A perfect elipse

[Stephen Hutchins]

I need to make a elipse 56 inches by 16 inches. Does anybody know how to do it without CAD or the string and peg method?

[mmd]

Here's a method from Junior High Shop Drafting:

Draw a right-angle "L" with one leg 8" high and the other leg 28 " long. Make a tic mark every 2" along the short leg length and number them 0 (intersection of legs), 1,2,3, end. Make a tic mark every 7" along the long leg length and number them 0 (intersection of legs), 1,2,3, end. With a straight edge, draw a line ("ray") to connect short leg tic 1 to long leg endpoint, SL tic 2 to LL tic 3, SL tic 3 to LL tic 2, and SL end to LL tic 1. Spring a batten from end to end and landing on each of the ray intersections along the apparent ellipse that the rays form. This will describe a quarter-ellipe. Either make a pattern from this & trace four times to make the full ellipse, or draw the above construction four times.

Note: this is not a true ellipse, but is close enough that most won't know. It'll only be a problem if it is to match with a true ellipse, such as if this is the hole you must cut in the deck of your container ship to be able to insert the angled anchor hawsepipe.

[ 09-20-2002, 05:18 PM: Message edited by: mmd ]

[Dave Fleming]

Ho Ho MMD, so you have been there to, laying out hawse pipe openings for the shipfitter/burners.
Thruster openings are also lots of fun... ;)

[Dave Fleming]

To give an answer to the gentlemen's question here is one way.
Assuming you are starting with a rectangular piece of... whatever.
Layout the length and width at their longest and widest points.
Mark the intersection of those lines, lets call them the Major (long way) and Minor( short way) Axis.
Cut a strip of stiff paper or cardboard about 3/4 of an inch wide.
It should be a bit longer than 1/2 of that Major Axis.
Lay out two points on the strip call them A and C, their distance from each other should be 1/2 the Major Axis Length.
Using point C lay out point B, B should be 1/2 the distance of the Minor Axis from C.
Lay the strip on the piece with the two lines with point A on the Minor Axis and point B on the Major Axis.
Now point C should be the point/s of the outline of the Elipse.
Lay out a series of point C's all around the edge until you have enough to connect the 'dots'.
There you are.
Am I making sense here?
PS: you could use a nice batten for the 'tick strip' too. A strip of Formica backing sheet makes a good flexible batten for connecting the dots and tick strip too. Or so says I. ;)

[Stephen Hutchins]

Thank you Dave and mmd. I guess I'll try both methods and go with whatever looks betta.

[RGM]

Are you in a big hurry for this? Do you want to just lay it out as the other guys have decribed or would you like a jig that you can use for this particular elipse and be able to adjust it for other elipses? Perhaps build a jig that will cut the elipse with a router? Are you going to do this more than once? We've got a couple of elipse jigs at work. Is that the kind of thing that you are looking for? If so, check out the following website. www.mindspring.com/~griffinmt/Oval/default.htm (http://www.mindspring.com/~griffinmt/Oval/default.htm) , this guy describes how to make an effective elipse jig. I think he even includes a material list. Good luck.

[ 09-20-2002, 08:10 PM: Message edited by: RGM ]

[Stephen Hutchins]

RGM,
I have to build a plug for a bait-well that will fit into the aft washboard of a sport fisher. I don't plan on building anymore anytime soon, however, being the jiggy sort of guy that I am, I'd like to read about your jig. Please, do tell.

[RGM]

Stephen, re-read my previous post, I edited it to include a website that describes how to make the jig.

[Dave Fleming]

Here is a shot of a bow thruster elipse I lofted out when foreman of a new build yard down here in San Diego. The vessel in question is a 105 foot Alumunum motor yacht.

http://www.imagestation.com/picture/sraid33/pf8f86e2a1ab18a7b13d8489dcbbc2ce4/fd40d4d7.jpg

I have more shots of this use of the elipse if anybody is interested.

