How to cut this curve

[John Nesmith]

Hello:

I am designing a low media cabinet, and have decided I want the bottom of the lower front rail to be curved. I don't want a circular arc though. I would like the ends to be more curved than the rest of it. An example of exactly what I have in mind is on this cabinet:

http://www.finewoodworking.com/Gallery/GalleryImage.aspx?id=32862

A circular arc would be fairly straightforward. This seems a little more challenging. I've thought about laying out most of it as an arc with some bendable stock, and then fairing in the ends with a french curve. That might make it hard to get both ends exactly the same though.

I've seen this type of curve before, so it seems it might be a fairly common design element. I don't want to reinvent the wheel. Have any of you done something like this? Any suggestions for me?

Thanks in advance!

[Lee Schierer]

You have the right approach. To get both ends the same, do your layout on a piece of foam core board and draw just half of the desired curved area to make a template for half the curved area. Flip the template end to end to make both sides exactly the same.

[Paul Atkins]

I use cardboard or thin plywood for templates, and do as Lee says. I usually go beyond halfway so I can feather out any bumps or flats. Another thing I do is after I cut one side, I trace the cutoff piece on the uncut side and see how it matches up with my drawing. Sometimes the saw cuts a bit different than I planned so I have a matched curve anyway if I follow my new line.

[Bill Arnold]

I copied the image of the cabinet into a drawing program, then scaled it to the approximate dimensions noted on the page you posted. I added a circle and sized it until its radius matched the curve on the cabinet door. I get a match with a radius of about 48".

You can draw the arc with a trammel arm or a piece of string and a pencil. Using the right door as an example, use the left side of the arc as the peak. Draw an arc that will be lower as you go to the right. I think you'll find this will give you a smooth arc that matches the cabinet.

[David DeCristoforo]

All of the methods mentioned so far will work. My preferred method is to hang a piece of heavy twine between the two extreme ends of the arch allowing it to form a "catenary arch" which looks very "natural". Just stand the stock on edge and use a couple of map pins to locate the ends of the arch. Tie the twine to one, drape it over the other and play with the "drop" until it looks like what you want. Then it's pretty easy to mark the line with a pencil.

[John Nesmith]

I copied the image of the cabinet into a drawing program, then scaled it to the approximate dimensions noted on the page you posted. I added a circle and sized it until its radius matched the curve on the cabinet door. I get a match with a radius of about 48".

You can draw the arc with a trammel arm or a piece of string and a pencil. Using the right door as an example, use the left side of the arc as the peak. Draw an arc that will be lower as you go to the right. I think you'll find this will give you a smooth arc that matches the cabinet.

Intriguing, but I'm not sure I'm following. I'm interested in the curve on the lower rail, not on the 2 doors. Does your recommended method apply to the curve on rail at the bottom of the cabinet? (Which does not appear to be a circular arc. I don't want a circular arc)

Thanks.

[John Nesmith]

All of the methods mentioned so far will work. My preferred method is to hang a piece of heavy twine between the two extreme ends of the arch allowing it to form a "catenary arch" which looks very "natural". Just stand the stock on edge and use a couple of map pins to locate the ends of the arch. Tie the twine to one, drape it over the other and play with the "drop" until it looks like what you want. Then it's pretty easy to mark the line with a pencil.

Is a catenary a curve like I described? (and what appears to be on the rail in the example I posted) In other words, a smaller radius at the ends, and a larger radius throughout the majority of the curve?

[John Nesmith]

I use cardboard or thin plywood for templates, and do as Lee says. I usually go beyond halfway so I can feather out any bumps or flats. Another thing I do is after I cut one side, I trace the cutoff piece on the uncut side and see how it matches up with my drawing. Sometimes the saw cuts a bit different than I planned so I have a matched curve anyway if I follow my new line.

Good advice there. Thank you.

[John Nesmith]

Actually, I just look up catenary arch. My geometry days are long gone, but it appears to be the opposite of what I am after. it appears to be an arch that has a tighter radius in the middle, and larger radius as you approach the ends. Like the Gateway Arch, for example.

[Frank Drew]

John,

If I understand correctly, it looks like what you want could be either a section of an ellipse or of a catenary curve, and narrow ones at that with such a faint curve to the long side. Shouldn't be difficult to layout the portion you want by drawing an ellipse full size or with David's twine method, or even by freehanding the drawing (keep your eraser handy.)

