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CAD tip CAD tip # 5757:

Question CAD 
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Q - question

How to draw a 3D arc or circle?

A - answer Planar (2D) entities can be drawn in 3D space using a temporary user coordinate system (UCS).

If you want to draw quickly an arc or circle from 3 given points in 3D, you can use the following LISP code (save it to a .LSP file and load with APPLOAD):

(defun c:3Darc (/ p1 p2 p3)
 (command "_ucs" "_w")
 (setq p1 (getpoint "Arc point #1: "))
 (setq p2 (getpoint "Arc point #2: "))
 (setq p3 (getpoint "Arc point #3: "))
 (command "_ucs" "_3p" p1 p2 p3)
 (command "_arc" (trans p1 0 1)(trans p2 0 1)(trans p3 0 1))
 (command "_ucs" "_p")
(defun c:3Dcircle (/ p1 p2 p3)
 (command "_ucs" "_w")
 (setq p1 (getpoint "Circle point #1: "))
 (setq p2 (getpoint "Circle point #2: "))
 (setq p3 (getpoint "Circle point #3: "))
 (command "_ucs" "_3p" p1 p2 p3)
 (command "_circle" "_3p" (trans p1 0 1)(trans p2 0 1)(trans p3 0 1))
 (command "_ucs" "_p")
100% *  CAD 
20.9.2007    26582x  
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applies to: AutoCAD ·

See also:
Tip 10249:Osnap mode Geometric Center in AutoCAD 2016.
Tip 10237:Arc bridges (gaps) in line intersections.
Tip 10138:Dimensioning a curve length or distance between two points on a curve.
Tip 10054:Boolean operations on 2D polylines.
Tip 7800:How to model a 3D hyperboloid?

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