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?
platinumLLC
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.
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.
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.
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.
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.
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
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.
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).
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 ½".
I can't remember the CM way to do it, but I will check my old note books when I get a chance.
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 ½".
I can't remember the CM way to do it, but I will check my old note books when I get a chance.
I have been doing arches like this for years. Don't know if this is still the way it's done.
View attachment 175778
View attachment 175778
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.
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.
I could put math on the things I hate thread, but it's more like an uneasy relationship.:whistling
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
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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