[Dave Fleming]

This one is a bit more self explanatory

http://www.imagestation.com/picture/sraid33/p0eb8cf781a0c9f3a2f019dee442ee792/fd40d2c5.jpg

Right hand is the stem and the bottom is...the
keel. ;)

Now as is evident, that is not a true elipse but, it comes close enough for most to get the idea.

[ 09-20-2002, 08:55 PM: Message edited by: Dave Fleming ]

[mmd]

Dave, my "trial by ellipse" came with designing the installation of thrusters and transponder nacelles in a 130-ft steel catamaran hydrographic research ship in 1985. I was about a year out of NavArch school, still exceedingly green, and not at all confident in my skills. I was soooo worried when they lit that first cutting torch....

Thankfully luck was with me and it went without a hitch. The layout skills have since been honed by doing warships, fishing boats, and most recently a few years of work on offshore oil supply boats, which are constantly being modified for new equipment. High point was being responsible for the design of a new wheelhouse, foredeck, and moonpool in a 280-ft OSV conversion. Doing "big steel" design, especially compound surfaces and odd intersections of developable plates and tubes (i.e., the curved funnel/antennae pipe mast interface) is a lot of fun, but on big boats you seem to do only one thing for a long time. Little boats are funner 'cause every day has a different challenge.

[htom]

RGM -- that's a great description of an ellipse trammel. You used to be able to buy them made from bamboo or aluminum in drafting supply houses, but never large enough to hold a router!

Stephen -- If you're willing to draw a lot of lines, you can do it very accurately. Go to the local Orange Store and get a 4'x8' sheet of something flat and cheap. You're going to use it as a large sheet of paper. Then follow the directions at http://mathforum.org/library/drmath/view/55085.html -- a neat math way, but only really practical with graph paper or a small drawing.

or
http://wwwoodcarver.com/Articles/EllipseArticle/Ellipses.html -- a more practical method, the "paper trammel"; first draw the major and minor axi and then use a marked straight edge (a paper edge on the drafting table, hence the name) as a trammel. The mark at the end marks the ellipse, the other two marks are at distances half the length of the major and minor axi.

or
http://www.usitt.org/tech.guidelines/issue21.htm
-- a discussion of five methods, including these.

[LoonyToo]

Since an ellipse is just the intersection of a cylinder with a plane, you could make an ellipse by projecting a 16 inch circle onto an inclined plane. All you need is the circle, a bright sunny day and a piece of paper-covered plywood. Mark out a box 16 by 56 and fiddle with the disk and the tilted plane until it just fits. Then mark the edges of the shadow on the paper-covered board and connect the dots.

Be sure to work fast enough that the movement of the sun doesn't mess up your ellipse. Of course, you could fold the paper into quarters, mark one quarter, cut it out, and then unfold your ellipse.

The folded paper trick might be useful if you're plotting the coordinates too.

Hal

[RGM]

Just to beat this dead horse a little more. To layout an elipse on a hull, say for a bow thruster or similar animal. Determine the diameter that you are working with, like 16 inches. Lay that out on the inside of the hull. Figure out where the center of 16" circle or pipe has to live happily ever after and drill a pilot hole all the way thru the hull, hitting the center both port and starboard. Adjust as necessary if you are slightly amiss. Drill the pilot holes out larger, say 3/8" or 1/2". Insert a long straight rod thru the new holes (same diameter as the holes). The rod needs to extend farther than the greatest dimension or length of the pipe. Fabricate a trammel that will fit snugly over the rod and be able to rotate around the rod. It will have an arm equal to the outside radius of the pipe, or whatever your installing. This "radius arm" will need a fixture on it's end that will allow a marking device to be fit to it. The marking device will need to be able to move in and out parallel to the axis rod. As you swing a "circle" around the rod with your 16" trammel and sliding marker, you slide the trammel arm in and out along the axis rod in order to keep the marking device in contact with the hull, or whatever suface you are working on. The result is an elipse marked on the surface. OK, I'm done, and I didn't use any paper or templates.

