I'm in the planning stages of my next gas forge and have decided to go with the doghouse style because I have the material on hand.  I found a design that i like, but there seems to be a disconnect in my brain and the arithmetic isn't working.  Seems like there's some fancy trigonometry or such going on, or maybe I'm just not seeing what I should be seeing.

I really like the ridge lines on the shell and would like to duplicate that look, but for the life of me I can't figure out how to lay it out.  Assume that the inside chamber measures 9x11.  You then have what appears to be 2" of insulation.  That puts the outside walls at 13" apart at the base and 15" tall.  

But, how does that translate into a the length needed to form a faceted parabola like in the picture?  Thoughts?

This isnt rocket science,  treat it like an oblong and average the radie

Edited September 19, 201510 yr by Charles R. Stevens

Simple way is to just draw what you want full scale on paper, then measure the facets and add them up. The small loss due to bending won't make enough of a difference to matter much, but if you feel it's critical, add some extra to the total and cut once you have all the bends done.

measurements don't have to be exact   treat it like two concentric half circles and calculate the thickness and length of the arc.    If you really want to be picky break each segment into trapezoids  draw out on graph paper and measure with scale ruler or draw full size on paper and measure. 

Just get a piece of string and lay the shape out and measure the string.

The actual math to calculate the exact circumference of an ellipse is fairly complicated, there are simpler equation that will be within 5%. Just do a google search for the formulas if you need it that precise. It works out to be 37.24 using the simpler equation.

 

Or take a strip of cardboard and using a blunt edge tool (we used to use a washer welded to a piece of pipe for a handle), score the facets and bend the cardboard as you want the steel to be bent. then flatten it out and measure. Sometimes getting fancy isn't worth the effort...(I used to build very odd custom boxes when I worked in a small custom wood shop...)

Does it have to be a parabola? A straight sided box with a semicircular vault is EASY to calculate. Seriously, box LxWxH + 1/2 pi.r.sq x L. Or if you have an example of the thing trace it on graph paper and add the little squares and triangles.

My preference if for technique #1 I'm a KISS kind of guy.

Frosty The Lucky.

I would resort to Frozenforge's "math" (the string) that said , that forge is going to 'eat you out of house and home' on gas! 

Make a full size drawing on paper. You actually only need to make 1/2 of the drawing on a folded piece of paper, then unfold the paper.

Transfer the drawing to cardboard strips, and use the cardboard strips as ribs like found on a ship. This will allow you to have a full size model to see if you like the design, ir has room, etc etc

 

Think I would just buy a large mailbox and cut it down... 

Still have to figure volume..

To figure volume the easy way, put a plastic bag inside and fill it with water.

Then measure the volume by measuring the water.

Glenn, that's cheating! Even by my lazy standards!

To figure volume the easy way, put a plastic bag inside and fill it with water.

Then measure the volume by measuring the water.

measure the water???? Surely weigh the water? You know those pesky "Frenchies" the ones that made it all so easy in metric. Ie. 1000Liters of water = 1000Kgs = 1 cubic Meter.

Once you have your metric answer then convert Your answer to your preferred unit of measurement be that Roods, Cubits, Bushels, Cubic feet etc..etc.

Edited September 20, 201510 yr by ianinsa

Glenn might have a distant relative by the name Rube Goldberg!

Ian, the ryme for those of us who use the "right" meserments is "a pint a pound the world round" lol

Edited September 20, 201510 yr by Charles R. Stevens

Charles, not many pubs that will still give you a pint for a pound

I imagine not! Lol, they short you 6oz stateside anyway!

Charles, Ian: come on guys pint, pound, litre, meter, who cares. I mean really, who drinks French beer?

Frosty The Lucky.

I would. Right after using it to measure volume. In fact, I think that's how I'm going to measure from volume here on out. Measuring beer is important enough to get finicky over. Forges - close enough works.

Edited September 21, 201510 yr by Nobody Special
Because I can't spell.

Belgian beer is really good certainly worth calculation!

Edited September 21, 201510 yr by ianinsa

you are half an ellipse (basically). long axis is 30" (your 15" height) You short axis is 13" (your width.). Thanks calcs to 72.63" for an ellipse. Take half and you get  36.31".

 

8 segmants of 4.5"

 

 

http://www.mathopenref.com/constellipse1.html

Half of this should get you to outline to fits the facets into.

Edited September 21, 201510 yr by Culver Creek Hunt Club

VaughnT, pm sent

You gents are fantastic.  Lots of great info, and lots of different ways of accomplishing the same goal.  Next time we meet, your first beer's on me!

The shape of the furnace looks like a catenary. Hang a chain with the dimensions (width and height) you want and then measure the length.

https://en.wikipedia.org/wiki/Catenary