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Why doesn't "Point on plane" relation exist in 2D sketch?

Question asked by Robert Silesius on Oct 31, 2018
Latest reply on Oct 31, 2018 by Sergio Monti

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I believe that it's a mathematical truth that two planes which aren't parallel will always meet in a line.

So why can't you add the relation "point on plane" in a 2D sketch when you select a non-parallel plane? (This relation is only allowed in 3D sketches in 2017.)

I know a workaround is creating an axis first between the sketch plane and the relation plane, but it feels as a glaring omission on Dassault's part.

EDIT: Apparently picture explanations are necessary. Neither Plane1 nor Plane2 in this above image is in any way parallel to any of the default planes (nor each other), but they do intersect:

2D sketch. No relations allowed.

3D sketch, relation is allowed.

Workaround: 2D sketch, relation allowed to axis created from Plane1 and Plane2.

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Glenn Schroeder Oct 31, 2018 7:39 AM
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I don't understand. You can create a coincident relation between a point and a plane in a 2d sketch. Doesn't that do the same thing?
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- Robert Silesius @ Glenn Schroeder on Oct 31, 2018 8:53 AM
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  Look at my edit.
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Rob Edwards Oct 31, 2018 8:12 AM
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Yep, I agree it would be nice. Also I just tried and you can't make a construction line co-linear to the plane (in lieu of the axis)

edited:
I'd probably just use a 3d sketch if on balance the convenience outweighed any limitations
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- Glenn Schroeder @ Rob Edwards on Oct 31, 2018 8:12 AM
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  Rob Edwards wrote:
  
  Yep, I agree it would be nice. Also I just tried and you can't make a construction line co-linear to the plane (in lieu of the axis)
  
  Rob,
  
  I can make a construction line colinear with a plane. I don't know why you don't get the option.
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  - Rob Edwards @ Glenn Schroeder on Oct 31, 2018 8:15 AM
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    Glenn Schroeder wrote:
    
    Rob Edwards wrote:
    
    Yep, I agree it would be nice. Also I just tried and you can't make a construction line co-linear to the plane (in lieu of the axis)
    
    Rob,
    
    I can make a construction line colinear with a plane. I don't know why you don't get the option.
    
    Hi Glenn
    I think the limitation only applies to non-Orthogonal Planes
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    - Glenn Schroeder @ Rob Edwards on Oct 31, 2018 8:22 AM
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      - Doug Seibel @ Glenn Schroeder on Oct 31, 2018 8:28 AM
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        Glenn Schroeder wrote:
        
        Is Plane1 perpendicular to your sketch plane?
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        Glenn Schroeder @ Doug Seibel on Oct 31, 2018 8:33 AM
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        Doug Seibel wrote:
        
        Is Plane1 perpendicular to your sketch plane?
        
        Yes, it was. Thank you for finally making this dumb ex-carpenter understand.
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  - Robert Silesius @ Glenn Schroeder on Oct 31, 2018 8:54 AM
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    Look at my edit.
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Josh Brady Oct 31, 2018 8:42 AM
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A 2D sketch can only "see" entities that "resolve" into lines or points when projected into its 2D space. An orthogonal plane or surface "projects" to a straight line, but a non-orthogonal plane doesn't. Any non-orthogonal plane would just "project" into all available X-Y space of the sketch. Another way of thinking about it... the 2D sketch relations only "work" as you view the sketch normal to it.

This is what allows you to make relations to geometry that doesn't actually intersect the sketch plane. In order for this behavior to be consistent, SW cannot consider 3D planes that are at some angle to the sketch.

For example, you can make a sketch line on a non-orthogonal plane that crosses the front plane. Then, make a sketch on the front plane and drop a sketch point. Make it coincident with that line. By 3D considerations, that point should be fully constrained, but it's not because it's only constrained to the 2D projection of that line onto the front plane.
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- Matt Peneguy @ Josh Brady on Oct 31, 2018 8:51 AM
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  Josh Brady wrote:
  
  A 2D sketch can only "see" entities that "resolve" into lines or points when projected into its 2D space. An orthogonal plane or surface "projects" to a straight line, but a non-orthogonal plane doesn't.
  Yes, a non-orthagonal plane does intersect in a line on a sketch plane. Two planes always intersect and make a line unless they are parallel.
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  - Josh Brady @ Matt Peneguy on Oct 31, 2018 9:23 AM
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    Matt Peneguy wrote:
    
    Josh Brady wrote:
    
