How do I cut a very gradual arc?

[rwburbage]

I'm trying to cut an arc thats 3/8" over 27". It's part of a faceplate for a microwave that's installed in a brick wall above a double oven stove system. I've been searching all over and as I would like to try and create a still bar and attach this 1/2 x 2 1/2 x 27" maple stock to a thin board it seems impractical at a radius of 21'. yes 21' was determined. Is there any other way? Thanks for any help!

[Wood_workur]

I just cut a 2" deep arc over 54" for a bed. I used a thin piece of scrap to mark it (I ripped a 1/8" piece off of a board) and put two spring clamps to mark the end points of the arc. I then bent the wood between these two clamps and pulled the middle up to the top of the arc. I marked this line, and then I cut it with my band saw, though if I didn't have one I could have easily done with a jig saw.

Once it was rough cut, I used a 60 grit sanding drum in my drill press to smooth the curve. I was very consistent with feeding it, and this helped to even out any roughness or waviness in my cut. After tha tI used a sanding block with 60 to even the whole thing out, then worked my way up to 150 and at the end I had a nice, smooth curve.

[cabinetman]

I'm trying to cut an arc thats 3/8" over 27". It's part of a faceplate for a microwave that's installed in a brick wall above a double oven stove system. I've been searching all over and as I would like to try and create a still bar and attach this 1/2 x 2 1/2 x 27" maple stock to a thin board it seems impractical at a radius of 21'. yes 21' was determined. Is there any other way? Thanks for any help!

If you could explain it better or post a sketch/drawing it would help.

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[Wood_workur]

If you could explain it better or post a sketch/drawing it would help.

My understanding of it (and the math checks out) is he has a board 27" wide, and he wants an arc on the bottom of it, that at the highest point, is 3/8" of an inch, so at the ends it is 2 1/2" board and in the middle2 1/8"

He was looking at using a router or similar setup to cut a circle, but the radius of such an arc is about 21' long.

[sciguy]

This may help:
http://www.woodmagazine.com/woodwork...smooth-arches/

[leehljp]

This may help:
http://www.woodmagazine.com/woodwork...smooth-arches/

Thanks for posting this. I saw it a few months ago somewhere but could not remember where. This is probably the way I would do it.

[cabinetman]

My understanding of it (and the math checks out) is he has a board 27" wide, and he wants an arc on the bottom of it, that at the highest point, is 3/8" of an inch, so at the ends it is 2 1/2" board and in the middle2 1/8"

He was looking at using a router or similar setup to cut a circle, but the radius of such an arc is about 21' long.

If the math works out for smaller layouts to get the radius, that's fine. If not, for 21', use a longer tape measure, and lay out the arc on a piece of substrate from a center point to 21', and strike the arc for the length that will work. The arc will not likely be that long. It may have to be done in the driveway or on the shop floor if you have the space.

I've had to do that for a long radius on a footbridge over a pond. I will post pictures Monday.

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[cabinetman]

I've had to do that for a long radius on a footbridge over a pond. I will post pictures Monday.

I just got a web cam for X-Mas, and experimented with taking a picture of a picture. The first picture is my results of experimentation. This long arc was drawn out to scale, as the drawing became the pattern for the parts.
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[Pappy]

This may help:
http://www.woodmagazine.com/woodwork...smooth-arches/

That was the method that came to mind. Think I first saw it used on either NYW or WoodWorks.

[cabinetman]

I just cut a 2" deep arc over 54" for a bed. I used a thin piece of scrap to mark it (I ripped a 1/8" piece off of a board) and put two spring clamps to mark the end points of the arc. I then bent the wood between these two clamps and pulled the middle up to the top of the arc. I marked this line, and then I cut it with my band saw, though if I didn't have one I could have easily done with a jig saw.

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

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[woodturner]

This may help:
http://www.woodmagazine.com/woodwork...smooth-arches/

Yes, that's probably the easiest way to do it.

This method will give you either an arc that fits between two points or an arc of a particular radius. If a homogeneous material such as plexiglas is used for the strip, the strip will bend uniformly to provide a particular radius. If a wood strip is used, the arc may tend to be a bit more parabolic, due to differing wood density, but usually will still closely approximate a constant radius arc.

The OP wants 3/8" at the deepest part of the arc, so he just uses a 3/8" block. The OP also indicated he wants a 21' radius arc over 27". He therefore knows the length of the chord and the radius of the circle, and can calculate the segment height for the block. The calculator here
http://www.1728.org/circsect.htm
makes it easy, and shows 3/8" is a good approximation, though 23/64" is closer. Given a 23/64" block and a homogeneous material for the strip, the arc formed will be the 21' radius segment of the circle.

[cabinetman]

Yes, that's probably the easiest way to do it.

This method will give you either an arc that fits between two points or an arc of a particular radius.

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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[All Thumbs]

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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You are correct.

If he was able to make some test cuts, perhaps that method would eventually work.

But bending a stick provides an arbitrary radius.

I'd cut a 21' long piece of string, tie it to a pencil, attach the other end to a stationary point, and use that to draw my arc onto the piece of wood to cut.

[phrog]

You are correct.



I'd cut a 21' long piece of string, tie it to a pencil, attach the other end to a stationary point, and use that to draw my arc onto the piece of wood to cut.

That's what I was thinking also.
By the way, C'man, that is one beautiful bridge.

[woodturner]

But bending a stick provides an arbitrary radius.

It depends on whether the material will bend uniformly. Wood typically does not, so the curve tends to be more parabolic than circular. Given a uniform material such as plexiglas for the strip, however, the curve will be a segment of a circle.

It's really not bending a strip between two points, BTW, but three - it's the three points that ensure you get a circle. The three points are the two ends and the "height" of the chord, which is the segment. Only one circle of one radius will fit those three points.

Recall from geometry that the chord of a circle defines a unique circle. Given the chord length - the 27" length the OP wants - and the radius of the circle, which the OP said is 21', the length of the segment of the chord can be readily calculated.

The length of the segment defines the size of the middle block, and setting that middle block and the ends gives you the desired curve radius.

This method will work for arbitrary circle radii.

Sounds complicated when explained, but if you try it you will see what I mean. It's really not that complicated and ensures you get a circular curve.

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.