# Framing arches



## wallmaxx (Jun 18, 2007)

I have been doing arches like this for years. Don't know if this is still the way it's done. 

View attachment 175778


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## digiconsoo (Apr 23, 2012)

What manner of sorcery is that???!!


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## wallmaxx (Jun 18, 2007)

So:

where S= the Span of the opening
where R= the Rise from the chord span to the top of the arch

Radius = (S^2 + 4R^2) ÷ (8R)

But that just makes it part of a circle. An ellipse/parabola is different entirely....unless you're framing for a math professor (like I did for a Rice U math prof once) you can eyeball half of it and flip it over to keep it symmetrical.

The RPN way is way more fun.


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## kiteman (Apr 18, 2012)

The framing method is what I do but I don't recognize the formula


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## wallmaxx (Jun 18, 2007)

Ellipse is like this:


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## tipitop (Dec 3, 2013)

wallmaxx said:


> Ellipse is like this:


Ellipse have 2 vertex not centre. And radius of ellipse is sum of distance from 2 vertex if I remember correctly.


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## wallmaxx (Jun 18, 2007)

tipitop said:


> Ellipse have 2 vertex not centre. And radius of ellipse is sum of distance from 2 vertex if I remember correctly.



Correct. Two foci.


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## hdavis (Feb 14, 2012)

tipitop said:


> Ellipse have 2 vertex not centre. And radius of ellipse is sum of distance from 2 vertex if I remember correctly.


An ellipse has 2 foci. For the special case where they are coincident, the ellipse is a circle. The radii are frequently said to be the two semimajor axis, although mathematically this is imprecise.

In any event, if you are thinking the "radius" is the sum of the two semimajor axis, this is obviously wrong, as the special case of a circle makes plain.


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## mnld (Jun 4, 2013)

hdavis said:


> An ellipse has 2 foci. For the special case where they are coincident, the ellipse is a circle. The radii are frequently said to be the two semimajor axis, although mathematically this is imprecise.
> 
> In any event, if you are thinking the "radius" is the sum of the two semimajor axis, this is obviously wrong, as the special case of a circle makes plain.


Obviously.













:sly::what:


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## hdavis (Feb 14, 2012)

I could put math on the things I hate thread, but it's more like an uneasy relationship.:whistling


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## AustinDB (Sep 11, 2006)

I tripled up a piece of string to make a large 27' radius. That cut the flex down and the surface I was pulling over was smooth and flat which helped. 

guess these are all different size openings and you can't use the same one 

I've had a HP48GX sitting on my shelf for 12+ years-it looks cool but is confusing


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## wallmaxx (Jun 18, 2007)

72chevy4x4 said:


> I tripled up a piece of string to make a large 27' radius. That cut the flex down and the surface I was pulling over was smooth and flat which helped.
> 
> guess these are all different size openings and you can't use the same one
> 
> I've had a HP48GX sitting on my shelf for 12+ years-it looks cool but is confusing


I bought the 48SX when I was an Aggie in 1990

Then the 48GX, 49g and now the 50g.

I was a calculator guy before the internet really took off.....they never let me down so I keep finding ways to make my calculator useful.


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## Rhode Island (Mar 24, 2015)

I just make a template and have at it. Granted I don't do a lot, but they come out good.


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## tipitop (Dec 3, 2013)

With all risk to get stones throw at me again that arch is not good enough. I can se uneven lines. Compare to this arch that I build past year. Btw sheet rocker should help with his work. Maybe he mess? That second arch in background look really clean.


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## Rhode Island (Mar 24, 2015)

I hope that wasn't directed at my arch. Maybe it's the shadows from the night pic, but they are very good. There's some serious arch to them rather than a minor bend over 6'.


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## Warren (Feb 19, 2005)

tipitop said:


> With all risk to get stones throw at me again that arch is not good enough. I can se uneven lines. Compare to this arch that I build past year. Btw sheet rocker should help with his work. Maybe he mess? That second arch in background look really clean.


I hate when the top of the arch is that close to the ceiling. Good luck if anyone ever wants crown in there.


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## tipitop (Dec 3, 2013)

Warren said:


> I hate when the top of the arch is that close to the ceiling. Good luck if anyone ever wants crown in there.


I actually agree with you. I had talk with women before I build arch exactly about space for future crown. She said that with crown space would be to busy. When all is done I actually agreed with her. It is a lot in and out on ceiling as I had to cower air ducts.


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## Warren (Feb 19, 2005)

Even without the regard for crown, it still looks too high.


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## tipitop (Dec 3, 2013)

Warren said:


> Even without the regard for crown, it still looks too high.


Yeah for my taste to. How much you would recommend space in middle for 10' of ceiling maybe something like this (but someone else frame that arch. I did only trim there). In photo is ellipse of course.


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## tipitop (Dec 3, 2013)

Rhode Island said:


> I hope that wasn't directed at my arch. Maybe it's the shadows from the night pic, but they are very good. There's some serious arch to them rather than a minor bend over 6'.


See what I see like problem with yours arch and for sure many would agree. It is “too curved”. That is problem for sheet rocker/ tapers. No matter how precise is your arch they will not be capable do quality work. Try to avoid short radius. I go usually to 20’ radius.


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