How to cut a perfect arc

[Zahid Naqvi]

For a current project I need to cut a very smooth arc, about 24"=30" radius. I have tried marking it and then cutting it with a jig saw, smoothing and sanding it later. But no matter how carefully I do it I never get the curves smooth without any bumps. Tools available are a router and jig saw. I am thinking about making my self a circle cutting Jig for the router. Any other options I should consider?
thx
Zahid

[Todd Burch]

The router on a trammel is your best bet. Cut close to the line with a jig saw first, then one easy pass with the router and you are done. Todd

[Chris Padilla]

If pressed for time or you are lazy (no problem there, I'm VERY lazy), there are some great although a tad pricey circle cutting jigs available for your router. In fact, I just bought one that does 8-18" circles at Woodcraft last weekend to make some discs for my dust collector. Worked very, very well and right on the nose.

However, if you only need 1 particular radii, you can easily make one to bolt up to your router baseplate with any length of plywood or MDF. 1/2 or 3/4" material works well although it depends on how deep a cut you need to do for your particular arc and your router bit cutting depth, etc.

Most routers will also have a way to bolt a fence on them and you can take advantage of those holes to make yourself a circle cutting jig as well.

As Todd said, cut it as close as possible with a jigsaw and then clean up the cut with a straight bit. Use the largest diameter straight bit you have...it'll provide the cleanest cut assuming it is good and sharp.

[Chris Padilla]

I took a pic of the circle-cutting jig I purchase so you have an idea of what it looks like and how it works.

The hole for the router has a small disc that snaps into place to allow you to center it (i.e. calibrate) on your router. You then pop the disc out and you are ready. Drill a clean 1/8" hole for the center of the hole/disc, tap in the 1/8" drill rod they provide for your pivot, start up your router (use a 1/4" bit as that is what the jig is designed for) and go to town! Couldn't be easier. Just be wary that the jig is designed to cut HOLES of the size indicated. If you want to cut a DISC of a certain size, you must add a 1/2" to the scale on the jig. In other words, you put the pin in the 17 1/2" mark if you want a disc of 17" diameter.

The reason I mention this is it caught me! I purchased this jig thinking I could make an 18" disc since the scale went up to 18 3/16". WRONG! It'll make a circular hole that big, but not a disc...the max disc size would be 17 11/16"....a whole 1/2" shorter! Boy, if you coulda heard the swear words coming out the garage when I realized this!

I doubt you can see it, but I ended up drilling another hole into the jig in order to get an 18" disc out of it (the hole is right next to the 1/2" mark on the y-axis scale on the far left of the jig). I did my best to guesstimate where to drill and ended up a hair short with a 17 31/32" circle!:( But, to fix that, I simply dug out my 1/4" plywood straight bit which is actually 15/64" in diameter and used that one to cut the next disc...bingo! Perfect 18" disc. :D

[Zahid Naqvi]

Chris,
That pictures is worth a thousand words. Gives me ideas about what I need to make. I am going to take a 1/2" thick plank and make a trowel my self. The hole allignments shown in your jig looks like a very practical idea, although mine will probably only contain a single row of holes.

thanks
Zahid

[Bob Hovde]

I doubt you can see it, but I ended up drilling another hole into the jig in order to get an 18" disc out of it (the hole is right next to the 1/2" mark on the y-axis scale on the far left of the jig). I did my best to guesstimate where to drill and ended up a hair short with a 17 31/32" circle!:( But, to fix that, I simply dug out my 1/4" plywood straight bit which is actually 15/64" in diameter and used that one to cut the next disc...bingo! Perfect 18" disc. :D

Chris,

That equates to about a 1/10" circumference error. For the next cyclone builder (maybe me), do you think it made any difference in the metal work, since you had to bend it by hand? With my normal techniques, I could be that far off even using your solution. :(

Bob

[Chris Padilla]

Zahid,

Glad to have sparked some brain cells! :) That jig above cost me $35 at Woodcraft so they aren't cheap but I didn't feel like spending time to make one. If I need bigger holes, I plan to make one as their jig for 18" to whatever is like $50! Still, very nice jigs and they are dead on accurate.

I made my own arc/circle cutting jig with a long piece of 3/4" MDF. It was for one specific arc...radius was around 5 feet. I posted that in the Design section: Floating Maple Shelf...I cut that arc with the jig I made.

[Chris Padilla]

Chris,

That equates to about a 1/10" circumference error. For the next cyclone builder (maybe me), do you think it made any difference in the metal work, since you had to bend it by hand? With my normal techniques, I could be that far off even using your solution. :(

Bob
Actually, Bob, it is dead on...check your calculations again and recall that you have to consider the DIAMETER which means you take twice the size of the bit. So using a bit 1/64" smaller in diameter would add twice that much to the finished diameter of the disc...hence my dead on 18"! :)

However, you are correct, that kind of precision when dealing with metal work of this kind probably won't amount to a hill of beans but since I had the chance to actually make it 18", I went for it. In fact, it might be better to make it a ~tad~ larger to help really pull the seams tight but whatever you use, make sure both discs are the same even if off a bit so everything fits in the end. :D

[Mark Singer]

I concurr with Todd....a router and tramell point

[Tony Falotico]

Here's a pic of what I used to make Thomas train track a few years back with a router. Cut out of 1/4" plywood, Worked great and cut perfect arcs.

[Chris Padilla]

Very nice, Tony! That one is just the ticket and the right price, too! :)

[Jim Becker]

I'm also a proponent of a router with a trammel, although you could theoretically use the same type of setup with a jigsaw. I've used the router for this--the example shown is for a cherry Shaker candle stand top.

[Zahid Naqvi]

Tony, that's a nice one. I ended up using a straight plank 6" wide somewhat similar to your tramel, but with a single hole. I think I will copy your design and make one for future use.