How do I cut a very gradual arc?

[Wood_workur]

That technique will give you a gradual arc, but not one in a specific radius.

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That is true, however I thought he wanted the arc, and calculated the radius of the circle from that. If he is after a true circle then he would need a 21 foot piece of string to do it right. FWIW, unless you are matching it to something you know is circular, I don't think the difference would be noticeable to most people.

[leehljp]

Simply bending a flexible strip between two points doesn't give a specific radius. If you've ever done it you would know it's not as easy as that.

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Yes that is true, but he is not trying to get the project to fit a specific and perfect radius. A perfect radius does not look well on all projects. The way that the thin board gives - usually makes for more esthetic appearances. People don't know all that is involved but they do appreciate that esthetics of a minute variance in radius that this method "usually" gives.

[All Thumbs]

The string method will work, too, it just won't give as accurate a circle, due to stretch in a 21' string. If you want to do it that way, a less stretchy material such as copper wire would likely give a better result.

If he is doing the math, he needs to get to the arc length of the segment.

And unless he has access to a copy of some CAD package and a plotter that will output at 1:1, I'd skip all that. Too many chances to introduce errors.

Just get a piece of string. And if he is worried about stretch, then I'd suggest using fishing line, not copper wire.

[LCHIEN]

my first comment is that fishing line usually being nylon monofiliment is quite stretchy.

My next comment is that I calculate a 3/8" rise over 27" to be a radius of 20.25ft
but its very sensitive to the rise, e.g. 15 thousandths less rise gives a 21' radius.

My last comment is that the link from sciguy that suggests putting some blocks to the ends and one 3/8" up in the middle and then using a flexible board to trace it will give a smooth line that's probably a very good approximation to the arc you desire.

But I don't know if you need an exact arc or just a smooth curve approximating the arc.

If you must have an exact circle section arc then it can be calculated knowing the chord length (27") and the radius (20.25' or 21', your choice). It can be calculated so you know the height at specific intervals along the 27" length... how many intervals is up to you but I would imagine for such a large radius every 3 inches would be OK. the calculations could be done in a spreadsheet so the number of points is only how much work you want to do. let me know of you need this spreadsheet.

[All Thumbs]

my first comment is that fishing line usually being nylon monofiliment is quite stretchy.

I use monofilament (fishing line) not because it won't stretch, but because it weighs practically nothing and requires very little force to bring it taught. You don't have to apply any more force than it takes to make the line straight.

[chopnhack]

Welcome to the forum! It looks like you have gotten some great info already. I will only add this, do your first cut by whichever method you choose on a scrap piece of ply, mdf, etc. and then if it suits your needs i.e. fits by the microwave or "looks" right then you can use that as your template with a router and pattern bit.

[jnesmith]

My last comment is that the link from sciguy that suggests putting some blocks to the ends and one 3/8" up in the middle and then using a flexible board to trace it will give a smooth line that's probably a very good approximation to the arc you desire.

Right. I doubt anyone would be able to tell that the curve is not a perfect portion of a circle. Even if you could tell, the curve would probably look just as good; maybe even better. Just keep it simple. Tack 3 brads or small nails, use a thin strip of wood, and be done with it.

[pelligrini]

If you are trying to match an existing arc, I'd just scribe the existing one to the board and then trim it to the line. I'd probably use a block plane and spokeshave if I wasn't cutting the inside of the arc. If I only had one board, I'd probably do what chopnhack suggested.


This is an image from Popular Woodworking showing an arc jig. I can't find the article, as it appears there is some problems with the website.


In my autocad, drawing an arc with a 27" chord length and a 3/8" rise with a 3 point process would have a radius of 20'-3 3/16" and a arc length of 2'-3 1/64"

[sscherin]

Lots of good ideas here..

If a accurate smooth arc was required and a bandsaw or jigsaw was not available I would have probably done something silly like get 22' of aluminum round bar, drilled a hole on one end for a pivot and and stick the other in the edge guide mount on my router.

[rwburbage]

Thanks for the all of the help!

The problem has become more complicated because this bottom piece to this completed face-frame needs to be cut and edge beaded to fit flush with the top of the GE profile oven and flush with the bottom of a table top microwave installed above. The tolerances are 1/16" at the most. I was forced to turn this project over to a local shop with a CNC machine. At least I tried to first setup a trammel arm made out of plywood at 20.5' and lay on the floor. This stretched the limits of setup diagonally in my 2 car garage. That, at least, allowed me to draw the precise arc needed. I freehand cut this in a small band saw with a cut friendly 3/4 piece of MDF. I was very careful to control this cut and it had some slight wavering. I placed it against the oven top to test fit and because the router edge bit follows the wavering, it didn't look precise as needed as well as the cut line. But the arc line was correct as it followed very close to the oven line, the oven top and the microwave edge lines are exact, this cut wood piece needed to follow it exactly. Now the client is like eeeeeeek on the local shop price of $250 to complete it and match finish. I didn't think it was that bad... thanks again.

[mpc]

Some ideas on making templates:
Can you lightly glue or double-sided tape a strip of sandpaper to the arc you need the wood to fit into or around? If so, then bandsaw or jig saw the workpiece slightly too big and literally sand it to fit. If the workpiece is going to be a thick chunk of hardwood, start with a thin (1/4 inch) piece of plywood instead... sand that to fit. Use that to pencil mark the real workpiece and make a rough cut with the band/jig saw again. Use double-sided tape (i.e. carpet tape) to tape the thin plywood template/pattern to the real workpiece and clean the workpiece edge with a router & bearing guided pattern bit.

If you need to fit to an arc that is "trapped" between other panels (like I'd expect to find in kitchen cabinets above a microwave or range fan) then make a half-template: go from the center (top) of the arc to one side. Then you can glue sandpaper to the cabinet and fine-tune half the template. Use that to then make a full template (with the router and bearing guided bit again) and use this second template on the real workpiece.

When cutting on a bandsaw or using a jig saw, stay outside the line... you'll never get a perfectly smooth cut with those tools and, as you noted, the router will repeat the wiggles in the final workpiece. Sand/file to the line. Although it's tedious, I would hand-sand the last bit of this (rather than using a belt sander, spindle sander, or random-orbit sander) just for the control. Use a wood or hard rubber sanding block and concentrate on keeping it flat against the edge - don't round over the edge corners.

mpc