# cutting Arches



## beltboy1

what is the best way to accurately make and cut arches for a face frame on an ET center? Approx. 39" wide 3" high. I have tried the thin strip of veneer but it never comes out even.


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## woodnthings

*Use a thicker strip*

maybe 1/16" or 3/32" or so and 3 points, in this case 39" apart and 3" high, on a template. The center nail must be on the opposite side of the strip. Trace the line from the center to one side and cut the template to that shape. Then using this as half your pattern trace it on a horizontal line, flip it about the center keeping the horizontal line and trace it again. That way any variations are duplicated on both sides.
A straight grained wood like maple or a homogeneous material like plastic or thin metal will give a more uniform arc using the 3 point method. This will eliminate the need for the above procedure. The advantage to this method is you can see the the arc as you adjust the center point up or down and design the arc to suit.
If you create your arc using the 3 point method on a material you can set small nails or push pins into to locate the strip that will help when tracing the strip, but a stiffer material will not flex when tracing and it gives a more predictable arc.
:thumbsup: bill


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## Gus Dering

With those dimensions your radius is 64 7/8".

I would scribe an arc on a template, cut it, sand it nice then use it to shape the actual piece with a flush trim router bit.

Using the template you can be assured that if you screw up the initial set up, you can make another at little to no expense. 

Here is a link to a good circle calculator.
http://www.1728.com/circsect.htm


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## Streamwinner

Just use a long, thin piece of hardboard and cut a small notch on each end. Then attach a string at each end and adjust the length of the string until you get your desired arc. Trace, cut, trim, yada yada yada...


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## Jeremy E

http://www.miterclamp.com/Woodworking_video_index.htm

I think these videos have been linked on this site before, but you can't go back to refer to this kind of stuff too often.:thumbsup:

The video on drawing ellipses will probably fix what currently ails you, but they're all worth a look see...


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## woodnthings

*Video on the use of a "fairing" stick to make an arc*

Wood magazine has some interesting free videos:
http://www.woodmagazine.com/woodvision/?bclid=1243638292&bctid=1805615843 :thumbsup: bill


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## ihackwood

i'm sorry i won't take advice from a guy missing 1/2 his finger, clearly he learned the wrong way hahahahahahahahaha


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## Leo G

For larger arches I will make half an arch, cut it out of a pc of cardboard (or whatever) and then draw 2 half arches. Then they will always be the same.


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## Leakygoose

Never use a strip of wood bent to form an arch ,your just asking for inaccuracy ,typical rough carpentry technology. 
Cabinet builders use techniques like finding the point of radius and use a router mounted on an adjustable piece plywood or a pencil mark and cut with a saber saw.

I'd have to look up the math for finding the radius point but sounds like post #3 did just that .

Best way is the way you find and are comfortable with .


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## Leakygoose

Heres one I did and it was the only time and last time I used a stick to mark the radius off the counter top and cut in place with a saber saw,all four.:laughing: Quicky job


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## woodnthings

*For all you hardcore scientific types here's the math*

*http://www.josephfusco.org/Tips/Finding_the_Arc_Radius.html*
*Topic: **Finding the Radius of an Arc.* 
This question comes up many times and you can use this formula: *R = ((((L/2)sqrd)/H) + H) / 2 if you have a scientific calculator * 


I couldn't get the formula above to work, myself, but the dimensions of 39" chord and 3" height plugged into the formula below gives the answer of 64.875 as Gus had posted. bill
Where L is the length of any chord that touches the arc.
Where H is the length of a line from the center of the chord L to the arc.
Divide L by 2 then *square* it.​
Then divide the answer by the value H.
Then add the value of H to the answer.
Then divide the answer by 2 and *that's the radius.*


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## frankp

woodnthings: I just wanted to make a minor correction to your formula based on your step 3. If you do it as your original formula it is just ((L/H) + H)/2. What you actually want is
*R = ((((L/2)^2)/H) + H) / 2

*By the way, that's great information. I am constantly trying to estimate radii of the boats I'm building, and now I know how to actually calculate them. Cool.


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## woodnthings

*Thanks Frank*

It isn't "my" formula, I just copied it from the source above. But I fixed the missing term by adding the word "sqrd" to it. :thumbsup: bill


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## cabinetman

woodnthings said:


> maybe 1/16" or 3/32" or so and 3 points, in this case 39" apart and 3" high, on a template. The center nail must be on the opposite side of the strip. Trace the line from the center to one side and cut the template to that shape. Then using this as half your pattern trace it on a horizontal line, flip it about the center keeping the horizontal line and trace it again. That way any variations are duplicated on both sides.
> 
> :thumbsup: bill




Basically the simplest method is to use a bendable strip, and create half the arch. Cut out the pattern and use it for the other half.

Another way would be to use the two nail and a string method used to draw an ellipse. Knowing the span and rise, an arch can be easily created with great accuracy. An advantage with this method is if the arch has to flow into vertical planes, the ellipse can be adjusted to straighten out at predictable points.

With this wall system, I used the half pattern drawing, as I hate complicated math formulas. I think it came out OK.
.


