How to cut a large arc

  • Advertise with us

« back to Woodworking Skill Share forum

Forum topic by bringitonhome posted 10-09-2011 12:42 AM 16175 views 0 times favorited 15 replies Add to Favorites Watch
View bringitonhome's profile


28 posts in 3219 days

10-09-2011 12:42 AM

I as wondering if anyone can share their technique in cutting arcs. I’m trying to cut one in a stretcher beam for some built-ins i’m making. It’s a ~40” span and a 1” rise – which gives me a 200” arc radius. Far too big for any trammel/jig I can imagine. My plan is to cut a template first from 1/4” MDF, and then use a flush trim router bit for the actual work piece (this technique has worked wonderfully for me on pieces that were small enough where the templates could be cut with a router compass).

I was able to draw a perfect arc on the template by tying a pencil to a 200”-long string that was anchored across the driveway, but that’s where things got messy. I tried following the line with a jigsaw and then smoothing it with a drum sander bit on my drill. I may as well have been jogging in place while i did it. Looked absolutely terrible.

Is there any way i can cut the template directly with a router? I’m trying to conceive of a way I can reverse engineer this technique: by attaching the router to the jig, etc.

Any ideas greatly appreciated!

15 replies so far

View Bill White's profile

Bill White

5069 posts in 4104 days

#1 posted 10-09-2011 01:22 AM

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.


View Sawkerf's profile


1730 posts in 3212 days

#2 posted 10-09-2011 01:42 AM

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.

-- Adversity doesn't build reveals it.

View jeth's profile


262 posts in 2982 days

#3 posted 10-09-2011 03:42 AM

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..

View Sawkerf's profile


1730 posts in 3212 days

#4 posted 10-09-2011 04:38 AM

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.

-- Adversity doesn't build reveals it.

View fussy's profile


980 posts in 3195 days

#5 posted 10-09-2011 09:20 AM

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 in KY. 44 years so far with my lovely bride. Think I'll keep her.

View Tootles's profile


808 posts in 2646 days

#6 posted 10-09-2011 01:41 PM

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.

-- I may have lost my marbles, but I still have my love of woodworking

View richgreer's profile


4541 posts in 3218 days

#7 posted 10-09-2011 03:26 PM

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.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View miles125's profile


2180 posts in 4149 days

#8 posted 10-09-2011 03:48 PM

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.

-- "The way to make a small fortune in woodworking- start with a large one"

View jim C's profile

jim C

1472 posts in 3242 days

#9 posted 10-09-2011 03:55 PM

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.

View Bill White's profile

Bill White

5069 posts in 4104 days

#10 posted 10-09-2011 04:01 PM

By the way. The device I described above is called a “fairing strip”.


View bob101's profile


321 posts in 3594 days

#11 posted 10-09-2011 04:10 PM

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.

-- rob, ont,canada

View TopamaxSurvivor's profile


18373 posts in 3820 days

#12 posted 10-09-2011 08:12 PM

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

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View bringitonhome's profile


28 posts in 3219 days

#13 posted 10-10-2011 02:31 PM

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…

View tenontim's profile


2131 posts in 3888 days

#14 posted 10-11-2011 03:10 AM

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.

View DS's profile


2997 posts in 2564 days

#15 posted 10-11-2011 06:27 PM

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.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

Have your say...

You must be signed in to reply.

DISCLAIMER: Any posts on LJ are posted by individuals acting in their own right and do not necessarily reflect the views of LJ. LJ will not be held liable for the actions of any user.

Latest Projects | Latest Blog Entries | Latest Forum Topics