Cutting curve with large radius

[eweber]

I would like to cut a decorative curve in a piece of cherry that will join to upper kitchen cabinets.
The piece will be 7' long and 6-8" wide.

Ideally I would like the curve to reach the middle of the board so the recess would be 3" in the middle.

How do I lay out such curve ?

Emmanuel

 

[Eli]

A 10' batten strip and some awls or nails would work well. Put three nails into your intended product, one at each end and one dead middle at your 3". Go long on the workpiece as the end nails have to be above the batten , and you'll trim the holey ends off. I have seen people use a really long piece of lumber perpendicular to the arc as a trammel, but the radius of the circle you're talking about is... (engineers, step in) about, um, I, um, 20 feet? (failed algebra twice, due to lack of attendance. I went to make-it-up-as-you-go school later on)

 

[Paul Farrar]

For the radius. By Pythagorus's Theorem:

R^2 = (C/2)^2 + (R-d)^2

where R is the radius of the curve, C is the chord length (7 ft here), and d is the maximum depth of the cut.

Rearranging,

R = [(C/2)^2 + d^2]/[2*d]

In your case, C = 84", d = 3"; so R = 295.5"

It may look better if you stop the curve a few inches from each end.

 

[eweber]

Thanks Eli for the suggestion. I figured the radius would be somewhat gigantic 

 

[Eli]

Paul deserves thanks too. I didn't have the real solution, just the shady tree solution. I'm grinning I was only off by 4 and a half feet though ;D

 

[eweber]

Sorry Paul I didn't mean to leave you out  :

 

[Eli]

Not only that, he's totally right about stopping the curve on the bottom edge. I forgot about that.

 

[gkeas]

With a BIG stinking compass. ;D

 

[Eli]

I thought about that first Garry, but think of the size of the pencil?

 

[gkeas]

Eli,

I think my old F250 could almost make that curve, (the turning radius sucked), so someone could sit on the tailgate and scribe a line.

 

[cdconey]

I researched this a few years ago on the internet, I think it was finehomebuilding.com had  an article about large radiuses / arcs which used a long board as a carrier for the router that was cut with the rise and run and used 3 nails offset to give you the arc.  Can't remember too much else about it though, but you should be able to find it with a search

 

[Tinker]

When i was a mason contractor, I often had to make arches with a long chord and low spring.  I would work out the math and then make an arch with wood for support as i did the masonry (usually brick. but a few of times cut field stone).  Let's say a chord of 8 ft with a spring of 6".  i would take a ten foot 2x8 and nail it to the floor.(the room at this point was always unfinished.)  Using exact measurement, i would locate the ends of the arch (leaving the extra at ends for a flat as temporary support before cutting them off before proceeding with the masonry).  Once the ends of the arch were located, i then used geometry (the formulae have left me at the moment) to locate the center point of the circle.  Using a steel tape measure & pencil, i drew my arch and proceeded to construct the rest of the form with supports to hold it in place.  Once all supports were in place, i then cut off the extensions at end of form and put on my mason hat and back to the biz I was getting paid for.

Oh yes, when making forms like that, I would use lots of shingles under the supporting legs so they could be easilly removed once the brick work was completed. 

(Corwin, i know you understand what i am talking about. are the supports i am mentioning sort of like the "pogo sticks" you have told me about?)
Tinker

 

[ccmviking]

Since that's such a large soft arc you can probably just trace something flexible.  I small piece of flexible molding, some 1/2" PVC pipe, a small diam. fiberglass rod.  See if you have something around.  If you had a 9-10 foot long board you could rip a piece like 3/16" off it and it would probably be flexible enough.  You may be able to come up with something to just do it quickly and adequately.

Chris...

 

[Michael Kellough]

As Tinker and Chris said, use a sufficiently long and evenly flexible stick to bend to the desired curve and trace the curve on the front of the board as a guide line to cut along with a jig saw. Then tack the flexible stick to the back of the board to serve as a guide for a router and a pattern following bit.

 

[Roland Chung]

Consider using Sketchup to help you determine the proper radius.  The program is free and some users are able to get up to speed fairly quickly.  People used to get a working copy that expired after 8 hours and people were buying it after the 8 hours.

There are forums that have people ready to help with Sketchup questions.  It would allow you to do a lot of testing an layouts without having to handle the full size material.  Once you find the look you want, you can just measure the parts with the included dimensioning tools and make up the full sized jigs.

The large oval type arc would be harder than a radius arc (for me).  I once set up a router on a 60 inch long trammel to cut parts for a cabinet.  If your project is more of a rough framing project, then you could use a jigsaw after laying out the arc.  The Sketchup could still help.  Good luck!

 

[eweber]

I experimented this morning with the flexible stick method on a smaller scale and that will do the trick.
I need to wait until I get my cabinets installed before I get the final dimension.

Next step is to train myself not to have a shaky hand when I run the board through the bandsaw.

I also haven't decided wether I should use a router to trim the cut or if I should run the board through the spindle sander (the later would save me from making a template but might be a tricky operation). I could also experiment with my rarely used spokeshave.

Emmanuel

 

[poto]

The methods with a bendy stick (a spline) or a giant compass will give you a curve with a constant radius like a circle. You might, however, want something that curves a little bit more at the far edges, and less toward the middle of your arc. It's pretty easy to do this elliptical shape using two nails, a pencil, and a piece of (non-stretchy) string. Lay your piece of wood beside another long piece that you can drive nails into, and clamp them together so they can't move relative to one another. Find the center of your arc, and then drive two nails on each side of the center, equidistant from the center, into the scrap board, right at the edge of your nice piece. Their heads should stick above the top of the nice piece. These will be the foci (technical term) of your ellipse. Make a loop with the string, and lay it down so that both nails are inside the loop, and adjust the size of the loop so that when you put a pencil inside the loop and stretch it to one side (in line with both nails along the edge of your nice piece) it reaches the farthest point of your arc. Now you can draw an ellipse on your good piece. Just move the pencil inside the string loop, keeping even pressure against the string as you go from one end of the board to the other. You should get a perfect ellipse. You can experiment with putting the nails closer together (you'll get a deeper arc or larger minor axis of your ellipse) or farther apart (smaller minor axis, shallower arc). You'll have to adjust the size of the string loop each time you move the nails. For such a shallow arc, you'll want the nails very close to the ends of the board. Always make sure they're equidistant from the center.

You may not like the resulting shape, but it's worth trying to see before you cut.

 

[Jim McFarland]

...
I also haven't decided wether I should use a router to trim the cut or if I should run the board through the spindle sander (the later would save me from making a template but might be a tricky operation). I could also experiment with my rarely used spokeshave.
...

95% of the time I use a router and a pattern bit all works well.  The other 5% I manage to get in a hurry or otherwise slip and tear out a chunk of material.  Very low risk of a such a catastrophic failure with the spindle sander. 

 

[eweber]

The good thing with the template is that once it is done I can use it for pattern cutting on the bandsaw and triming with the router.

 

[Corwin]

(Corwin, i know you understand what i am talking about. are the supports i am mentioning sort of like the "pogo sticks" you have told me about?)
Tinker

Hi Tinker,
 Yes, sort of.  But we were talking in circles rather than on the level  , so you can see how the pogo sticks would be oriented a little different.  Must have been kind of scary with all that overhead -- our jig left the pogo sticks and jack-and-turn methods in the dust.             Corwin