Posted by dxli on
URL: https://forum.librecad.org/Isometric-Circles-tp5711165p5711177.html

Hi,

I didn't read carefully enough.  it's about circles in isometric.

You can use ellipse ( Center and two points, the first method in Draw -> Ellipse) together with command line (calculator) to get inscribed ellipse.

The basic idea is that: the projection to isometric does not change scale ratios in the same direction ( say, along a line before and after projection).

I will guide you through an example:

0, Edit -> Current Drawing Preferences-> Grid : choose isometric
1, start a new drawing;
2, snap to grid only in the snap toolbar, no restriction
3, Draw->line -> two points;
4, use command line (Ctl-M to activate or just click on the command line) to draw a square contour (square in 3D), i.e. a rhombus. (0, 0) , (200/sqrt(3), -50), (400/sqrt(3), 0) , (200/sqrt(3), 50). or (0, 0), (100, 0), (100, 100), (0, 100) in 3D;
4, Draw -> Ellipse -> Center and two points ;
5, use command for center: ( 200/sqrt(3), 0), i.e., the rhombus center;
6, set the end of major axis: which is 1/sqrt(2) to the rhombus vertex (400/sqrt(3), 0) which is relative to ellipse center ( 200/sqrt(3), 0). therefore, in command type in relative coordinates: @200/sqrt(3)/sqrt(2), 0
7, type in coordinates for the end of minor axis: again , 1/sqrt(2) factor relative to vertex ( 200/sqrt(3), -50). type in relative coordinates to ellipse center: @0, 50/sqrt(2)

Since the ellipse orientation and its major/minor radii are just 1/sqrt(2) of known values, the construction it quite straight forward.

isometric_circle.dxf

earthcaretech wrote

Mike, thank you for this.

I followed steps 1 - 3 rigorously, selected the "Draw Ellipse Inscribed in a Quadrilateral" tool but could only select 3 lines, not the fourth.

Am I missing something? An ellipse appears after clicking on the third line, and the command line shows "Specify the fourth line" but it won't select.