Autocad create 3d arc.

[craigdughan]

Hi, would like to know if it's possible to create a 3d arc using AutoCAD 2021 (not civil 3d). 

Work as setting out engineer and spend lots of time working out different elevations on radius kerbs & wondered if there is an easier way. The way i do it's a straightforward calculation (arc/segments then manually enter the elevation for each point) but it's time consuming. Recently figured out there is such a thing as 3d polyline and wondered if this could be used as an alternative? 

Look forward to any input/replies. 

[Kent1Cooper]

One variety of what one might call a 3D Arc is a HELIX.  Can that do what you want?

[Kent1Cooper]

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@craigdughan wrote:

.... AutoCAD 2021 (not 3d). .... 

---

[By the way, AutoCAD is a 3D program, whether or not you use the 3rd dimension in it.  There's no separate variety to work in 3D.  If you meant "not Civil 3D," that could be relevant, since that has specialized functions that may do what you're after.]

[cadffm]

Hi,

>>" to create a 3d arc using AutoCAD 2021 (not 3d)"

What is a "3D arc"?

An arc rotated in 3D world is still an arc.

And what is "AutoCAD 2021 (not 3d)"?

There is no 3D xor not-3D version.

DWG/DXF file format is 3D, you can create an arc in every position you want with all AutoCAD/LT/Toolsets.

What Product/version so you really have? Please share a screenshot of dialog, command: ABOUT

>>" Recently figured out there is such a thing as 3d polyline and wondered if this could be used as an alternative? "

That's something you have to know that.

But it sounds like YES!!

>>" The way i do it's a straightforward calculation "

AutoCAD can do calculation too.

_

Share a sample (DWG and how you calculate it until now)

BTW: Isn't CIVIL3D the best tool for your tasks?

[cadffm]

_

Share a sample (DWG and how you calculate it until now)

BTW: Isn't CIVIL3D the best tool for your tasks?

[craigdughan]

Hi Kent, Thank you for the speedy reply.  Using AutoCAD not civil 3d. 

[richard_387]

Usually, a radius kerb is calculated in 2D format.

Rotating an arc into 3D might produce some interesting shapes!

[craigdughan]

Hi Cadffm, 

Hi,

>>" to create a 3d arc using AutoCAD 2021 (not 3d)"

What is a "3D arc"?

Apologies, think i'm getting mixed up. Unsure how to  to explain it correctly. But here goes.. 

An arc/radius with an elevation (z) attached at the start middle and end. So it would be possible to click anywhere on the arc and AutoCAD would do the maths to figure out the z value. 

An arc rotated in 3D world is still an arc.

And what is "AutoCAD 2021 (not 3d)"?

There is no 3D xor not-3D version.

DWG/DXF file format is 3D, you can create an arc in every position you want with all AutoCAD/LT/Toolsets.

What Product/version so you really have? Please share a screenshot of dialog, command: ABOUT

AutoCAD 2021 op 3d was referring to civil 3d. 

>>" Recently figured out there is such a thing as 3d polyline and wondered if this could be used as an alternative? "

That's something you have to know that.

But it sounds like YES!!

>>" The way i do it's a straightforward calculation "

AutoCAD can do calculation too.

_

Share a sample (DWG and how you calculate it until now)

BTW: Isn't CIVIL3D the best tool for your tasks?

Possibly,  trying to figure that out. 

[craigdughan]

Hi Kent, 

Its something will look into. Thanks. 

[cadffm]

>"AutoCAD 2021 op 3d was referring to civil 3d. "

Thx for explaination, so we talking about plain AutoCAD

>>"So it would be possible to click anywhere on the arc and AutoCAD would do the maths to figure out the z value."

Command ID will always report the Z-Value of each point you are click to.

IF you drawed the arc at the correct position (so the different Z-Value).. not a problem.

But I thing I understood wrongly.

