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How can I cut the arc as shown in the photo?

I have a circular saw. I have a jig saw.

I thought I might calculate the center of the arcs circle and (using a line tied to my circular saw) cut the curve on a clamped down 1 x 4.

I don't have a very steady hand when it comes to using my jigsaw so I don't want to draw an arc and then cut it.

How do professionals cut arcs?

The arc I want to cut looks like this:

enter image description here

Source

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    You won't cut that arc with a circular saw. Even if you keep the blade depth shallow you'll have scarf/kerf marks that will be difficult to remove. – isherwood Aug 28 at 13:06
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    Using a stick/rod/rigid item is the only way to get reasonable results here - using a line/string/rope/flexible item is not going to produce decent results, nor will a circular saw. Jack is right on - a router is the way these are done, cleanly, professionally, and to a high quality standard, unless you want to get into really weird old hand tools you don't have and will have a hard time buying. – Ecnerwal Aug 28 at 13:39
  • Do you have a band saw? – TylerH Aug 28 at 14:22
  • you can draw the arc with a yardstick and a few nails. You hammer the nails to hold the stick straight, then pound a nail just below the top of the arc, then bend the yardstick to catch over the top nail. This gives sturdy, traceable, and evenly arced lines. – dandavis Aug 28 at 16:31
  • What is it you wish to make, based on the what the picture is and what tools you have? – Jack Aug 29 at 3:14
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Whenever I want an arc that accurate I use my 3hp router (a smaller rated router will do too, just for smaller scale work) screwed to a shop made pivot (trammel) and make repeated passes, next pass deeper than the last to complete the cut.

A circular saw may make that tight of a curve if you set the saw shallow enough to just cut through the material. It will be quite rough, not recommended for finish work

Here is the formula to figure the radius of a given arc.enter image description here

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  • The shape op is looking at is elliptical and will require additional calculations to draw . But +1 for using a router and multiple passes – Kris Aug 28 at 12:48
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    I don't see this as an ellipse based on the photo and the question. Where did you get that impression? – isherwood Aug 28 at 13:06
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    I also don't see the ellipse. I don't know why for a decorative piece of trim, someone would chose an ellipse rather than a large radius circle. – SteveSh Aug 28 at 13:25
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    @whatsisname , The diagram is to aid in the calculation of the radius of any given arc. It is not an ellipse. If the arc I drew looks like an ellipse, it was not intentional. The arc is only for illustrative purposes to show that part of the calculation is derived from half the cord length, and the other part of the calculation is base on the rise of the arc. This is a simplification of an algebraic formula. – Jack Aug 29 at 3:03
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How do professionals cut arcs?

They don't use a saw to cut them.

If you're the IKEA kind of professional, you have a monstrous milling machine. You program the arc into your CNC machine, and it just happens.

If you're a carpenter, you rough-saw to the basic shape you want, plus a bit spare. Then you use a spokeshave, which is essentially a plane for curved surfaces, to bring it to the exact shape you want.

Quite apart from the accuracy issues, a saw always leaves marks on the cut surface. If you care about the finish, you don't use a sawn surface as-is - you always use some way to take off a bit more wood and get rid of the saw marks, as well as getting the shape fully accurate.

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I used an arm screwed to my router to cut a curve in a benchtop, it worked well.

The same technique could be applied to a jigsaw.

screw a wooden block to the side of your jigsaws foot screw an arm to the block coming off at 90 degrees to the cutting direction.

mesure the radius you want from the blade along the arm and put a pin (nail) through it at that location

clamp to your work another block same height as the first with a hole to accept the nail at the axis of the arc

Put the arm nail in he block hole

The jig saw will now be able to cut a reasonably steady arc.

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    If going the jig saw route, use a new blade. – JACK Aug 28 at 12:48
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Your jigsaw can do the job, but it's far too prone to wander for a perfect line. But you can rough cut it below the line and then use some sanding to get it to the line you want.