[John Coloccia]

With this magic tool, that gets lots of use in my shop:

http://www.woodcraft.com/Family/2020439/2020439.aspx

I have the shorter one, but I don't know how long your item is.

Two ways to do it:

1) do 1/2 of the curve, flip it over and match the second half. I would just eyeball it but you can measure some control points if you want
2) precisely lay out control points along the whole length, and make the tool go through those points. This requires more precision to get a symmetrical curve.

I like method #1, although it's always a little frustrating to get the the 2 halves to match up perfectly. The secret is to remember freshman year calculus: "The derivative of a function is 0 at maxima and minima", i.e. the highest point and lowest points, or generally any point where the curve switches from going up to going down (or the other way around), is parallel to the floor. To make the two halves match, scribe a line parallel to the floor at the height you want the top of the curve to be. As long as the curve you lay out blends smoothly with the straight line, it will blend smoothly when you flip the curve over.

Generally, anytime I want to blend 1/2 of a curve to another half, I lay out the tangent of the curve at that point first, and then make sure that my pattern blends smoothly with the tangent.

[David DeCristoforo]

"...a smaller radius at the ends, and a larger radius throughout the majority of the curve?"

No. But it's a simple matter to add the smaller radius at the ends of the arch.

[johnny means]

My technique is a little less technical than some.

I draw the arch, curve, squiggle, or whatever freehand. I then stand back, look at it, figure out what I don't like, then make corrections. This process repeats itself until my shape is just right.

Sometimes us guys have a tendency to over engineer things.

[Mike Henderson]

I do the same thing Johnny does. I draw half then flip it over.

Mike

[George Beck]

I made a display case for a customer who collects these antique Santas. I originally planned on a standard square base but I thought it made the case look heavy like an aquarium. So I decided to add a curved bottom. Here is how I did it: I first made a drawing of the curved corner I wanted (about 8 inhes in just to the straight part) on poster board using a french curve. I then cut out the pattern and used it to make a full size template on 1/4" 5 ply plywood. I carefully cut out the template and smoothed it on a drum sander using the pattern to check both sides. Once I had the template the way I wanted I used it to layout the base and cut close to the pencil line on the bandsaw. I then use the template(attached with 2 sided tape) to guide a laminate trimmer to smooth the piece. I used the pattern again to make the smaller end pieces template and repeated this process. It worked pretty well and now I can duplicate this base curve when needed. I also remembered that my Great Uncle who was a cabinetmaker and primary a chair maker had hundreds of templates hanging from the ceiling in his shop with all manner of curves marked with lines like "Queen Anne back top" and such. You can see the finished piece at this old thread. Anyway just a suggestion.
George

[George Beck]

oh here is the finished case.

George

[Mike Cruz]

A true artist's eye...

[Mike Cruz]

You know, If we all hadn't uttered those famous words in 10th grade, "Why do I need to learn Geometry? I'm never going to NEED to use it in real life....", we'd be able to answer this question with an equation. :rolleyes:

[John Nesmith]

I made a display case for a customer who collects these antique Santas. I originally planned on a standard square base but I thought it made the case look heavy like an aquarium. So I decided to add a curved bottom. Here is how I did it: I first made a drawing of the curved corner I wanted (about 8 inhes in just to the straight part) on poster board using a french curve. I then cut out the pattern and used it to make a full size template on 1/4" 5 ply plywood. I carefully cut out the template and smoothed it on a drum sander using the pattern to check both sides. Once I had the template the way I wanted I used it to layout the base and cut close to the pencil line on the bandsaw. I then use the template(attached with 2 sided tape) to guide a laminate trimmer to smooth the piece. I used the pattern again to make the smaller end pieces template and repeated this process. It worked pretty well and now I can duplicate this base curve when needed. I also remembered that my Great Uncle who was a cabinetmaker and primary a chair maker had hundreds of templates hanging from the ceiling in his shop with all manner of curves marked with lines like "Queen Anne back top" and such. You can see the finished piece at this old thread. Anyway just a suggestion.
George

I think that is basically how I am going to do it. I came across a product called a "ship curve" that would probably be useful, but not necessary.

Thanks to everyone for the replies.