Marking out an oval

[Bodgy]

I want to make a large, oval table, say 1000mm x 1800.

I can't seem to think of a way to mark out the circumference of the oval top.

I don't want two straight sides with semi circles at either end.

Should I just experiment with a bit of string on a drum that winds itself up as you swing the circle/oval and becomes a progressively sharper curve?

There must be a more scientic way of doing it. Anyone tried?

[Rocker]

There is an easier, but just as primitive, way of doing it. Drive two nails into a sheet of MDF; take a length of string, and tie its ends together; place the loop of string over the two nails; then place a pencil inside the loop and draw it tight. Move the pencil round until you have drawn an ellipse. If its shape is not what you want, move one of the nails, or vary the length of the loop of string.

Rocker

[Tex B]

Bodgy,

Have you looked at an oval maker's jig? Just up your alley to build, two intersecting channels, big stick, adjustable pencil holder.

I'll try and find a link for you.

Tex

[jarrahhead]

I have been using 2 nails, a pencil, and a loop of string. Vary distance between the nails to change the shape of the oval. Simple, cheap and reasonably effective.

Recently been given directions to make an ellipse jig... search homemade jigs forum for oval cutting jig if you want something more accurrate.

[Skew ChiDAMN!!]

Two nails, a set distance apart. A length of string, each end tied to one nail. Pull the string tight with a pencil and draw your ellipse.

From memory this draws a parabaloid ellipse, but it has been a LOT of years since I did maths at school and don't really remember that much about it... except for the basic method 'cos I still use it in infrequently. [shrug]

There's an associated formula, of course, that you should be able to plug the basic dimensions of the desired oval into to predetermine the spacing of the nails and the length of the string, but any vestige of memory I had about that is long gone. Apart from the obvious: the further apart the nail spacing, the more elliptical, the longer the string, the bigger the average radius. :rolleyes: Sorry about the sparsity of info, but... :o

(Would be interested to hear if someone can work out what I'm on about though, and post the formula... I'm tired of trial'n'error myself. :D)

[Tex B]

How many of us can post at one time? Looks like we tried to find out.

Tex

[Skew ChiDAMN!!]

:D

I'd replied to... ahem... the "other" thread which had disappeared by the time I hit submit. :rolleyes: So cut'n'pasted it into this 'un. Couldn't be bothered editing to match.

[javali]

Bodgy: if you want to avoid the trial-and-error, space the nails 1500mm apart, and create a loop from a string 3300mm long. This will be close enough to the 1000mm by 1800mm you are looking for.

For the mathematically inclined: if x is the width of the required oval, and y is the length, the spacing between the nails should be S=sqrt(y*y-x*x), and the length of the string should be L=y+S

Just do not forget to verify my math. I have been known to make mistakes...

[Bodgy]

Great response, as always. Thank you

Looks like the nails have it.

Accurate is not moi, close enough is good enuff, I'm ashamed to say.

[Lignum]

Bodg, make a trammel marked with half the length of the major axis AB and half the minor axis BC. And if point C is kept on AB and point A on BC then point B will cut (bung your router on ) the elipse.

Hope ive got it right:o

[Bodgy]

Bodg, make a trammel marked with half the length of the major axis AB and half the minor axis BC. And if point C is kept on AB and point A on BC then point B will cut (bung your router on ) the elipse.

Hope ive got it right:o


Ligs

Could you expand this idea a bit please.

Two beers and just opened a Red for dinner - Chicken Tarragon meatloaf I think -couldn't quite catch yr drift.

[Lignum]

Well the blue line is the trammel. A peice of ply about 3/4 of the length of your table and wide enough to fix your router to the end.

Get a sheet of crap ply or mdf the size of the table top you are going to cut and rout two chanells (the yellow line)

Under the trammel fit two small blocks that slide snugly in the routed channels. (make sure they swivle)

To get the correct distance for those blocks, starting from the point of the router cutter ( point B ) measure half the length of the table (900mm) and that is point A. Then measure half the width of the table (500mm) thats point C, and attach your two blocks at those measurements.

If you then put those into the routed grooves, point B will travel around in an elipse.

So with router attached you can rout the perfect table top (even after two beers and a bottle of red:D )

[soundman]

There are a number of ways to mark out an "oval" well an ovoid shape realy because a proper oval is something specific.

proper oval.... two circles connected by tangents... straight sided.
compass and a ruler.
(when I say a compas I mean anything that will scribe a circle)

oval with curved sides as above but with regular curved sides.... again a compass and a ruler... scribe the two circles the desired distance apart... join the centres with a line then scribe a line thru the centre line perpendicular to it.... chose a suitable arc length and swing it between the circles
or drive in a nail at the would be tangent points of the circles.... take a long bendable straight thing and bend it against the nails pusing out till the desired arc is achieved & scribe.

parabaloid elipse as others mention

or fiddle about with combinations of the above

I'm not keen on the parabaloid for a dining table as its too pointy at the end....I also recon a true oval looks a bit..... regular.

cheers

[Bodgy]

Ligs

Got it, I'm a bit slow sometimes (and yes maybe it should have been a White with Chicken?)

Soundy

Oval, ovoid, how about egg shape?

I think with your method the intersection between the arcs at either and and the middle arcs would be noticeable, whereas the other methods seem to have a smooth transition?

You may well be correct re the shape being a little unattractive, but won't know till its marked out.

[soundman]

You could always just get a long piece of steel strapping... join it into a loop and poke & prod it till you like the shape.

cheers

[Lignum]

The one i described is the standard jig for elipses. Just about all the tables and coffee tables you see in mags are cut using this method.

Their is another way. A bloke who posted the great bandsaw cutting jig here about 6 months ago Con Anton from Frontline engineering .com .au has this remarcable ribbed plastic strip you get in 2.4 mt lengths and you bend it to the shape you want and staple it to some MDF and with a special "half" plate that attaches to your router, you then rout the shape.

I was talking to him at the recent W W Whouse open day and checked it out and i must say it is BRILLIANT He will be at the Melb woodwork show if anyone wants to check it out:)

[Tex B]

Lignum's got your answer Bodgy. Check your email. That is the typical oval drawing jig. Even saw some on flags in the city today for some reason. It will make a lovely oval, with no breaks or kinks, and no flat spots.

Tex

[Rocker]

I agree that a true ellipse is liable to be a bit too pointy at the end; but an easier way than having a complete loop, as Soundman suggests, is the following: First mark out the two axes at right angles to one another on a sheet of MDF slightly larger than the size you want your oval to be. Then use a thin lath, say 5 mm x 20 mm of hardwood to mark out one quadrant of the oval, by clamping one end where the short axis meets the edge of the sheet, and bending till you get a pleasing curve (preferably with a helper or two), and then clamping the other end where the long axis meets the edge of the sheet. Scribe the curve onto the sheet, and then make a template for the quadrant the same shape as the curve, and, using the template, mark out the other three quadrants. Obviously, to avoid sharp angles, you need to ensure that the lath is perpendicular to each axis where it crosses them. I used this method to make this table.

Rocker

[AlexS]

Check out the drawing in this thread, look at the attachment on post 12.

http://www.woodworkforums.ubeaut.com.au/showthread.php?t=34307&highlight=isometric+ellipse