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Logic of Sweep Base point

I am trying to figure out the logic behind the base point option of the sweep command. In my case I am attempting to create the thread of an auger. I initially tried to put the profile at the end point of the helix, to indicate the base point. But, in my V14, that has no effect on the default base point. Though, in more recent testing, I find that I can get predictable results by moving the profile to the end of the helix, turning ALIGN off, and not use the base point option.

My earlier attempts involved keeping the profile off to the side, and setting Align to On, and then using the BASE option, and choosing what I want to be the base... but it put the base point on the inside of the helix. Next I mirrored the profile, and chose the same base point as before, and this time it worked. But, the trial-and-error approach is taking a lot of time. The user manual does not address this topic at all, other than to say, "Defines the point on the sweep entity that follows the path."

So far, after a few hours of effort, I have been unable to figure out the logic that the software is following. Can anyone explain?



  • Thank you for the link to the explanation, though I still need to do some trial-and-error to settle things in my head. That is the nature of describing visual things using words.

    One thing I would like to clarify. Reading about surface normals on Autodesk's web site, I see that the normal of a 3D face follows the "right hand rule". This would be done by curling the fingers of your right hand in the direction of the point order, and seeing that your thumb points in the direction of the surface normal. Is that correct about closed poly lines as well?


  • The normal of a polyline does not depend on the order of its point. It is a separate property.

  • Is that correct about closed poly lines as well?

    No clue.
    (I think not, same as Roy)

    Face Normals are important for "3D Faces" only.
    I think a Polyline itself (in ACAD) has no "Fill" and is no surface or Face
    and has a own setting for its direction.

    If you create a Region from a Polyline, I think it gets a ACIS "3D" object
    and Face Normal doesn't matter so far.
    And if you convert Solids to Meshes, it is the converter who cares about
    setting all Face Normals to point outwards.
    But if you draw a 3D Face, I think the drawing direction will matter
    for the definition of the Face Normal.

    Standard 3D Apps that work Mesh base see 3D Polygons only from one side.
    So it is important that Face Normals are aligned correctly.
    Otherwise you can view through those Faces.
    An example would be a Cube Mesh for an Interior Scene.
    Flip its Face Normals to see the Space from inside, but you can still look into
    the cube when you put your camera outside of the cube.

  • Most entities in DwgCAD have a normal. This normal defines the Object Coordinate System of the entity. It should not be interpreted as a 'face normal'.

  • After some experimentation with sweeps, I found the following;

    I confirmed that, unlike 3D faces, the sequence of the points of a polyline has no bearing on the direction of its normal.
    1. The "normal" of a polyline is the Z-direction of the UCS in existence when it was created.
    2. The centroid of the closed polyline is the default base for the sweep. (not in the v14 user help file under "sweep", but this is described in the "What's new" area.)
    3. The X direction of the profile is set to be the same direction as the X direction in effect when the 2D polyline profile was created.
    4. If you use a 3D polyline for the path, there is no apparent orientation, and it seems random. (though I suspect there is a logic to it that is not apparent, nor described in the manual under "sweep".)


  • @Joe Dunfee:
    As far as I know what you say under #3 is not correct.

  • How is the rotation oriented then? At least in my simple test, the rule #3 worked. Though, when I later tried to do a sweep on a helix, I could not figure out any of the rules. I just tried different random things until it looked right.


  • edited December 2019

    AFAIK the X axis of the OCS is used. Since the OCS is stored as a single vector, representing the direction of the +Z axis, the other axes have to be calculated using a certain algorithm. Only in very specific cases will the X axis of the UCS that was current when a polyline was created have the same direction as the X axis of its OCS.

    One of the obvious cases is if the current UCS is the WCS. This is the reason for my advice to create the profile entity in the XY plane of the WCS.

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