I’m about to make a new top for a large table. It will be 10′ X 57″. But, I want the ends to be about 1/2 of a 48″ round. How to calculate the oval ( & any other tips, as this will be the largest piece I’ve ever made)? It will be in 2 or three pieces & able to be lifted off of the, smaller, underlying table, to which it will be attached (how?). I’ve been unable to find any plans for anything similar. Any sources?
Thanks,
Morty
Replies
I don't quite understand. If the table is oval (straight sides, semicircular ends), then if the table is 57" wide, don't the ends have to be half of a 57" circle, not a 48" circle? On the other hand, if the table is an ellipse, then the curvature is constantly changing, and there is no part that is truly circular in shape.
-Steve
I misspoke. It IS an elipse, tapering (curving) down from 57" at its widest to the 42" - (1/2 of a circle).
Morty
That's not an ellipse. As I mentioned, there is no part of an ellipse that is circular. The curvature is constantly changing from point to point along the perimeter.
A true ellipse that is 10 ft long and 57 in wide has a radius of curvature at the "pointy" ends of about 13.5 in., quite small, and probably too small to be practical for a table.
So, it appears that you need a curved shape that doesn't have a simple formula. There are a lots of shapes that would probably work for you, but most of them are kind of complicated to figure out numerically. Probably the best thing to do would be to sketch things out, maybe at 1/4 scale or so, big enough to give you a good idea of what the final table will look like, but small enough to be manageable. Sketch out a shape that appeals to you, then redraw one quadrant of it at full scale.
Next, use a thin, flexible piece of wood to approximately follow the sketched out line while producing a smooth curve. The process of creating a nice-looking curve this way is called lofting by boatbuilders. I tried to find a good online resource that describes the process, but I couldn't come up with anything.
Once you've got a fair curve for the quadrant, just reflect it twice to create a complete outline of the tabletop.
-Steve
Edited 6/10/2009 11:53 pm ET by saschafer
Something like this?
View Image
YES, Dave, exactly! The question is, how to draw it to made the trasitions smooth & esthetically pleasing.
Morty
I just banged that out this morning before leaving for work. If I were drawing it full-sized, I'd draw a bit more than the half-circles at the end. Then I'd take a long, flexible batter and some weights (spline ducks would be nice) and layout the curve that joins the two half circles.As was already mentioned, make a quarter of the shape and for a template and flip and flop it to get all four quadrants done. I'd layout half so that you get a nice fair long curve.
To draw a true ellipse:
Work with a right angle that has one leg the half the length of your table and the other leg half the width of your table. You can lay this out on a piece of ply or clean cardboard that is 5' long by the half width. Starting at one corner, divide the long side into 30 parts. In this case - 60" - they would be 2" apart. A short line every 2" along the 5' side will suffice. Then, do the same on the shorter side (axis) - divide it into 30 parts. If it is 57", divide by 2 for the half width (not half wit) for 28.5" and divide that by 20 for 1.425" per part. Mark out the shorter side.
Now the fun part. Take a 5'+ long straight edge and connect the point that is one line (2") from the corner to the furthest point (28.5") on the short leg. Draw a line. Then scoot over 2" on the long leg/axis and go the second furthest point on the short leg/axis. Continue this until you have connected all the points. Wa -La, as they say, an ellipse is formed before your eyes. Or a quarter ellipse to be more accurate. You can use the quarter ellipse to lay out the half and the whole. Dave S
http://www.acornwoodworks.com
Thank you, all, for your time & ideas. We'll see how it goes. Then, of course, I have to, actually, make it! Once I get it glued up, I'll have to cut it out. I'm envisioning using a jig saw to rough cut & then a template with a router to finish it off-- none of which I've ever done, before. Wish me luck!
Morty
When doing the template routing, you must register the router base on both the top and bottom of the table for alternate quarters. Otherwise, the bit will attack the sharply exiting grain with disastrous tearout. If you've only one bit (either top bearing or btm. bearing) you can move your template to the alternate surface but registration becomes difficult.
If you want to drive yourself a little crazy check out the definition of the elipse and oval on Wikipedia.
I suggest using a cad program to draw it.
I print full size patterns on multiple sheets.
ASK
There is a kind of curve called a superellipse. (Imagine taking an ellipse, putting inside a rectangle and gradually "inflating" the ellipse so that it gets closer and closer to the enclosing rectangle.)
One of them might be suitable for your table:
View Image
-Steve
Thanks, Steve. That's helpful.
Morty
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