[ 09-21-2002, 12:19 AM: Message edited by: RGM ]

[mmd]

RGM, you are oh, so right. The most elegant - not to mention cost effective - method is the simplest. In my career I have been fortunate on several occasions to work with shipbuilders who really knew their stuff. It is a pleasure to work with fabricators who how to get complex things done without the need for reams of drawings and geometrical developments and sequential instructions.

However, I think that a new wrinkle that has entered the design end is the recent increase in the use of CNC-cut plating directly from the designer's files. This has put us designer guys in the precarious position of having to determine all the little details like developed cut-outs and spiling and such stuff without the access to the accrued experience of the guys such as yourself who actually build with the stuff. Several of my recent aluminum boat designs have been sent from my computer to the plasma cutter, then the cut & numbered plates are shipped to the builder for assembly. It is always a nervous time as you wonder if a stupid mistake has rendered thousands of dollars of plate useless. (Happily, I'm pitching a perfect game so far. smile.gif )

Finally, I hope that this is interesting to Stephen, 'cause it ain't related to his question (sorry, Stephen). If I recall, he's building a bait well, not a hawsehole. :D

[htom]

RGM -- Perfect. Automagicly compensates for the surface you're drawing onto, too.

[Victorious]

Why not use CAD?
go to www.rhino3d.com (http://www.rhino3d.com)
download trail copy of Rhino.
Draw/plot and print the required elipse. (you can even find print shop locally that will print to exact size)

:D

[casem]

Or why not just mark out a big grid, plot a bunch of points on the ellipse and connect the dots with a batten? I think an ellipse is x^2/a^2+y^2/b^2=1.

[htom]

You can slide two trammel points along the arms of a big steel square, too, and the other end of the trammel will sweep out the ellipse. (The trammel points are at the semi-major and semi-minor lengths from the other end, which would have a pencil, pen, rounter .... This really takes three hands to do well, though, one on each point, one holding the pencil point. There's a very old-school thing where you use four trammel points on a T-square, two on the ends of the T, holding the other two just barely off the board, which are guided by the right angled triangle, and then the pencil off toward the end doesn't "float".

[Stephen Hutchins]

Thanks for the help, everybody. I ended up downloading rhino onto a friends computer and drawing and printing it. I came up with a scale factor and will be drawing it full size tomorrow. Russel, will that program make me a table of offsets?

[Dave Fleming]

Don't think so but a fella in the PacNoWest bye the name of Cliff Estes, I heard he wrote a plugin to do that. IIRC, he is mentioned on the Rhino web site.

[On Vacation]

Gee weeze guys and Dave, take a five gallon pail and trace it onto the surface and cut it out. Nothing to it. if you need it bigger, take duct tape and wrap around till you get the desired hole. Or then again, take a piece of plywood one inch bigger than the hole and find the center with a sheetrock screw and then draw the opening by placing a pencil scribe to half the size. tongue.gif

[pjwalsh]

MMD - your high school shop method is actually a parabolic envelope. I saw this in an old book on lofting aircraft, was intrigued and figured out the arithmetic that it describes (or describe it). It is a parametric equation for a parabola. If you force the ends of the batten to be tangent to your L sides then it does approximate an ellipse only a bit flatter at the corners.

You can lay out a true ellipse by drawing two circles from the same origin, one the major diameter, one the minor diameter of your desired ellipse. Draw diagonal lines or rays radiating from the origin at even angle intervals. draw a horizontal line from the intersection of each ray and the minor circle and a vertical line fron the intersection of each ray and the major circle. the intersection of the horizontal and vertical lines for each ray is a point on the perimeter of the ellipse. draw as many points as you need and use a batten to string them together.

Rhino is cool, and useful but give me a clean lofting surface, pencil, squares and batten any day - it is just good for you soul.