Dome Question

[leemc]

Well I did the mockup of my dome this morning and it looks a little flat to me. See the pictures below of mine and a picture out of the Pompeii oven plans. The dome mockup pictured in the plans looks a lot more structurally sound than my layout. My dome is a 36" and according to the plans should be 18" at the top of the dome. I am thinking I need to do a half soldier for the bottom instead of a full brick to get some more "vertical-ness" to the dome walls.

Also on another note - I looked but could not find posts that dealt with how to layout an ellipse. I found a good one at the following link -

How To Lay Out an Ellipse Video | General Woodwork Videos | Howcast.com

Lee

[SpringJim]

Re: Dome Question

Interesting but it can also be done by simply using two nails (two points along your long axis) and a larger loop of string. As the loop of string revolves around these two points it forms an ellipse.

That method was used to cut an oval hole out of a concrete wall.

[egalecki]

Re: Dome Question

Lee, I laid my dome out flat the way you did- I only used half bricks for the soldier course. It worked out pretty well to get the right height in the middle- I think it came out just over 18 inches.

[leemc]

Re: Dome Question

Elizabeth

That's what I ended up doing - using a half soldier on the first course. See the picture. I ended up being a much better arch IMO. I plan to do a center mount chimney so I think the more arch in the dome will be stronger to support the weight of the it.

SpringJim

I tried the string method but couldn't get it right. I think I was confused about the location of the two nails. The method on the video worked flawlessly though. You might want to make a video of your string method and post in the videos section.

Lee

[nissanneill]

Re: Dome Question

Hi leemc,
the 2 nails and the loop of string works fine for most elipses and a lot simpler than using the geometric method of drawing an elipse.
I just set up a sheet of card 900mm wide with marks on the bottom at 100mm (4") apart. I initially chose the location for the 2 nails at 200mm and 700mm, made a string sling that went from the left hand nail to the right hand edge of the card (900mm mark). I then put a pencil inside of tyhe string pusing it outwards and drawing an elipse from one edge th the other. This elipse measured 400mm high
next I reduced the distance of the 2 nails by 50mm (2"), 25mm each nail closer to the centre. I then had to reduce the length of the string sling to position the pencil on the outer (900mm) edge The elipse drawn was 415mm high.
I then repeated the exercise by another 50mm (2") again reducing the string sling to the outer edge of the sheet from the lh nail. The elipse drawn was 430mm at the highest point.
The last one was when I reduced the nails to 325mm in from each side (250mm or 10" apart). Again reducing the string sling to suit the card width and produced an elipse 450mm high.
Obviously the closer the nails, then the nearer the elipse becomes a circle.
I have attached a diagram to show the first and last result
Hope this helps.

Neill