View Full Version : How do you cut an oval?
I want to make an oval shaped dining room table top, about 7 feet long. Any advice on how to cut the oval. Expensive jigs not an option -- hopefully, this will be the only oval I will ever cut.
11-27-2001, 07:32 PM
If you are asking how to layout the oval, I have some information written down out in the shop that I can retrieve for you. I use a string with a couple of nails on the major and minor axes. You remove one of the three nails and hold the string tight with a pencil as you mark the oval. I used to know how to explain this method without my notes, but time has taken its toll...
11-27-2001, 08:08 PM
If you take a ruler and draw a line from one focus to any point on
the perimeter of an ellipse, and then a second line from that point
back to the other focus, the total length of those two lines is a
constant, independent of which point on the perimeter you choose.
This gives you a simple way of drawing an ellipse. Take a wooden board,
and hammer in two nails to represent the foci (plural for focuses, not a dirty word or a Star Wars critter). Fasten one end of a
piece of string to each nail, then push a pencil against the string
and draw the ellipse keeping the string taut at all times. You can
draw ellipses of different eccentricities by adjusting the length of
the string relative to the distance between the two nails
Cool beans daddy-o...have fun!
11-27-2001, 10:05 PM
Do the same stunt except instead of using a pencil, you use the round body of your router...
11-27-2001, 11:37 PM
Where the hell do you put the string?? Around the router? This could be a handy thing...if it's safe. Guess I'll try it...if I die or lose an appendage, we won't recommend this :)
11-28-2001, 10:17 AM
Yep. I'd put the string (actually I'd use something a little stouter than just some random twine. Perhaps a thin metal cable (stranded) with a crimped end?) around the body of the router and the two foci (?) pins. You pull the cable taut and it'll inscribe an oval automatically. You'ld have to let the cable slip and slide a bit on the foci pins, but that should work.
I'd draw it first, then bandsaw it to get close and then router it to get the finished edge. That way you're only going to take off 1/8" or so.
11-28-2001, 05:05 PM
You guys did an admirable job of describing how to draw an ellipse, but that man asked about an OVAL! An ellipse is ovoid with different radii at each end. An oval has equal radii. To make an oval, draw 2 identicle circles and connect with straight lines.
11-28-2001, 11:54 PM
Hmmm...ok...you have driven me to confusion on these terms...(yea yea...I know...it was a short drive )
When I used the described method, I had the nails on the scrap nearly as far apart as the table was wide, and moved the board into position so that the string (and pencil) would reach the end of the table in the middle of the arc (at the top of the arc eh). Drew the ellipse, oval...whatever, then repeated this for the other end of the table. Since both ends were (basically) equal, what was it? Oval?...or an ellipse?...or a table with rounded ends? I'd like to call it by the correct name eh....
11-29-2001, 07:38 AM
The string technique for geometrically constructing this type of shape, always makes an ellipse. As far as an oval, I am not sure about the proposed definition of an oval, there are many types of ovals, the extended circle idea, along with other mathematical and non-mathematically described enclosed curves. I have stayed out of this one due to the differences in what is asked versus what is wanted.
"If it is worth doing, it is worth overdoing"
You've got to have a college degree in mathematics to know there is difference between an oval and an ellipse. I guess I want to cut an ellipse after all. I guess MadMark's suggestion is the way to go, "mad" as it may be. I just can't figure how to make the metal string run around the router smoothly enough for the cut to come out right. The metal string will have to be tied together to get the right length, and won't the knot cause a bump when it goes around the Router.
11-29-2001, 08:19 PM
I'm pretty sure ol MM hasn't actually done this...I think we were just kinda yakking...if ya know what I mean. I guess it would work..in theory...but by the time you get all the logistics worked out you could have just drawn a pencil line and cut it out.
11-29-2001, 08:29 PM
OVAL: 1. Resembling an egg in shape. 2. Resembling an ellipse in shape; elliptical. --o·val n. 3. An egg-shaped or elliptical form or figure.
ELLIPSE: A plane curve, especially: a. A conic section whose plane is not parallel to the axis, base, or generatrix of the intersected cone. b. The locus of points for which the sum of the distances from each point to two fixed points is equal.
