Framing Arches

 
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Old 04-27-2015, 11:47 PM   #1
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Framing Arches


I'm working on a job that has a ton of arches in the walls. A soffit along metal beams with 5 sections between the steel posts that are all arches. Doorways with arched tops. Arches in walls for displays. Arches everywhere. I've done arches in the past and most of them at this job are done. I made them out of plywood and to lay them out I used a piece of FRP paneling J trim that is flexible to get the shape of the arch and traced it. But after doing a dozen arches today there has to be a better way to lay these out. What are your tips or tricks for doing arched doorways, arches in the walls or soffits where the bottom is arched? Especially how you get the arched shape laid out? I was working by myself most of the day and some of the longer arches were a little difficult to get the shape laid out by myself. Are there any special tools to get the arch shape laid out on a sheet of plywood?
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Old 04-27-2015, 11:51 PM   #2
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Re: Framing Arches


There are formulas, theorems, calculators etc...

In the end a great deal of it depends on how the heck it looks.

Something that can't be calculated.

I've done it your way with no regrets....

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Old 04-27-2015, 11:51 PM   #3
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Re: Framing Arches


Radius arches or elliptical?
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Old 04-28-2015, 12:11 AM   #4
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Re: Framing Arches


They were mostly radius arches. The way I did it was with a piece of flexible trim. I screwed a few temporary screws in at the ends and the center point and used the flexible trim to get an arch. So I'm sure they weren't perfect "circle" arches but closer to radius then elliptical. I will try to get some pictures tomorrow.
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Old 04-28-2015, 12:13 AM   #5
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Re: Framing Arches


Strings screws and pencils. For radius one screw. For elliptical two screws. Just gotta figure out the math. I usually hang the rock over the square opening, mark the arch, cut it out and fill in with blocks, glue and screws.
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Old 04-28-2015, 12:19 AM   #6
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Re: Framing Arches


I always use the formula, calculate radius, lay out on floor, drawing radius on rip of plywood.
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Old 04-28-2015, 12:26 AM   #7
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Re: Framing Arches


For ellipses, I use the trammel method. This page shows the method: https://www.uwgb.edu/dutchs/MATHALGO/Ellipses.HTM

It's very simple, and the method makes it very easy to make multiple arches with the same height and different widths, which is a fairly common requirement.
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Old 04-28-2015, 08:59 AM   #8
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Re: Framing Arches


Strings will stretch. If you want an accurate circle or ellipse, use the trammel method Bob posted or a single stick for laying out a circle.

In remodeling, you may have to enlarge an existing rectangular opening to put an arch on top. You either have to put some structure in the opening to be able to use a trammel or stick, or you can take 2 straight boards, put one nail through at the ends and clinch it over so they act like a big bevel, put nails in the wall at the locations you want the ends of the arch, then move the hinged part up the center line of the opening until it's at the height of the arch you need. Put some screws in it so the angle is now fixed. Draw the arch by letting the legs slide on the 2 nails while holding a pencil at the point of the angle between the legs.

For smallish circles, you can do that with your framing square.
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Old 04-28-2015, 09:46 AM   #9
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Re: Framing Arches


We cut arches ahead of time on the CNC and deliver them with the framing package.

Depending on the situation it's often less expensive than laying them out on site, and more accurate.

We also cut TDC and level chord indicator marks in for easy alignment.
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Old 04-28-2015, 10:04 AM   #10
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Re: Framing Arches


I've use a string unless it really large like a garage door then I use build calc to plot points and connect them with a flexible straight edge
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Old 04-28-2015, 03:10 PM   #11
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Re: Framing Arches


Invaluable:

http://www.amazon.com/Circular-Carpe.../dp/0941936481
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Old 04-28-2015, 06:12 PM   #12
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Re: Framing Arches


I use the basic calculator at handymath.com. You can also lay them out on the ground by selecting a starting point (shoulder) and a finish point at the top center. On a remodel, sometimes with a big radius it can be difficult to find the room to swing the radius. I have done a few at home on the concrete, and then taken them to work.
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Old 04-28-2015, 06:25 PM   #13
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Re: Framing Arches


http://www.finehomebuilding.com/item...an-ellipse-ish
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Old 04-28-2015, 10:11 PM   #14
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Re: Framing Arches


Old carpentry handbooks frequently started off with geometry and trig, and then covered how to layout conic sections (at least some of my favorites do).
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Old 04-29-2015, 09:51 AM   #15
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Re: Framing Arches


That formula where you write in high of arch and span. Then nail at center and regular carpenter tape. Work for me.
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Old 04-30-2015, 06:07 AM   #16
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Re: Framing Arches


replace the string with piano wire no stretching
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Old 04-30-2015, 08:53 AM   #17
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Re: Framing Arches


Quote:
Originally Posted by tipitop View Post
That formula where you write in high of arch and span. Then nail at center and regular carpenter tape. Work for me.
That works great for an arch, but for an elliptical you need string-or wire.
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Old 04-30-2015, 09:51 AM   #18
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Re: Framing Arches


OP said mostly arches....
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Old 05-01-2015, 09:40 AM   #19
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Re: Framing Arches


Quote:
Originally Posted by mnld View Post
That works great for an arch, but for an elliptical you need string-or wire.
Definitely will try piano string when will do arch next time. Pencil/tape method have some play.
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Old 05-02-2015, 01:25 PM   #20
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Re: Framing Arches


RPN for HP 50G Calculator

Find Radius for Arch

<<
"SPAN INCH" "" INPUT OBJ-> 'S' STO
"RISE INCH" "" INPUT OBJ-> 'R' STO
S SQ R SQ 4 * + 8 R * / 'RADIUS' ->TAG
{ S R } PURGE
>>

So a 72" wide opening with a 16 inch rise from where the arch starts at the edge, up to the peak, would be a radius of 4'-0 ".

I can't remember the CM way to do it, but I will check my old note books when I get a chance.

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