Take a thin piece of wood, add a string to each end, bow the wood to the desired arc, tie it off. There's your marking device. Cut to the pencil line that you scribe and sand to the finished line.
Bill

 

I' ve done that sorta like Bill suggests, but without the string.

I used a piece of MDF larger than my arc, laid out and marked the arc endpoints and top of the curve, and ran screws at the three points. Then I put a strip of 1/4" ply on the outside of the top of arc screw, and inside the endpoint screws and used it to mark the arc on the MDF.

Cut and sand to the line and you have your template.

 

But it seems his problem is the result he got trying to cut and sand to the line.

You could use a thin bendable strip of wood/plastic, clamped to the correct offset for your router base/cutter, it should be fairly sturdy under tension, then clamp supports that touch the back of the arc at intervals, till the arc doesn't deform with moderate pressure, use the actual arc as a guide for the router. Would probably be easiest with a small trim router with a round base..

 

Then it's about the cuting and sanding technique. Trying to get a smooth curve with 200" of string has to be quite a challenge. It's way to prone to stretching.

 

Draw your arc but make it larger by EXACTLY the distance from the edge of the bit to the edge of the baseplate of the router. Make a temporary fence on the arc by hot-melt glueing wooden blocks every few inches and then glue a thin strip to those so that they conform to the arc. Use the fence as you would use any fence, rout in steps, and knock off (carefully)+ the fence.

Steve

 

There have been a couple of project posted similar to this one about how to draw a large arc. How about trying to modify this method to hold the jig saw to do the cut?

A good idea that I saw recently for sanding internal curves is to glue some sand paper to thin ply (or mdf) and then to sand with that using enough pressure to flex the ply to the shape of teh curve.

 

Here is a picture of a jig I used with a router and a straight bit to cut an arc with a radius of 6'.

If it were important, I would take this jig out long enough to handle a 200" radius. You would want 3 or 4 saw horses to set it on.

I can virtually guarantee you that you will get good results.

Only take about 1/4" in depth with your router with each pass and make multiple passes for better results.

 

A hand held belt sander with a 4"x24" belt is your friend in this situation. I say get one and learn to be proficient with it in your hands. That skill will be invaluable and time saving to you for years to come.

 

Buy an 8' toe kick, drill a 1/4" hole on each end, thread a string through the hole on one end, thread through the other end and pull it to the desired radius you want. Clamp or tie the string in place. Large radii are almost limitless.
Scribe/mark your radius on the piece, jig or bandsaw roughly, but close, and finish sand to the pencil line with an oscillating sander, like the ridgid table model, using the largest barrel/arbor. Do a final light hand sand with a flexible hand sander.
Excellent results and easily controllable.

 

By the way. The device I described above is called a "fairing strip".
Bill

 

make a large drawing bow with thin flexible strip of wood and string i have both manufactured and home made of various lengths . they work well , so the same as above posts, i guess.

 

A slight arc on your sanding block for finishing up should help

 

Wow - lots of great responses here. Thanks to all.

Just to clarify - my problem is not at all in laying out and making the line. I just can't get a straight cut, so I really want a jig or fence that will guide the router. There also is the possibility that the template MDF is just too soft, and that's why the cut is so crooked. 3/4 maple may be easier to follow the line…

 

I use a compass plane to smooth out large arcs and someone mentioned a belt sander. Either of these will work, or work both to the line, after you make your cut.

 

Here's a high-tech approach that has worked for me on extra large arcs…

I draw the arc in AutoCAD with a 1" background grid and print them 1:1 on 8-1/2" X 11" sheets of paper.
Then, I tape the pages together using the grid as an alignment guide and tape the arc to my workpeice.

It beats trying to strech a string 200" and keeping everything steady while trying to draw with a pencil.
This also works for other complex geometry in addition to arcs.

For cutting, I sometimes cut the pattern in 1/4" plexiglass then use the flush-trim router bit to transfer the pattern to the workpeice. If you have an accurate radius on the template, you could reposition it several times to get extra length of cut.

The ultimate in accuracy is to send the AutoCAD data to a CNC machine which will take about 10 seconds and produce a perfect cut over and over again. About $200k will get you started! heheh.