# cutting to fit an archway

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• posted on July 31, 2005, 4:46 am

Thanks for the link. I enjoyed that site.

registered
hope
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• posted on July 31, 2005, 9:25 pm
So CW, Do you know of a formula?
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• posted on August 2, 2005, 2:17 am
Yes, posted to A.B.P.W.

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• posted on August 2, 2005, 2:58 am

See if DJ's site will help. http://www.delorie.com/wood/chord-radius.html
Mike O.
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• posted on August 2, 2005, 12:56 pm

Here's the math:
Radius = R, center height = h, full chord length = C.
Pythagoras: (R - h)^2 + (C/2)^2 = R^2
Simplify: R^2 - 2Rh + h^2 + C^2 / 4= R^2
2Rh = h^2 + C^2 / 4
R = (h^2)/(2h) + (C^2 / 4)/(2h)
* ******************* * * R = h/2 + C^2/8h * * ******************* *
This can be combined into one fraction, but why bother ...just stick the numbers in the above.
So with h = 3" and C = 36", you'd have
R = 3/2 + (36 x 36)/(8 x 3) = 55 1/2"
Also easily checked with a CAD program.
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• posted on August 2, 2005, 9:07 pm
Thank you, I very appreciate it.
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• posted on July 30, 2005, 3:48 am

While I agree that, with the problem as originally posted, a perfect arc will probably not fit into most work done by a mason, I'll have to disagree with your statement above. With the with design and manufacturing techniques used today there are a lot of instances where the mathematical solution will yield very good results when trying to work with existing architectural elements. I was part of the earlier discussion about finding a radius for a window while knowing just the cord length and height of the arc. I can testify that not only did the formulas work great but the resulting jam extensions and trim pieces were very accurate. The time savings involved in not making a template (or using trial and error to find the radius) makes it well worth keeping a few formulas in the notebook along with the calculator.
Mike O.
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• posted on July 30, 2005, 6:13 am
I'm a machinist. Your idea of fit and mine obviously differ. At work, I have two calculators. One at each end of the shop and they are used extensively. At home, to fit something like what was under discussion, I would not blindly rely on the numbers, even though I know they are correct, Variations are very common. In the case of your windows, you have a lot better chance of getting a good fit by formula when fitting to factory made windows which are jig built and machine made. With something that was built in place, I wouldn't chance it. A test piece would be a good idea and don't be surprised if hand fitting is needed.
wrote:

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• posted on July 30, 2005, 1:28 pm

You are correct. There has to be an element of both; practical and theory for the best advantage. Been there, done that in the steel business. In particular I assisted my brother [structural steel] by doing calculations on an arch for the recovery after the 911 disaster. His practical knowledge far exceeded [he's now passed on] mine, but he said I saved him many hours of work. On another occasion he couldn't get the CAD dawings to give him what he expected. I saw the flaw through calculation that the had been given two sets of unmatching detail on two different drawings. Both are necessary for a better job: practical experience and base theory. He knew there had to be a mistake, and I found it. It's not enough to use a sledgehammer to get the motor started. You have to know where to hit it.
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• posted on August 13, 2005, 6:26 pm

They sell caulk by the truckload. Rabbit
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• posted on July 28, 2005, 1:30 pm

Look up "scribing" to fit and you'll get some ideas on how to do this.
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Larry Wasserman Baltimore, Maryland
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• posted on July 28, 2005, 2:49 am
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That's the only way to do it, and at the same time to nicely avoid one of those endless discussions on how to find the radius from a perfect arc.
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• posted on July 28, 2005, 1:41 am
js5895 wrote:

Get you a strip of hardwood say 1/8" thick by 5 or so feet. Tie a string to one end. Now loop it around the other end and form a bow. Put it in the arc and let it spring fit to your opening. Tie off the sting so the bow will not spring back when removed. Mark the center of the bow from the center of the arc. Do the same with the ends. All you do then is lay it on your wood and trace.
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• posted on July 28, 2005, 1:59 am
Jody wrote:

Elegant solution, Jody!
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• posted on July 28, 2005, 2:30 am
Dhakala wrote:

but unlikely to make more than an approximation of the curve.