Trouble understanding constraint hierarchy

[brian_w_yates]

I'm attempting to draw a 2 point rectangle and then move it. I place the first corner at the origin of the sketch and the other at an arbitrary grid point, let's call it a 2 x 2 square. When I attempt to Move the square I am unable to , which I believe is because of the constraint automatically applied to the first corner. There are two Coincident Constraints applied to the first corner, one for the origin (constraint on the left) and one for the corner of the square itself (constraint on the right). 

   If I delete the Coincident constraint on the left (coincident to Origin), it disappears and the Coincident constraint for the corner remains, and I am free to move the square along the sketch plane. However if I delete the Coincident constraint on the right (corner of the square) both constraint symbols disappear from view but I am unable to move the square. The  Coincident constraint to the Origin remains in effect, yet I cannot see or delete it. 

    Why does this occur, and how is it intended to benefit the user?

[kacper.suchomski]

Hi

I think the better question is why do you want to move it?

[brian_w_yates]

Because it would be more useful to me in a different location.

Because sometimes I make a mistake in where I need to place a feature

Because I am trying to understand the behavior of constraints

[jeff_strater]

there is no "hierarchy" in sketch constraints.  All sketch constraints are equal.  In your case, assuming that the rectangle is dimensioned, what is preventing movement is the constraint between the center point and the sketch origin.  It can be a bit tricky to find out which constraint that is, but trial and error can get you there.  At some point it becomes easier to re-draw the rectangle, but that is up to you

[TheCADWhisperer]

@brian_w_yates 

[brian_w_yates]

Jeff,

Thank you for your reply and video, however the scope of my original post was regarding a 2 Point Rectangle, not a Center Rectangle. I was replicating the steps you showed in your video, but if you read the end of the paragraph of my post " if I delete the Coincident constraint on the right (corner of the square) both constraint symbols disappear from view but I am unable to move the square." I'm trying to understand why the Coincident Constraint coupling the square to the Origin is still active, yet hidden and unable to be selected and deleted.

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[brian_w_yates]

Thanks for the reply  and the video. I see how the scope of the constraint is highlighted in blue when a constraint is selected, yet the trouble I am having is why do both constraints disappear  if I delete the one on the right, even though the constraint on the left is still active?

I will try to post my own screencast here soon, but I live in rural West Virginia and our upload times are abysmal.

[jeff_strater]

Sorry - I'm not sure why I thought center rectangle - maybe because that is a common thing to do - center a rectangle on the origin.  Sorry about that.

But, @TheCADWhisperer 's video illustrates the correct solution.  There is a coincident constraint that ties the point to the two lines (the one on the right), and one that binds the point to the origin.  Deleting the one on the right separates the point from one of the lines (it is arbitrary which one).  This leaves the other point still connected to the origin:

[brian_w_yates]

So after further experimentation I have isolated the behavior. I thought the Origin might have something to do with it, but it does not. 

In the attached video, I start with an empty sketch and then insert three Sketch Points. I use Coincident Constraint to join all three together. I then use that point start a 2 Point Rectangle. When hovering over each corner of the square a Coincident Constraint symbol is visible, except for the bottom left, where there are actually 4 overlapping constraints (One for the corner of the rectangle, and three for the sketch points). When I delete the Constraint for the Rectangle, the symbols for the three points all disappear, yet when I move the bottom right point of the rectangle and drag the bottom line out of the way, the constraints are visible again.  It's as if the point of the rectangle is covering up the other three points. 

[jeff_strater · Accepted solution]

I can explain this, or at least explain how it works. I won't try to justify why it works this way, but I can explain it.

One tool that is super useful in understanding geometry is Select Other:  A long left mouse button press over the graphics area will bring up a list of everything that is under the mouse at that point.  For instance, in the case described above, this is what exists for the lower left corner:

you can see that there are 4 points there (the 3 that you placed + 1 from the rectangle).  This will be useful later

A couple of concepts:

• point merging.  When two curves share an endpoint at creation time, those points are merged.  That is why you won't see two points at the corner of a rectangle
• in some cases points are not merged.  For instance, in your video, when you made the 3 points coincident, they were not merged.  The assumption here is:  You created those 3 points on purpose, you must want them to still exist.  Arc/circle/ellipse center points are also not merged
• the list of coincident constraints that comes up when you hover is only for the item currently under the cursor at 0:52 in your video:
  
  is for the rectangle point (the last point placed, which has coincident constraints to the 3 points in the point stack.
• when you delete that constraint, it form another point, which you can see in Select Other:
  
  now there are 5 points there.  It turns out that the top point is now attached to the lower horizontal line, as you see when you drag it
  
  the other 4 points are still in the point stack, the top one is from the vertical rectangle line
• "When I delete the Constraint for the Rectangle, the symbols for the three points all disappear".  They are still there.  They are just associated with the point that is now underneath the point on top.  If you were to hover over the original point after moving the bottom line, you can see this:
  

Hope this helps a bit

[brian_w_yates]

" If you were to hover over the original point after moving the bottom line, you can see this:", Yes I do this at 1:16 in my video. I just didn't realize that the points can hide the constraints in that manner.

    Thanks for the thorough explanation. I've worked for years with other CAD software and the emphasis that Fusion puts on constraints is a bit baffling at times. 
I also realized that there is an indestructible point that always exists at the Origin, which explains why the extra Coincident constraint was showing up in my original test sketch. 

I'm using Fusion primarily to help my welding students (High School Vo-Tech) design and build things, and I've been struggling trying to explain to them why things are working the way they are, but this helps a lot. Thank you.

[TheCADWhisperer]

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@brian_w_yates wrote:

 I've worked for years with other CAD software ....

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For the benefit of your students, you might let them know that geometry constraints work essentially identically in Fusion, SolidWorks, Inventor and Creo (Pro/E) and also in AutoCAD, but very very few even know they can exist in AutoCAD let alone know how to use them.  The key is geometry constraints are first principles rules of geometry..