3d-3 point arc 2

If you want to draw a 3D 3 point arc, first align the UCS with the plane passing through the 3 points. If you want to draw a helix, then you can draw a 3D polyline. However, it is unfortunate the you cannot extrude along a 3d polyline, such as to draw a spring.

I am away from my house for another month but for the sake of asking, can a 3d polyline be incorporated into a solid and then modified using the various solid modelling commands?

1) Draw the arc in the x-y plane and use the command rotate3d to position it correctly.<br>
2) Draw a 3D line from the start point to the end point of your arc. Use object ucs and pick the line. Change the view using plan, currrent ucs. Draw your 3 point arc.

The first suggestion - to align the UCS with the three points and then draw the arc - is the simplest and quickest method. It gives you an arc that has a consistent radius (not elliptical) and is easy to manipulate (it can be trimmed etc).<br>
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The second message regarding a helix however, describes the situation that often drives one to make a 3d curve. A helix can be modelled using AutoCAD. The trick is in creating a group of nodes to use with a 3d polyline. Once a 3d poly line is used to &quot;connect the dots&quot;, the solid&gt;extrude&gt;path command can be used to make a solid helix, like a spring. Making this a &quot;true&quot; curve may not be possible, I do not think AutoCAD will extrude along a spline path. It will extrude along a 3d polyline however. (the distinction to make here is that this method creates a &quot;helix like&quot; object, one that consists of a group of straight line segments, not a true curved object.)<br>
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Creating a group of nodes in 3d space can be done using a spreadsheet. Simply make a spreadsheet that calculates the x,y,z coordinates for each node. Import the coordinates using a script file. Once imported, create a 3d polyline that passes through each node. Draw a circle perpendicular to the node at one end. Extrude along a path and select the 3d polyline for the path. A helical solid will be created.

MikeVV gave a very good answer, I just have one thing to add to it.<br><br>Before you extrude along the 3D polyline path you can 'fit' the poly line (curve, cubical, or quadratic fit).&nbsp;&nbsp;Doing this will make the polyline look more like a spline (experiment with the type of fit and number of points used in the polyline to get the results you want - eg: you could draw a 'circle' with the polyline command using say 20 segments, but you may get as good or better results using 6 segments and then using a curve fit).&nbsp;&nbsp;One word of caution - 'fitting' the polyline essentially adds more points to it, which in turn adds to the time needed for AutoCAD to do calculations with it.

Thanx, I'm trying to draw a compound spring for a shock absorber. I'm much obliged. I did experiment with rotating sections of a torus to obtain a smoother 3d spring section, it wasn't completely successful but it's an approach that other people may find useful for other applications. I'm glad I'm kind of toying with a personal project, or I'd be way past the deadline. HA!! HA!!