# Using a rotary table to cut an approximate ellipse?

• posted

Years ago, Errol Groff posted some pictures of some packaging prototypes
he had made. The bottle top was shaped in an ellipse. Can anyone give
the procedure for fabricating an elliptically shaped object using a
rotary table, given a rectangle size that contains the ellipse?
Kevin Gallimore
• posted
I expect you probably have to break it down into 4? separate arcs and cut each one in a separate setup with the part and mill positioned the proper distance from the center of the rotary table.
• posted
I would use the "Egyptian CNC" method, it is probably the easiest.
It took me less than 1/2 hour to cut a circle just turning dials according to instructions from my perl script.
i
• posted
I was looking at the 5 centered arch graphical method, thinking that given an ellipse bounded by a rectangle, one could mathematically determine offsets and radii. My head hurt. It then occurred to me that someone in the bad old days before CNC must have done this. Hence my appeal to the collective knowledge pool.
Kevin Gallimore
• posted
Does your method leave stair steps in the result? I can't imagine being able to manually move two axes in small enough steps for a smooth result without going bonkers. With a rotary table, the four arc will be entirely stepless.
• posted
No doubt. But the object is to use the new toy (rotary table).
Kevin Gallimore
• posted
It was actually not bad, due to the fact that the endmill is round and its diameter is much bigger than the step. Think about it. Visually the circle looked very round. Check the pictures.
i
• posted
Since these are the good new post CAD and CNC days, I'd draw it up in CAD and let the CAD measurement tools tell you the radii and whatnot.
• posted
If you really want to do it mechanically without moving two axes independently, you need two rotary tables mounted one top of the other. The top is offset by half the distance between the foci of the ellipse. Then rotate the two tables at the same rate.
• posted
If you tilt the blank up at an angle a long vertical endmill will cut a mathematically correct ellipse. The major axis ends will be beveled so the thinner the stock the better it works.
jsw
• posted
...
Glad I got a CNC so I just put in the Gcode and cut.
"I THINK" if you also tilted the head, the above suggestion would give you an elipse with square sides.
Karl
• posted
P.S. You'll have to raise/lower quill as you rotate to keep tool at same depth.
• posted
You can make the edge square on one end but it will be worse on the other.
• posted
I can remember drawing an "ellipsoid" using a compass back in grade school, was part of some kind of drafting thing, Was drawn as 4 separate arcs, I remember that Left some sharp points at the ends of the major and minor axes, which could be dubbed off afterwards if you were actually cutting stuff. Don't remember much more than that. Maybe an old drafting text would have the procedure.
Or you could build an actual ellipse generator, I've seen some for cutting photo matts, too. Isn't much more than a pair of cross slots with sliders and a bar linking the two. As the bar goes around the end traces out an ellipse that depends on how closely the two sliders are linked for major and minor axis ratios I suppose it could be hooked to some sort of pantograph linkage to get smaller ellipses.
Stan
• posted
There is a simple method for DRAWING one that involves the use of two pins and a length of string, but after drawing it, you would have to manually cut it. If anybody is curious enough, I'll write it up and post it; that is, unless someone else does it first.
Flash
I can remember drawing an "ellipsoid" using a compass back in grade school, was part of some kind of drafting thing, Was drawn as 4 separate arcs, I remember that Left some sharp points at the ends of the major and minor axes, which could be dubbed off afterwards if you were actually cutting stuff. Don't remember much more than that. Maybe an old drafting text would have the procedure.
Or you could build an actual ellipse generator, I've seen some for cutting photo matts, too. Isn't much more than a pair of cross slots with sliders and a bar linking the two. As the bar goes around the end traces out an ellipse that depends on how closely the two sliders are linked for major and minor axis ratios I suppose it could be hooked to some sort of pantograph linkage to get smaller ellipses.
Stan
• posted
If you mounted a tilt table on the rotary table, you could tilt the head so one major axis end was square. After milling half the elipse, you could tilt the tilt table the other way by the same amount. This would make the other major axis end square and eliminate having to raise/lower the quill.
Dan
• posted
There is a device for a BP mill called a Volstro quill adaptor that will allow you to mill an elipse.
• posted
Indeed. Leigh at MarMachine has two of them for sale IRRC. \$1200 and humm..\$2000 for the bigger one.
Gunner
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