#1  
Old 08-02-2009, 12:28 PM
Peasant
 
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Location: Draper, Utah
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Question Problematic Pompeii Parabola Plan

How are most of you building your domes as high vaults or low vaults? From what Ive found on this site, a low vault parabola is the desired shape, while the high vault is basically a semi-circle the height is the same as the radius.

Im about to start building the dome for a 39 oven. I want to make my dome a parabolic shape, but not quite as low as the plans call for (about 18 high vs. 15 for a 39 oven). Is there a good way to draw a parabola other than freehand guessing? Ive found a bunch of web sites showing how to plot a parabola based on a formula, but I cant find anything showing how to draw a parabola when you know the desired radius and height.

If the low vault parabola proves too hard to figure out or build Ill go with the high vault using one of the jigs I found on this site. Is the overall performance of a high vault oven significantly different than a low vault oven?

Thanks for the help and info.

Freddie
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  #2  
Old 08-02-2009, 12:41 PM
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Default Re: Problematic Pompeii Parabola Plan

Quote:
Is the overall performance of a high vault oven significantly different than a low vault oven?
It's said there's not much difference. (I haven't cooked in a low dome) A hemispherical oven has slightly less top-browning, but it's a matter of balance, which you learn when you use your oven. You can always lift your pizza for a few seconds if your bottom is browning faster than your top.

A hemispherical oven has lots of room for fuel, so you can fuel your fire just a few times while heating it up. There's also lots of room for roasts, in retained heat cooking. The low dome is more pizza specific.

The parabola (the conic section) is similar to the catenary (the line formed by a chain suspended in two places). I've used CAD so long that I've forgotten the old methods of generating curves.
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Old 08-02-2009, 06:04 PM
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Default Re: Problematic Pompeii Parabola Plan

Freddie,
I too went with a parabola shaped oven. I liked the idea of a flatter dome pushing more heat down on the food. I built my sides completely veritcal, up to about 9''. This allows me to push pots and pans right up to the side with no gap at the bottom and still have volume enough for large roasts.
Now, to make a jig I used a technique I learned from woodworking. You need a piece of tempered hardboard, 1\4'' thick. Cut a strip about 2'' wide and about 5' long. Put a 1\8'' notch in the middle of botton ends. This notch will hold a string at both ends. Attach the string at one end by tying a large knot that will NOT slip through the slot. Make the string a 1' longer than the strip of hardboard. Now, to make a parabola, pull the string through the slot on the other end and bend the hardboard and use the string to keep it in place to make the curve you want. It's going to look like a very crude bow and arrow, without the arrow. Make sure and mark the center of the hardboard, this will mark the peak of your oven. Using this marking tool, you can make your own dome making jig. I used a piece of plywood for mine.
The important thing to remember is to use something that will bend consistently thoughout its length. Good Luck!
I thing a may have pictures in the photo gallery.
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Old 08-02-2009, 07:34 PM
Peasant
 
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Default Re: Problematic Pompeii Parabola Plan

Thanks for the info and the ideas. Im trying to find the perfect balance between a pizza oven and an oven that will cook bread and anything else. My wife is an avid bread maker. She even teaches a class to other women in the neighborhood. I was hoping she would like the idea of a wood fired oven in which she could bake bread. Unfortunately, her opinion is that this is the worst idea Ive had since trying to make home made rootbeer using real roots and spices (an absolute disaster). I figure if I can find the best shape that will work for both pizzas and breads, then maybe shell change her mind (unlikely, but dare to dream).

Freddie
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Old 08-03-2009, 05:11 PM
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Default Re: Problematic Pompeii Parabola Plan

Freddie,
Here's a photo of a couple of pages of a book I own. I have only included the written directions for the first ellipse as the roughness of construction using bricks would make spending the time to draw the more perfect ellipse a "waste of precision" (like measuring a the length of stud for a wall to the 1/1000th of an inch.)

Anyway, here's how to do it with a pair of dividers and straight edge. Just like the old time masons used to do.

Hope it helps,
Wiley
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  #6  
Old 08-03-2009, 07:07 PM
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Default Re: Problematic Pompeii Parabola Plan

I've already been accused of being pedantic this week, but an ellipse (cylindrical section) and a parabola (conical section) are two different things. Ellipses are pretty easy to draw with two nails (focii) a circle of string and a pencil.
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Old 08-03-2009, 07:23 PM
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Default Re: Problematic Pompeii Parabola Plan

Quote:
Originally Posted by dmun View Post
I've already been accused of being pedantic this week,
Dmun,

If the sharing of factual information counts as pedantry, then by all means keep it up. Your efforts help keep this a Reality Based Forum.
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Old 08-03-2009, 07:59 PM
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Default Re: Problematic Pompeii Parabola Plan

Right you are dmun, my bad.
Wiley
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  #9  
Old 08-03-2009, 10:44 PM
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Default Re: Problematic Pompeii Parabola Plan

Freddie,
I was considering the cylindrical vs conical and the implications of the focii origin , but in the end I went with a "squished ellipse thingie" (Ken's words, not mine) Here is the link to a pdf file I made #132. http://www.fornobravo.com/forum/8/ba...html#post45993 (BA's Dome! Let the games Begin!)

But seriously, you've got a good questions and there are some good answers above. You want your oven as structurally sound as your abilities allow and most of us are only building 1 oven so we want to make pizza and bake bread as well as feed the thing a medium size turkey once a year.

I think an 18-19" tall dome on a 39" oven would be a fine compromise over the perfect sphere. Also, if you make your 1st 3rd of rings more vertical, then arch over (the shoulder rings), then go flat for the last 4 rings at +18 or 19" you would have a tall enough oven for baking & roasting and a flat-ish dome for excellent pizza. There have been quite a few WFO's built in the last few years on this forum and they have all turned out great. If you don't do a perfect half-sphere but do some sort of parabola but without the pointy cone at the apex, (IMHO) you'll have a great dome.

Hope this helps, Dino
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Old 08-04-2009, 04:29 AM
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Default Re: Problematic Pompeii Parabola Plan

The ellipse drawing method:

File:Elipse.svg - Wikimedia Commons

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