I was going to make a circular breakfast table from old barnwood, but my client (WIFE) has now decided that it must be oval. She wants it about 7 feet in long axis and 42 inches short axis. I had planned to use the router mounted on a long sheet of 1/4 inch ply, pivoting around a nail to cut a perfect circle. Now that I’m in oval mode, is there any method for cutting a perfect ellipse? While you are answering, perhaps someone could review for me the best method of first drawing such an ellipse. Thanks very much.
Jay
Replies
Yes, there are at least two methods. What you probably want is an ELLIPSE, which is a special case of an oval and has a specific mathematical formula (the set of all points, the sum of distance from two other points (the foci) are the same)
The simplest is to use a beam with two pivot points (e.g., nails) and a pencil. Sort of like trammel points if you have them. The distance from the pencil to the nearest nail is the minor radius, and the distance to the second nail is the major radius. Lay a square, like a framing square or plywood corner in the center of the ellipse. Keep each nail in contact with each leg of the square and trace 1/4 of the ellipse. Move the square and repeat for each quadrant.
The second way just automates this process with a device sometimes called a "smoke grinder" or "do nothing machine" In this device, sliding dovetail keys fit in a dovetailed frame with tracks at 90 degrees to each other. Same sort of arrangement, just the nails fit into a hole in the keys.
The other way is to put nails at the foci of the ellipse and loop a string around them. Then keep a pencil in the loop and trace it around. http://mathforum.org/library/drmath/view/54949.html You have to do some math ( http://mathforum.org/library/drmath/view/55323.html )to locate the foci and I find it's difficult to get an exact tracing using this method.
I realize pictures would be valuable, but I don't have any available at the moment.
see also
http://mathforum.org/library/drmath/sets/select/dm_ellipse.html
By the way, I'm on your wife's side; I find elliptical tables very elegant.
Edited 2/17/2007 6:47 pm ET by byhammerandhand
Just to clarify, the inside corner of the framing square would be at the center of the ellipse, with each arm along one of the axes of the ellipse - correct? Thanks very much.
OK another question: When routing with a fluch trim bit along the edge of the mdf template, the bit will be going against the grain for 2 of the 4 quarters of the table, assuming the template is tacked to the bottom. How do you minimize tearout? Also, how can you be sure the 4 quarters are perfectly aligned, since the quarter template has to be removed and reattached 4 times? Thanks.
Jay
Yes, that is right. I tend to use the outside corner and edges, but either works.As far as tearout goes, I generally scroll saw out, leaving a fat 1/8" then use the router to trim. When I do circular cutouts, I sometimes just use a 1/4" straight cutting bit and plunge it in about 1/8" per turn, then reduce the radius by 1/16" and do it all over again. I am not sure there is a right way. You may want to "climb cut" on the light cuts against the grain.Re: removing and repositioning the quadrant. Just be very careful :-)As far as making a jig like pictured, route a dovetailed channel at right angles to each other on on a sheet of scrap plywood. Then cut keys a couple of inches long that fit relatively tight in the dadoed grooves. Wax them up, drill a hole in the center and use a beam with nails or screws.
Repeating my answer to another post:
Another approach is to go to a Kinkos type place and have them draw this in CAD and then print it full size on a wide format printer. It's a simple procedure and shouldn't cost much.
You only need a quarter of the design printed and you use it and then flip it a few times.
I do many layouts this way
good point, somewhere in a parallel thread (pardon the pun)
http://forums.taunton.com/fw-knots/messages?msg=34583.1
"....any method for cutting a perfect ellipse?"
Hammer has given ways of drawing it/setting it out, so now you want to cut it out nice and neat.
I suggest you use one of those methods to set it out onto a sheet of 1/4 mdf-but you can save material by dividing it into quarters and use only one of these pieces as a template which is used to mark out the whole thing onto your top-and it is then used as a guide for routing the edge after you have cut out the top with portable jig saw or similar.The template/router guide is merely pinned in place on the under side for each of the four quarters.
The mdf is cut out with bandsaw if you have one, as it is only 1/4 of the top and therefore managaeble, then a compass plane is great for fairing and squareing the edge.
That is a way of doing it if you were not mass produceing them....
Do a search for drawing an ellipse,you will find several easy methods. I have only done this on door and window heads,not tables.I suggest that you make a hardboard template first.Trace that on the table and rough cut with a sabersaw.
Clean up the edge with a router and bearing bit riding on the template.The edge profile can then be cut with the router.
mike
Jay:
An oval and elipse are not quite the same thing to many people even though they are geometrically equivalent. Make sure you know which one the wife really wants before you start cutting. Many dining tables are oval. Here are some links to images that are called oval:
http://upload.wikimedia.org/wikipedia/commons/f/f7/Oval1.PNG
http://en.wikipedia.org/wiki/Image:Oval2.PNG
http://en.wikipedia.org/wiki/Image:Oval3.png
Hastings
Hastings is correct. OP said both oval and ellipse, so that's where I was neaded. Another option is just straight sides and semi-circular ends (a racetrack oval). This is a simpler form as you can use a router extension sub base with a nail at the center, compass, or trammel points (or several other methods) to cut/trace each semi-circle.
http://www.microfence.com/pages/Ellipse%20Jig.html
Old technique but simple to make.
Looks interesting, but since I'm not planning to do this repetitively, don't want to buy a jig like that. If it is so simple to make, can you elaborate on the details of construction? Thanks very much.
Jay
I certainly wasn't suggesting you buy the Microfence elipse jig. Basically it's a x with crossing dovetail slots and two pieces of dovetailed slides connected by the long arm. Adjusting the spacing of the slides on the long arm defines the elipse. You can use it like a tram beam or just put a pencil in it to draw your elipse.
Jay
I have a 68"diameter teak table that I built from solids with two 18 " rectangular extensions that I insert when I want the table longer. I usually only use one extension. This oval (not elliptical) table is really great because it allows more comfortable seating than an elliptical table when there is a crowd for dinner. Ovals are also easy to build since they are only semi circles and straight lines. JL
I would go with the nails and the string, make a template, perfect it with a rasp and a file and then transfer the pattern to the tabletop. Rough cut it with a handheld jigsaw or a band saw and then use a spiral flush trim to make it perfect. Attach the template with a little two stick tape and you are good to go.
After I have the shape printed out full size I glue this paper to a sheet of 1/4 inch MDF and cut along the line through the paper and MDF on the bandsaw. Then I fair the shape by sanding. This template is then used to guide a router with a bearing guided bit to cut the final workpiece.
I have used this technique for many shapes including elongated ovals, hearts, and serpentine curves. I'm going to cut some harp shaped chair splats this way as well.
It's really worthwhile learning CAD.
If you go to Jim Chestnut's (of Clam Clamps fame)website, he has a interesting video on drawing and cutting an ellipse with a router.Incidentally his videos are quite humorous too.
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