# Fixture for a large radius curve



## mcpowell (Apr 29, 2014)

One of my upcoming projects is going to require a 280" radius curve along an 80" 2x12. The end result will be the "knee knocker" of a walnut dining table, which will join the legs. Imagine the "knee knocker" to be shaped like a rainbow. 

I have a band saw, but it doesn't have the ability to make this cut. My thoughts on how to produce this rainbow shaped board:
1) Do this outside on a flat concrete surface (driveway or garage)
2) fashion a rigid extension to my router that is ~23 feet long and adjustable
3) Rout a template from a 2 x 12 x 96 framing lumber. (Actually, I think I'll buy about 3 to begin with)
4) Once my template is complete (nice smooth edges) use the template to cut my final piece of walnut. 

It's going to be awhile before I actually have to produce this board, but I keep thinking about it as I don't have it all puzzled out yet. I have access to long pieces of angle iron, which I could bolt together for the 23' extension, but that's going to be really heavy. I've done a few searches but haven't found any better ideas. However, I thought I would roll this out to the forum to see if anyone has better ideas?

I'm open to suggestions. 

Thanks in advance. 

Mark


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## jw2170 (Jan 24, 2008)

Hi Mark,

Just my 2 cents worth.

I would not make the template from 2 x 12 lumber, I would use 1/2 MDF otherwise your are going to need a very long cutter.

Also, I did see somewhere on the web a way to calculate your arc so that you do not need a 23' extension. Will have another search.


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## mcpowell (Apr 29, 2014)

Thank you James. That seems like such a simple thing, I'm a little embarrassed I didn't think of that myself. 

Thx again.


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## Cherryville Chuck (Sep 28, 2010)

I don't think this curve doesn't need to be perfectly symmetrical, I think it just needs to look symmetrical. You can draw the curve to scale on some graph paper and then multiply up to the right scale and mark your board accordingly. If it looks good then it is good.

If you really want to do it the other way I would lay out a working platform out of OSB (which is fairly cheap) on as flat a surface as possible. Join the sheets together so that if forms a single surface. Mount the board on one end and set the pivot at the other. Laminate 6" wide strips of the OSB with staggered joints 2 layers thick 24' long. Mount the router on one end , put the pivot on the other and route the curve.


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## JOAT (Apr 9, 2010)

I have the feeling that if I were going to make something like that, I would mark the curve on the piece to be cut, then cut it close with a sabre saw, then sand it to the line. I like to keep things simple if I can.
:dance3:


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## jw2170 (Jan 24, 2008)

The Complete Circular Arc Calculator

Try this and draw the arc with a flexible strip.


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## Mdawson (Mar 25, 2012)

Hi Mark

How about using a batten, 2 pins on the the outside of the curve to push against, push the middle forward until you get the curvature you want and pin it there. If you make the initial piece over long with the 2 outer pins outside the finished length you'll get the curvature right to your finished end without a flat spot.

You should then be able to stabilize the batten and run you router along using the batten as your template.

Mark D


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## 64 ford (Apr 21, 2013)

Hi Mark
We've done similar on porches and such. Two ways, snap a line on concrete longer than what you want. Lay your 2x at one end centered and perpendicular to the line and measure out the radius and mark the pivot. With a piece of wire (or anything that doesn't stretch) swing the arc and mark with a pencil and bandsaw it out. Otherwise, just screw together some 1x2s to length needed and put some thin flat material on the end and mount your router and cut the arc. We did try James math but found in our case,our way worked fine.
Dennis


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## tooler2 (Aug 11, 2012)

64 ford said:


> Hi Mark
> We've done similar on porches and such. Two ways, snap a line on concrete longer than what you want. Lay your 2x at one end centered and perpendicular to the line and measure out the radius and mark the pivot. With a piece of wire (or anything that doesn't stretch) swing the arc and mark with a pencil and bandsaw it out. Otherwise, just screw together some 1x2s to length needed and put some thin flat material on the end and mount your router and cut the arc. We did try James math but found in our case,our way worked fine.
> Dennis


FWIW I tried using a 20'string directly on the router and ended up abandoning the idea. I have done 12' with a metal flat bar and it worked perfectly.
Rob


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## mgmine (Jan 16, 2012)

I would just make a trammel out of piece of 1x2, mark the arc and cut it with a jig saw then sand it with sander (drum,.belt or whatever). If you want to use the router then add a wider piece of material to the end of the trammel to replace your router base.


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## OPG3 (Jan 9, 2011)

Mark, This is an interesting question, and here is your answer... You need a CAD prodram to draw the arc (exactly to scale, but certainly NOT to full size). Break it into segments the length of a segment of wood that you can bend to that radius. *Mark from Australia (Mdawson) hit the nail on the head!* My exception would be to use sticks about 3 or 4 feet long (1-1.3 M for you metric guys & gals). You will have a few nail holes in your workpiece, but with careful planning; the nail holes can _mainly_ be positioned on the waste sides of your cut. Hint this is NOT a job for a router!

further reading: Google "chords of circles, chords of arcs, chords of concentric circles".
Also be mindful that many buildings are built with curve radii of 500' to 800', nobody is actually nailing a stick to the center point - it is done with math that is everyday work for surveyors and civil engineers.

Good luck!
Otis Guillebeau from Auburn, Georgia


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## delmirj (Jan 10, 2013)

Get a geometry book or look up on the web "laying out a curve using offsets."
We did it all the time laying our road curves when I was a surveyor. (60 years ago.)


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## tooler2 (Aug 11, 2012)

delmi said:


> Get a geometry book or look up on the web "laying out a curve using offsets."
> We did it all the time laying our road curves when I was a surveyor. (60 years ago.)


Will that hold the router properly to cut the curve?


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## Selwyn Senior (Jan 11, 2014)

Mark

I think making a template is a good idea but I belive it could be much thinner and perhaps made from MDF which would give a nice smooth surface. To trace the arc I just saw the following method used well but it requires a helper.

1. Mark the bottom end points at each end of the stock.
2. Mark the height of the arc at its centre point.
3. Using flexible stock such as a piece of moulding have a helper hold it at one end while you hold the other end. Have your helper pull the flex stock up to the centre mark while you trace the arc.

The person I saw doing this had a shorter arc than yours but it worked very well.

Pete


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## mcpowell (Apr 29, 2014)

Some great ideas. Thanks guys.


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## vindaloo (May 30, 2009)

Something like this only a 'little' longer maybe?


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## papasombre (Sep 22, 2011)

*Hope this can help.*

Hi, Mark.
My wife wants an anti mosquitoes net for the crib I am doing but we do not want something neither round nor square so, as Dick wrote, I took my secondary textbook on drawing and searched for arches construction.

This is a work in progress but the basic idea is shown in the pictures. The first ones show the arch tracing following the textbook instructions. Hope to finish this very soon.


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