I would buy a new blade for this and make sure it has a high tooth count and/or is listed for fine or scroll cuts (if you're a masochist, you can try with a metal cutting blade, but that will likely take multiple blades for this size cut). Then take your time and err on cutting the part you're removing. For such a large cut, I would expect to take 30-45 mins slowly working your way around the circle. Then you can take your sander and hand tools and work out the remaining parts.

If you have a ton of time, just cut the bottom of your circle out with your circular saw, then cut the arc with a coping saw. But that has the same net effect as your jigsaw, just slower for the sake of precision.

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    Shouldn't wander too much once it's in the wood, just gotta move slowly. I'd prefer a band saw for this personally. – TylerH Aug 28 at 14:23
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    Yeah, a band or scroll saw would be ideal, but the odds they have one, or would buy such an expensive tool for one project, are probably low – Machavity Aug 28 at 14:24
  • In addition to wander, I find sometimes a jigsaw blade won't cut truly vertical. Maybe following wood grain somewhat. This is even more reason to leave a good buffer to the line. – UuDdLrLrSs Aug 28 at 17:14
  • The motion of the jigsaw blade, in the open air below what you are cutting, will make it wobble. That is what bends the path. This is why they are better for thinner stock. Similar to how hand sawing can sometimes cause the blade wobble though the cut (happens when your arm moves slightly out of alignment). – Matt Aug 29 at 13:59
  • @Marinaio You might consider drawing an oval instead of a circle. My jigsaw has a locking mechanism when released allows for the blade to be swiveled on the unit with the other hand. I'd probably also take my angle grinder and remove some of the material on the back end of the jigsaw blade to get it closer to the width of a coping saw blade from tooth to smooth end. Then if you have a drill, you can buy a round sanding drum to smooth the surface. – J D Aug 30 at 6:01
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Since no one else has suggested it, a bandsaw would work well for this curve.

Depending on how big of one you have access to, you might need to increase the size of the material support plate. And, in this case, having a wide blade would help prevent wandering. You could easily rough it in fairly closely with the bandsaw, then use a large spindle sander or belt sander to finish it off.

You could even build a jig to help with the cut, similar to how people cut round table tops with a bandsaw, except that the scrap piece is the part you want to keep. This is just one example of a jig I found, but there's many out there to find.

https://www.youtube.com/watch?v=N6_2h-vo0jA

To find the center of your circle/arc, draw a straight line between any two points on the arc, then find the exact middle of that line. Starting at that midpoint, draw another line at a 90 degree angle towards the center of the circle. Repeat the process again and where the 90 degree lines cross, that's the center. Repeat the process again to be more accurate, and to find out if you have an ellipse (oval) or an actual circle. With an ellipse, the 3rd and later 90 degree lines won't cross at the same point as the first 2. To make sure that you aren't accidentally creating a false center, make sure you don't reuse any points on the arc.

https://www.instructables.com/id/How-to-find-the-center-of-a-circle/

This is likely how a carpenter or handmade woodworker would make this curve. Not everyone that would do this kind of cut is IKEA (as mentioned in another Answer) and have access to a CNC router. I built my own 4'x8' capacity CNC router, but that's beside the point.

I realize not everyone has a bandsaw or large sanders, so you might want to find a local makerspace to see if the have that stuff. Makerspaces often have pretty good woodworking tools, but that varies with the organization. I've seen makerspaces that just have 3D printers or just have computers for internet and other IT related uses.

You might (but not likely) be able to find someone willing to help you with this one project, but becoming a member will give you access to all that equipment (based on their rules for it all). It'll also give you access to more build space as well as lots of people to help you figure out problems and even help with your projects. Yes, they will be willing to help other members (depending on their time and experience), even if they won't help the random walk-in.

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It's possible to cut coves on a table saw. This is kind of like a cove but on a very narrow piece of wood. But with the right amount of support I think it could be the same effect.

You can cut coves on a table saw by putting the wood through the blade sideways with lots of support on the front and back of the blade. Here's an example.

It's certainly not trivial and a jig saw/band saw/router would be easier but it's another option to think about.

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