet tall. All the best, Steve Stewart

I see you did yours with 13 segments, ~13.8°. Looks good.

But it makes the boss and surround harder to draw without awkwardness where they meet, although you could always use an unusual number of segments in your circular arcs for boss and surround - say 52 for a full circle.

Some such niches also use a flat ‘flute’ between the semicircular segments. I must try one of those later today, to see how it looks.

1 Like

Just for fun, I redrew this with a couple of changes.

13 segments with a flat (in profile) fillet in between the curved pieces, and a bottom half circle edge section.


Clamshell Niche with fillets.skp (504.3 KB)

In retrospect, perhaps the half circles at the ends of the segments would look better with 8 rather than six segments - barely visible in the image, but if you look closer, it doesn’t quite look rounded.

But 6 keeps the poly count down, and if this were incorporated as part of a building, you wouldn’t notice at normal viewing distances.

Not sure about the straight down extension of the boss, either. I didn’t have a real model to follow.

2 Likes

Hello! I wanted to give an update and thank you for your help! I was able to follow your instructions well, and they helped me finish my design.

I love the way it turned out, and I would’ve been stuck for much longer without you, so thanks a lot!

2 Likes

Looks well, and not just a slavish copy either.

One very small point might improve it even a little further.

There are no visible intersections between the central boss and the fanned out segments. IF (and I do mean if) you want to show them as I did above, you could open the boss for editing and use Edit/Intersect faces/With Model to create the sharp intersection lines. But you may prefer the softer look without the intersection lines.
Your model
image

Mine
image

That came out great! Nice work!