I don't know what you mean by a "true, mathematically accurate eccentric".
An eccentric circle simply means off centre, so you can just translate it
relative to the axis of rotation.

If you mean an ellipse then that is mathematically the same as a circle
scaled on one axis. E.g.

module ellipse(xr, yr) scale([1, yr / xr]) circle(xr);

On Wed, 28 Apr 2021 at 03:46, Ron Wheeler via Discuss <
discuss@lists.openscad.org> wrote:

On Wednesday 28 April 2021 03:10:06 nop head wrote:

I'm not Ron Wheeler, and I show no msg from him that you are replying to.
Is this list coupled?

That looks as if it should do it except that will make a single lobe (I
think but what do I know, my algebra teacher was far more interested in
getting the girls to the wet panties stage with his off color jokes than
in teaching the class algrebra, one of the reasons I quit school in the
middle of my freshman year and went to work fixing tv's for a living
in '48, ending up switching to broadcasting in '62, with a sign on my
saying Chief Engineer for most of the last 30 years of my working life.
I am also a C.E.T.) it needs to be like the usual woodworkers ellipse
layout tool, consisting of 2 pins and a string loop to fix the reach,
the string is dropped over the two pins, and a pencil is then used to
make the outline mark by using the string as a "thats as far as I can
reach" and stay inside the string. I want to do that in openscad so it
can be scaled and printed on a 3d printer. In this particular case an
Ender 5 Plus. At about 100mm in diameter, so the ellipse will have two
lobes. With about a 6 to 7mm total diff between x and y.

A hint about $fn here, I could see and feel in the movements, the facets
of the circle at an $fn=360, so I stepped it up to $fn=1440, which then
made a 50 meg stl it takes cura several minutes to process. But the
result is noticably smoother motion.

Thinking about that, and realizing as I watch the printer move, that I do
not have the electrical cableing routed correctly which is putting an
extra bind on the bowden tube, but the assembled printer is too heavy
for my back to move around easily, it occurs to me that while I am
printing bearings, I may as well do one as big as the Ender 5 Plus's
glass, 13" plus, and use it for a Lazy Suzan bearing in a 30" diameter
table top, with a quad of casters at the top of the 24" legs to serve as
weight bearing helpers for the lazy suzan bearing.  So thats on the
agenda too, but with bb's for bearings in such a beast, I'll have to get
another jar of them. Since I have another large but bed slinger printer
awaiting parts to repair its burned up controller board, I may as well
make 2 tables since the computer has more usb ports, it may as well
drive both printers. It will take this printer several hours to build a
L.S. bearing that big, but it will still be way cheaper than buying a
stamped steel one that big. Humm, with the casters bearing the weight, I
can use a broom handle dowel pin in the center for the locator pivot.
Even cheaper.

But the need for the ellipse continues. I'm making the bearings circular,
then insert the ellipse made on a mill from alu, to make it
non-circular. I have one made, but with the bearing groove too small, so
its a bit tight when filled with bb's and I can stretch it plenty far
enough to do this with only a couple ounces pull with my thumbs.

It also crosses my mind to go back to the external facing spline gear,
make its ID about 2mm smaller and inlay in that 2mm a pair of bearing
grooves effectively making it the bearings external race and removing
any possibility of slippage wear at that point.

As you can see, my mind, whats left of it, thinks better while typeing.

Thanks everybody.

Fugly stuff but attached. Comments welcome.

Cheers, Gene Heskett

"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.

• Louis D. Brandeis
  Genes Web page http://geneslinuxbox.net:6309/gene

Gene, I’m afraid it’s still far from clear (to me, at least) what exactly you’re trying make. Ellipses don’t have “lobes”. Perhaps you could post a hand-drawn sketch of what you want to make?

Gene Heskett wrote:

Gene:

Can you make the shape you want by using hull() on two circles or
cylinders of different diameters spaced a bit apart?

On 4/29/2021 7:59 AM, doug@milmac.com wrote:

It seems to me that Gene is describing an ellipse:

layout tool, consisting of 2 pins and a string loop to fix the reach,
the string is dropped over the two pins, and a pencil is then used to
make the outline mark by using the string as a "thats as far as I can
reach" and stay inside the string.

