bisecting an oval

[sawduster]

i have a 20" x 30" oval drawn on a piece of cardboard. I need to find the center and then bisect it into four equal pieces ( or vice versa ) . I'm sure there is an easy way to do this. i have a framing square, a measuring tape and a few other odd tools ....no compass that large without making one
How do I do it simply and with as little math as possible ? :mrgreen:

eventually this will be a wooden template but for now I need the pieces manageable

Thanx for any and all input :icon_thum

 

[Mike Davis]

How did you draw the oval? Or did you trace it?

If you construct the oval with string you will have the two pin points, find halfway between them and make a right angle there. Without some reference points I wouldn't know where to start.

 

[sawduster]

traced it ....that is the problem

 

[Mike Davis]

Well, you can get close by finding the longest axis, double and triple check by measuring to the line across the short axis. One you have that long line you can cut it in half and make a 90 there.

 

[nelsone]

Is it on a board or on paper? If on a board, get some tracing paper and trace the oval. You should be able to fold the paper so both sides match and the crease is your mid point.

 

[Mt. Gomer]

Is it on a board or on paper? If on a board, get some tracing paper and trace the oval. You should be able to fold the paper so both sides match and the crease is your mid point.

Yeah, that's what I was thinking too....

 

[sawduster]

It's on thin cardboard currently . Eventually it will be a template for cutting mirrors . My first thought was the folding thing cause I want a centerline so I can cut that to transfer and cut 1/2 at a time on the scrollsaw in 1/4" BB
I seriously doubt I could trace it accurately enough but I think it may be my only option :dontknow:

...gotta find paper big enough as well

 

[walnutjerry]

You stated you have a 20 x 30 oval-------------draw it out from scratch using the string method. Not much math involved, just geometry.

Jerry

 

[sawduster]

can you give me a 101 on the string method or the best place to look it up ?

...heck man I can't even spell [STRIKE]goematree[/STRIKE] [STRIKE]geeomtary[/STRIKE] [STRIKE]geeometrie[/STRIKE]

....phooey :BangHead:

 

[Gotcha6]

For a 20 x 30 oval, follow closely:

1. Draw 2 perpendicular lines bisecting each other 30" w x 20" long. You should end up with 2 segments 15" & 2 segments 10" @ right angles to each other.
2. From the extreme ends of the 10" segments make 2 diagonal lines 15" long down (or up) to the 15" lines.
3. Place 2 nails at these 2 points.
4. Attach a non stretching string to each nail (I prefer gift wrap ribbon) and make it long enough that the string will touch the end of the 10" line.
5. Start drawing.

 

[Tarhead]

Redraw the oval on some light paper and tape it to a big window on a sunny day. Cut a 20 X 30 rectangle out of the same paper and draw out dividing lines in the direction you need the oval divided. Lay it over your oval, lining up the edges of the rectangle with the oval. Poke a hole through the intersecting points. Remove the rectangle and connect the dots.

 

[walnutjerry]

Looks like you already have 2 replys as to "how" so I will not confuse the issue with another.

My wording would not be the same but after reading the other two---------I think they will work. Go to this link to see it.

http://www.blocklayer.com/oval.aspx

Jerry

 

[sawduster]

I think I ( finally ) got it ! Thanx guys , your input was invaluable :notworthy:

Does this site rock or what ? :headbang:

 

[walnutjerry]

Glad it helped, Robert.

I always used the length of the minor axis to mark the P1 and P2 on the major axis when I had to fit a given size. I don't know how the author came up with his length for that ----I just don't have that many smarts.

Jerry

 

[nelsone]

Glad you got it figured out. Can't wait to see the finished product!

 

[sushinutnc]

http://www.blocklayer.com/oval.aspx

Jerry

FWIW, that site won't give an accurate oval. For some reason, they left off some significant decimal places there.... Using 20 x 30 as the example, line segment C-P2 shows as 11. That should really be about 11.18. If using inches, it'd be off by about 3/16". If you set up your string to start at point E (the height), you'd wind up with an oval that was only 29-3/4" wide. Might be ok, if you're cutting your own mirrors... not so good, if you are cutting a wooden frame for a 20" x 30" mirror. :thumbs_do

Dennis' method is the "normal" way of drawing ovals with the string method. Works fine, like has been said, AS LONG AS you don't have any stretch to the string AND can deal with the end of your pencil trying to stay attached to a string.

This is reason #438 I use a computer for all my designs.

 

[walnutjerry]

Thanks for the information Mike---------When I was using the oval thing was when I was forming ash hoppers and had to form around eliptical openings. I only measured the two axis. Then drew them to bisect each other at center and used the minor axis to get P1 and P2 then allowed for the thickness of the covering on the form when I attached the pencil to the wire, usually 1/4" plywood. Yes-----tie wire works pretty good, and NO stretch at all. I never tried to figure a different length to use for marking P1 and P2. It worked for me.

Jerry