How to Draw an Oval

      Pros share methods for laying out elliptical shapes. May 4, 2005

Question
I have been looking for instructions on drawing an oval using two points and a string. What is the rule of thumb to position the pivot nails to get the size oval you want?

Forum Responses
(Architectural Woodworking Forum)
From contributor P:
There are formulas for drawing an ellipse. A little difficult to get it all straight in your mind. Maybe you really don't want to know what they are. But I do have a free spreadsheet to download at this website. You enter in the semi-major axis and semi-minor axis, and you will get the distance between the foci, and the length of the string needed, to draw the ellipse you are after. There are also some drawings to give you an idea of making a router jig, if you want to go that route.

Spreadsheet Calculation Program



From contributor K:
Draw two lines which bisect each other at right angles. Measure out from the center line and establish a point that is half of the short diameter. From this point, strike an arc which is half of the long diameter, where it crosses the other line, or perpendicular axis. This is the placement for the pins. The length of the string is the long diameter, which is the same as the hypotenuse of the two congruent back to back triangles described by these steps.

I hope this answers your question. I would not use this method for actually making patterns, but only for quick and dirty drawings. Also use only very thin low stretch line like spider wire fishing line.



From contributor R:
I sometimes need to draw an oval that has to fit a particular opening that a mechanically drawn oval doesn't work with. I draw bisecting lines on tracing paper, mark the horizontal and vertical extremes, then freehand sketch the oval. I then choose the quadrant that looks best and best fits the needed space. Next I fold the tracing paper in half along one axis and trace the selected quadrant. Then I fold along the other axis and finish the tracing


From contributor D:
I learned to do it with string in the shop, but now I just use ACAD and print full size.


From contributor L:
To draw an oval to a precise size using the method of a loop of string as already explained by the other contributors... I have found that there is a simple calculation of how far apart the two pins on the base line need to be placed. This is as follows: (The Square root) of ((Width squared) less (Height squared)) = the base distance between the two pins. For example, if the desired Width of the oval is 12, and the Height is 8, the calculations are: 12 squared = 144 less, 8 squared being 64 = 80. The square root of 80 is 8.944272, which is the width of the required base line.


From contributor A:
E

A P C P B

D

Steps to drawing an Oval
1. Get exact center of your desired oval and draw vertical and horizontal lines. Mark C.
2. Measure Width (AB) of desired oval size. Center this on your board.
3. Measure Height (DE) of desired oval size. Center height on board.
4. Calculate Ĺ of the distance from AB(=AC or BC). This will be the length of your hypotenuse (the diagonal line PE).
5. From point E with a ruler, find the spot on AB where PE intersects it with a length of AC.
6. Repeat for the other side, so you now have 2 point Pís along the horizontal axis AB.
7. Nail holes at P, E, and the other P.
8. Take non-stretchable string and connect it P E P, typing the string together for a triangle tightly.
9. Take a Pencil and pull out E nail hole. Replace the E hole with a pencil inside the string.
10. With the string as your guide, move around the oval. The max size of the string will create a smooth line as you finish drawing it out.
11. You will have to maneuver around the nails when you get to point A and B.
12. You now have a perfect oval, cut out along the newly drawn outside perimeter.



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor B:
The large radius is the hypotenuse and the short radius is the adjacent side and the distance from the center point on the long diameter to the pin is the opposite side. To determine this, take the square root of the large radius squared minus the short radius squared. This is the distance from the center to the pin. The pins are equidistant from the center of the large radius. The string length is large radius times two plus the total pin distance. With the string around the two pins and a pencil stretching the string draw an oval of the size needed.



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