A video of this technique: https://www.youtube.com/watch?v=Et3OdzEGX_w

In which case the answer has already been posted by Nop Head (module
ellipse(xr, yr) scale([1, yr / xr]) circle(xr); ), and any confusion about
lobes or asymmetry is just that.

On Thu, 29 Apr 2021 at 13:05, jon jon@jonbondy.com wrote:

// 3 "eccentric"s

module disc(r) { cylinder(r=r,h=2) ; }
module rod() cylinder(h=4,r=2) ;
translate([60,0,0]) {// A circle is a circle. Can't be anything else
// if you rotate elsewhere than its centre, then it's motion is eccentric
rotate(360*$t) translate([5,0,0]) disc(20);
rod() ;
}

translate([-60,0,0]) {
// An elongated circle is mathematically the same as an ellipse
// if you rotate it about its center it has two "lobes" in Genes parlance
translate([0,40,0]) {
rotate(360*$t) scale([1.2,1,1]) disc(20); // ellipse 40x48
rod() ;
}
// you can rotate it about one of the foci, in which case it has one "lobe".
focus = (sqrt(201.2201.2-2020)); // distance of focus from center
translate([0,-40,0]) {
rotate(360*$t) translate([focus,0,0]) scale([1.2,1,1]) disc(20);
rod() ;
}
}
// a "hull" round two circles (with plenty of overlap) is a traditional
"cam" as in camshaft
rotate(360*$t) hull() {disc(20); translate([0,18,0]) disc(8);}
rod() ;

Best viewed as an animation, f.eks. FPS 5, steps 25
[image: image.png]

On Thu, 29 Apr 2021 at 14:29, Will Hardiman bill.hardiman1995@gmail.com
wrote:

On Thursday 29 April 2021 08:05:34 jon wrote:

Close, but no cigar, Jon. See the drawings to the right of the hull
discussion in the docs.  That leaves a straight line top and bottom the
same length as the center separation in the example shown, and that is
not an elliptical curve.

Take a 4" circle made out of rubber, pin the left edge at its ID then
pull it out of round by a by stretching over another pin 3/16" larger
than the circles diameter. This will be a pretty close approximation of
what I want. This is a spline made out of triangles, running inside an
identical spline with the triangles rotated 180 degrees. The tips of the
elipse forces the splines to be fully engaged at 180 dehree intervals.
while the pullin at 90 degrees is just enough to make the splines clear
each other as the tips of the splines pass each other because there 2
more splines on the outer ring facing in, than on the inner ring facing
out.

Thanks Jon, you made me look it up to see if I miss-interpreted it.

Cheers, Gene Heskett

"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.

• Louis D. Brandeis
  Genes Web page http://geneslinuxbox.net:6309/gene

On Thursday 29 April 2021 08:29:03 Will Hardiman wrote:

I believe thats correct, however ellipse is not listed in the cheat
sheet, so where do I find its docs?

searching the net I found the MCAD-master.zip on gh, downloaded the zip
and unpacked as a subdir into the path openscad was looking in for its
library page. But I do not see a load library anyplace in the editors
menu's.  Can someone enlighten me how this is done for the linux
AppImage 2021.01 (git 41f58fe) ?

Thank you.

Cheers, Gene Heskett

"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.

• Louis D. Brandeis
  Genes Web page http://geneslinuxbox.net:6309/gene

http://forum.openscad.org/Drawing-Ellipses-td536.html

If you google "OpenSCAD elipse" you will find a lot of references.

I think that you are making this a lot harder than it really is.

In what way is an ellipse different from a circle scaled by different
amounts on each axis?

Free advice: Pay a lot of attention to nop head. I don't know him but
from watching the forum, there are not many things about OpenSCAD that
he does not know. If he says "try it" then you should try it.

On 2021-04-29 12:43 p.m., Gene Heskett wrote:

--
Ron Wheeler
Artifact Software
438-345-3369
rwheeler@artifact-software.com