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## Bob Wingard

Go here .. .. .. 

http://www.delorie.com/wood/chord-radius.html

and plug in your numbers .. .. ..


Sounds kinda weird, but when I need to cut large arcs, here's my setup .. .. ..

I have some pieces of square tubing that fit inside each other .. a couple of nuts welded to the larger piece allows a pinch bolt to be installed .. one piece has a large hinge welded to it's end in the vertical orientation, the other has a router plate mounted to it. I clamp the hinge to the upright of the overarm guard/d.c. tube on the table saw .. mount and adjust the router in the other end .. telescope the two 'till I get the right radius .. mount the stock to the saw table (elevated by about 3/4") .. make final adjustments to the stock position .. rout away using my giant trammel arm. Perfect results every time with never a need to "Sweeten" the curve with a strip sander or any such nonsense.

Same setup can be made of wood scraps, but I tend to over-engineer my jigs.


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## Leo G

I cheat, I draw it out on my CAD and point to the 3 point arch and it tells me the radius. :shifty:


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## mikeswoods

Hi ,Every body.
Try this link.I tried this jig and it worked well.Videos - Fine Homebuilding


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## woodnthings

*There are many types of arches including eliptical*

From Wikipedia: http://en.wikipedia.org/wiki/Arch
Construction
Isometric view of a typical arch
An arch requires all of its elements to hold it together, raising the question of how an arch is constructed. One answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s). The interior and lower line or curve of an arch is known as the intrados.
Old arches sometimes need reinforcement due to decay of the keystones, known as bald arch.
The gallery shows arch forms displayed in roughly the order in which they were developed.

Triangular arch
Round arch or Semi-circular arch
Segmental arch
Unequal round arch or Rampant round arch
Lancet arch
Equilateral pointed arch
Shouldered flat arch -see also jack arch
Three-foiled cusped arch
Horseshoe arch
Three-centered arch
Elliptical arch
Inflexed arch
Ogee arch
Reverse ogee arch
Tudor arch
Catenary or Parabolic arch

The most common arch is just the radius of a circle, using the chord as the base and a height determined by the design/designer. An elliptical arch may be most suited to the design over a simple chord and the video shows a neat way to construct one. Thanks for posting the link! 
:thumbsup: bill


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## beltboy1

mikeswoods said:


> Hi ,Every body.
> Try this link.I tried this jig and it worked well.Videos - Fine Homebuilding


 
This is FAR & AWAY the easiest and possibly the best way to draw the perfect arch. This takes seriously, like 2 min to complete. 1 million times easier than a string!!!


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## Old Skhool

Gus Dering said:


> With those dimensions your radius is 64 7/8".
> 
> I would scribe an arc on a template, cut it, sand it nice then use it to shape the actual piece with a flush trim router bit.
> 
> Using the template you can be assured that if you screw up the initial set up, you can make another at little to no expense.
> 
> Here is a link to a good circle calculator.
> http://www.1728.com/circsect.htm


x2 on the technique. Thanks for the link to the circle calculator.


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## thinknow

*formula for finding the radius of an arch*

The formula for finding the radius of an arch is
R=h÷2 + w•w÷8h where h=height from chord line to the highest point of the arch and w= the width of the distance where the chord intersects the arch. 
For example, if you wanted to trim an arch window that is 48" wide with an arch that is 6"high the following would figure the radius:
6÷2=3 so 3+ 48squared ÷6(8)
So 3+48=51
51" is the radius which can be traced using the simple expedient of a 51" piece of string, a central pivot point and a pencil attached to the string. Measure exactly 51" from the point where your pencil touches and pivot off of that point. Easy. 
This formula always works, you can even create a calculator for figuring radius of arches so that all you need to do is plug in numbers. They are readily available on the web too, so long as you know the formula is for figuring the radius of an arch. As long as you can cut along the line you have drawn, it will fit perfectly. Hope this helps, I have always liked math, it has made me more efficient than anything else I have learned or been taught.


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## Vexorg

beltboy1 said:


> This is FAR & AWAY the easiest and possibly the best way to draw the perfect arch. This takes seriously, like 2 min to complete. 1 million times easier than a string!!!


Anyone have the link to this video?


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## SteveEl

Magazine tip I read somewhere... if you do the string trick to get the curve, consider using that beaded pull chain for lights instead, since it will hang better than string


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## Warped bored

You guys are funny. ....arches are so easy that I don't even want to waste time posting the easy way......If someone still wants to know the proper way. ....PM me. ...

Proper elipsess might be a different story. ....


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## Mikeymoo

*Arc Cutting*



beltboy1 said:


> what is the best way to accurately make and cut arches for a face frame on an ET center? Approx. 39" wide 3" high. I have tried the thin strip of veneer but it never comes out even.


 I have an Arc Radius App on my phone and it says your radius is 64.875 inches. So if you can layout and draw a radius of 64.875 inches... it will be perfect. I deal in Radius cuts every other day and the App is a life saver. I have a long bench for such layouts. Good luck!


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