Sorry

[leeminardi]

Please clarify, by "3d arc" you mean 

1. A true arc but it cannot be assumed to be parallel to the XY plane.

or 

2. A curve which appears to be an arc in the top (plan) view but in fact is not on the XY plane and therefore is really an ellipse.

When defining road kerbing, is the specified radius true to the arc (#1) or as it appears in the plan view drawing (#2)?

What is the typical data that you have that details the "3D arc"?  E.g., 3 points along the arc, end point and center point, , 2 points a radius and additional information defining the plane of the arc, or something else?

A drawing of how the arc is specified would be helpful.  

You can use the divide command to create a bunch of points along the arc once yo have it.

[Kent1Cooper]

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@craigdughan wrote:

Its something will look into. Thanks. 

---

A Helix with equal bottom and top radii will be a true arc in plan view, with different Z coordinates at the ends, but you won't be able to specify a Z coordinate at the midpoint -- that will necessarily be halfway between those at the ends, which may not always match what you want.  But you could make two Helixes [or should that be Helices?], one from an end to your midpoint, and another from there to the other end.

A Helix is not the same as a tilted Arc, which as already pointed out would be elliptical in plan view.

Once drawn, you can use ID and NEArest Osnap to find the Z coordinate at any location along the Helix.

A 3D Polyline can be made of only line segments, so I expect that's not going to serve your purpose, unless you can use one that "looks" curved but is actually made of lots of short line segments.

[leeminardi]

The following code assumes that you want an arc in 3D space (and not an ellipse).   Given 2 arc end point in 3D and an arc radius it determines the arc's center point as the average z cooridnate of the endpoints. 

(defun c:ArcIn3D (/ p1 p2 r pm a b u du uper p3 nrm pz
		    p1t p2t p3t)
; creates an arc given the two end pointS and a radius.
; the center of the arc will be set to a value midway between
; the z valueS of the two ends.  The arc will lie on a plane
; defined by the two endpoints and the arc center.
; The solution provided depends on the order of specifying
; the two endpoints via CCW.
;
  (command "_ucs" "w")
  (setq	p1   (getpoint "\nEnter 1st arc endpoint.")
	p2   (getpoint "\nEnter 2nd arc enpoint.")
	r    (getreal "\nEnter arc radius: ")
	pm   (mapcar '/ (mapcar '+ p1 p2) '(2. 2. 2.)) ; midpoint
	a    (/ (distance p1 p2) 2.)
	b    (sqrt (- (expt r 2) (expt a 2)))
	u    (list
	       (- (car p2) (car p1))
	       (- (cadr p2) (cadr p1))
	       0.0
	     )
	du   (distance '(0 0 0) u)
	u    (mapcar '/ u (list du du du)) ; unit vector from p1 to p2
	uper (list  (cadr u) (* -1. (car u)) 0.0)
	uper (mapcar '* uper '(-1 -1 -1))
					; unit vector perpendicular to u
	p3   (mapcar '+ pm (mapcar '* uper (list b b b)))
					; center of arc
  )
  (setq nrm (cross (mapcar '- p1 p3) (mapcar '- p2 p3)))
  (setq pz (mapcar '+ p3 nrm))
  (command "_ucs" "za" p3 pz)		; ucs for arc
  (setq	p1t (trans p1 0 1)
	p2t (trans p2 0 1)
	p3t (trans p3 0 1)
  )
  (princ "\np2t = ")
  (princ p2t)
  (command "_arc" "c" p3t p1t p2t)
  (command "_ucs" "w" "")
  (princ)
)


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Compute the cross product of 2 vectors a and b
(defun cross (a b / crs)
  (setq	crs (list
	      (- (* (nth 1 a) (nth 2 b))
		 (* (nth 1 b) (nth 2 a))
	      )
	      (- (* (nth 0 b) (nth 2 a))
		 (* (nth 0 a) (nth 2 b))
	      )
	      (- (* (nth 0 a) (nth 1 b))
		 (* (nth 0 b) (nth 1 a))
	      )
	    )				;end list
  )					;end setq c
)