Man, it's tough when the definitions only make matters worse...hehehe
11-29-2001, 08:37 PM
Probably the easieast way to picture it is an elipse is a diagonal cross section of a cylinder and an oval is a diagonal cross section of a cone.
Another suggestion that might yeild more accurate results would be to use a pice of 1/4" or 1/8" hardboard, draw the line as mentioned above, cutout with a band or jigsaw, trim up and smooth the edges, trace this pattern to the table top, rough cut with a jig or band saw then use a pattern bit on your router to create the exact shape you want. This way you can experiment on the right shape using hardboard instead of a more expensive top.
11-29-2001, 09:25 PM
OW ! Now my head hurts from trying to think. I think Breakaway has the right idea.... Keep playing with the shape till you get what you like and the hell with the terminology and formulas and such! This is supposed to be fun! If you really need perfect geometric shapes, use a CAD program and a CNC machine. Woodworking is about putting your soul into your craft and each piece is a reflection of the craftsman. That's why no 2 pieces are exactly alike. Each is a labor of love for the wood and is shaped to bring out the beauty of that wood as seen by the craftsman.
11-29-2001, 09:43 PM
Hey thanks Marc for mucking it all up.... LOL...
11-30-2001, 08:43 AM
Muhuhuhahahaaa......I couldn't help myself...
What's funny is *I* certainly don't go through all this chit...I just hammer a couple nails in a board, attach the string, and start messing with it til I get a shape that looks good to me! Don't take long either :)
12-02-2001, 08:01 AM
Ever seen a "Vermont BS grinder"? It's perfect for this thread.
It's hard to describe, but it works the same as an elipse jig I saw once. It wouldn't be worth building for just one top.
Breakaway 9's solution in post # 12 is the way to go. I've made guitar bodies that way and it works great.
I don't know about that "strap around the router" thing. Disaster waiting to happen if you ask me...
I have made ellipse tables in years past, using a router. Get a piece of particle board as big as your table. Cut a dado about 1/2" wide x about 7/16" deep, in the middle running the length of the sheet. And a dado across the sheet. You will have a cross. Cut a couple pieces of wood about 2or3 inches long that fit in the dados and will slide. Wax the dados. Attach a 3/8" dowel, 2" long to the two pieces of wood. Now make a long extension for the router such as you would do for cutting large circles with a router. From the inside of your router bit, measure out 1/2 the width of your table, drill a hole so the dowel will fit into. And another hole 1/2 the length of your table. Insert dowels that are attached to small blocks and set the whole thing on top of your sheet with the cross. As you turn the router, the blocks slide along both dados, thus cutting with 2 axis points.
With a good plunge router, you're good to go.
12-14-2001, 10:05 PM
Hey look, the idea is to keep it as simple as possible . All these gadjets and jigs can fowl up ( and please notice when they do, they cut into the shape, not out of it to be cleaned up by a second pass.)I've made a number of ovoid shapes and just used the string method to lay out the pattern on quarter inch hardboard, cut it out rough with a jig saw and then sanded it to the lines with a belt sander or,if available (happy day )a stationary sander.Once the shape pleases you, then you just double sided tape it to your precious table top lumber and rout it to shape with the appropriate follow bit. Easy.
12-28-2001, 05:23 PM
Keep in mind that most people that are cutting a large ellipse table are going to be putting on a 1 1/2" wood edge around it. To make the one- 1 1/2" wide template for the 4 pieces needed to go all around the table, the 2 axis router jig will be needed. The template can be cut to fit the curve of the table with no sanding.
After glueing up some wood for edging, trace the template onto the board or boards. cut about a 1/16" or so bigger than the tracing on a bandsaw. Then you will need a shaper or router table setup with a bearing flushtrimmer. Screw the template to the bottom side of the edging and run through the flush trim setup.
To make a better cut, go with the grain direction. This is done by using 2 bearings on the flushtrim setup. On a shaper, you would have a bearing above and below a cutter that is a little bigger that the thickness of the wood edging. Make sure that the template is several inches longer than the wood your pushing through. When you get to the point where the grain is parallel, turn the assembly over and crank the cutter up so the template is riding the other bearing.
You get the idea.
A person can make these tables for years, perfect curve, uniform edgebanding and never do any sanding,to make the parts fit.