    A 2D sketch can only "see" entities that "resolve" into lines or points when projected into its 2D space. An orthogonal plane or surface "projects" to a straight line, but a non-orthogonal plane doesn't.
    Yes, a non-orthagonal plane does intersect in a line on a sketch plane. Two planes always intersect and make a line unless they are parallel.
    I may have hit the wrong post to reply to... Of course any non-parallel plane intersects another plane. "On plane" would be a geometrically valid type of constraint to add to 2D sketches. "Coincident" to a non-orthogonal plane wouldn't be right. The non-orthogonal plane has no entities to PROJECT into a 2D sketch's space. As I said, all the 2D sketch relations work as though the entity is PROJECTED ORTHOGONALLY onto the 2D sketch's plane. Otherwise, you could never make coincident etc. relations with entities that don't actually cross the sketch plane.
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    - Dan Pihlaja @ Josh Brady on Oct 31, 2018 9:30 AM
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      Josh Brady wrote:
      
      Matt Peneguy wrote:
      
      Josh Brady wrote:
      
      A 2D sketch can only "see" entities that "resolve" into lines or points when projected into its 2D space. An orthogonal plane or surface "projects" to a straight line, but a non-orthogonal plane doesn't.
      Yes, a non-orthagonal plane does intersect in a line on a sketch plane. Two planes always intersect and make a line unless they are parallel.
      I may have hit the wrong post to reply to... Of course any non-parallel plane intersects another plane. "On plane" would be a geometrically valid type of constraint to add to 2D sketches. "Coincident" to a non-orthogonal plane wouldn't be right. The non-orthogonal plane has no entities to PROJECT into a 2D sketch's space. As I said, all the 2D sketch relations work as though the entity is PROJECTED ORTHOGONALLY onto the 2D sketch's plane. Otherwise, you could never make coincident etc. relations with entities that don't actually cross the sketch plane.
      Interesting.....I didn't think of that.......Does that mean that I could be way off base with this?
      Convert Entities from Reference objects
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    - Matt Peneguy @ Josh Brady on Oct 31, 2018 9:36 AM
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      Josh Brady wrote:
      
      Matt Peneguy wrote:
      
      Josh Brady wrote:
      
      A 2D sketch can only "see" entities that "resolve" into lines or points when projected into its 2D space. An orthogonal plane or surface "projects" to a straight line, but a non-orthogonal plane doesn't.
      Yes, a non-orthagonal plane does intersect in a line on a sketch plane. Two planes always intersect and make a line unless they are parallel.
      I may have hit the wrong post to reply to... Of course any non-parallel plane intersects another plane. "On plane" would be a geometrically valid type of constraint to add to 2D sketches. "Coincident" to a non-orthogonal plane wouldn't be right. The non-orthogonal plane has no entities to PROJECT into a 2D sketch's space. As I said, all the 2D sketch relations work as though the entity is PROJECTED ORTHOGONALLY onto the 2D sketch's plane. Otherwise, you could never make coincident etc. relations with entities that don't actually cross the sketch plane.
      That makes sense and does explain the situation...But, what about the Pierce relation? Wouldn't this be similar to that?
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Doug Seibel Oct 31, 2018 8:43 AM
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Put this in the "Top 10" area and also submit it as an enhancement request.
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Matt Peneguy Oct 31, 2018 8:43 AM
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Back to what I think is Robert Silesius' point. My interpretation was he was referring to a face, not a plane:
If I try the same thing with a plane I can make a coincident mate:
This seems like inconsistent behavior. If a face is flat, it should be the same as a plane, correct?
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- Robert Silesius @ Matt Peneguy on Oct 31, 2018 8:53 AM
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  Look at my edit.
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  - Matt Peneguy @ Robert Silesius on Oct 31, 2018 9:01 AM
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    I see your point. This seems like it belongs over at Consistently Inconsistent Solidworks UI. In my post, I thought I'd selected a non-parallel (or perpendicular) plane, but I didn't. When I select a different face I get the same results as you. I don't know if it's a bug, but definitely not consistent behavior.
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Dan Pihlaja Oct 31, 2018 9:07 AM
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Sounds like an idea for the top 10.
If you don't want to add it, I certainly will...I would love to see this functionality.
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Sergio Monti Oct 31, 2018 9:41 AM
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I agree with you Robert, it's an omission.
I don't see any reasons why, for instance, if you sketch on X-Y plane in 3D-Sketch, you can make a point coincident to a non-orthogonal plane, but you can't if you are sketching on the same plane (front plane) in 2D sketch.
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Why doesn't "Point on plane" relation exist in 2D sketch?

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