V21 constraints - how to determine which part moves.

[jhiker]

In the new constraint tool is there any way to control which part moves and which remains stationary without first using the 'anchor' tool.
For instance, when I apply a concentric constraint 9 times out of ten the 'wrong' part moves no matter which part I select first.
Thanks.

 

[DavidJ]

It has always been recommended to anchor the first or 'most major' part in the assembly, then constrain other parts. Otherwise there has always been the risk of parts 'flying off into space' - valid solutions for the constraint manager are often unexpected or undesired.

Are you noticing significantly different behaviour in v21 from v2019 ?

 

[DavidJ]

Generally, the part selected FIRST remains stationary, the one selected SECOND moves to satisfy the constraint.

 

[idslk]

Hello David,

Generally, the part selected FIRST remains stationary, the one selected SECOND moves to satisfy the constraint.

have you tried this?
Regarding the location in the space, it seems to be the order, in which the parts were inserted.
The one inserted first remains in place, the one inserted later moves.
It seems independently from the selection sequence.
Regards
Stefan

 

[DavidJ]

Yes I did try it - but my selection order may have matched the insertion order. I'll re-check.

[EDIT - it isn't consistent either way. I have seen both behaviours in the same session ]

[EDIT - and sometimes neither matches what happens - I've seen last inserted part remain still, despite the one inserted first being selected first for the constraint]

 

[jhiker]

Generally, the part selected FIRST remains stationary, the one selected SECOND moves to satisfy the constraint.

I think that might be a good way of doing things if it could be coded in

 

[AlexB]

I may be mistaken but from what I've witnessed if a part has more contraints than the other part it will remain stationary.
Also, if the parts have the same number of contraints, the part added to the assembly later will move. Is this the case for you? Edit 2 - This doesn't look it's the case after all, a part will move with the same number of contraints irregardless of when it was added to the assembly.

Edit - If neither of the parts have any contraints, the part you click first will move.

 

[sz0k30]

Generally, the part selected FIRST remains stationary, the one selected SECOND moves to satisfy the constraint.

"Generally" is not good enough. Because as others have complained, you never know? You should always know. Seems it should be coded in that the first part selected should remain stationary and the second part move to the first!

 

[JST]

It does seem that the one with more constraints will stay steady, and the other will move. That may not always be the case.

Almost every similar operation I perform in Alibre seems to "know" what I want, and does the reverse! If all the selection rules were inverted, maybe Alibre would always do what I want?

 

[idslk]

Hello colleagues,
"generally" only a thought...
Even if a foreseeable reaction would be desirable, this "issue" would not occur if you constrain every part directly when inserting it.
(starting with the first one!)
Sample:

• Insert a housing -> contrain it to the coordinate system (or fix it).
• Insert a cover -> constrain it to the coordinate system
  (more "stable", but complex and you have to track each change) or to the housing (less stable but more easy)
• Insert a washer -> constrain it to the cover (see above)
• Insert a screw -> constrain it to the washer

...
other thoughts?
Regards
Stefan

 

[HaroldL]

The "Best Practice" that I was taught is to constraint the first/base part to the Origin and/or the Default Planes. Then all other parts will move to it no mater which order they are selected. I think the Help manual used to state the order of selection but it is on in the current version. It does have some nice animated examples of the new constraints though.

Stefan, I agree with your examples except that I would constrain the cover to the Reference Geometry of the housing. That way if the housing changes or needs an "adjustment" the cover will follow it.

 

[JST]

You CAN constrain to the origin and planes, but a simple "anchor" also works well, with the advantage that if you have to undo it to release things in a higher level assembly, it is one easy action.

 

[HaroldL]

There is always more than one way to perform a process. Need to be careful though, anchoring a part can lead to problems later. I have found in a couple of rare instances that the part that was anchored was not aligned to any plane or the origin. It caused a lot of grief trying to constrain in to the next level assy until I found the problem.

Where I worked it was required that all base parts were mated to the default planes and it's a habit I've stuck with. That assured the parts were locked onto the origin and square with the default planes.

 

[Hunter]

I also just anchor the first part I install, then re-anchor other parts later as required. Personally I've not noticed that the parts jump around unintuitively when applying mates, but now that I'm aware of the issue, I may...

 

[JST]

As I have seen it, the parts continue to obey all constraints. At worst, the part comes in at one position, and then is corrected to the proper place.

Where you might get into trouble is if you act in strict "academically correct" fashion, and always avoid constraining to any part feature, instead constraining the parts to and with original reference geometry (I think you can actually do that, I never have even tried it). In that case, you would be required to build the parts constrained to the original reference planes. If they were allowed to move relative to those original planes they were built in reference to, then everything would rapidly fall apart.

As for "jumping unintuitively", it annoys me that now every constraint is instantly chosen and applied as soon as the surfaces are selected, even before one can apply modifiers (limits, free, angle, etc).