Proper Assembly Technique

[BigPicture045]

In an assembly I have used a few parts to constrain most of the parts to. I have edited one of the parts by redoing the extrusion sketch and now, many of my constrains in the assembly are "sick". 

The only solution I know of is to individually fix each constraint. 

Is this somewhat normal workflow? 

[samanthalynnwatkins]

Been there so many times.  So many times I had to redo constraints because one part changed.  I have run Design Doctor and some constraints seem to heal themselves without going thru everything.

[mluterman · Accepted solution]

Yeah, that's why you're "supposed to" constrain everything by their planes/axes, and not their actual faces and edges. But, I can't think that way on-the-fly and like to assemble everything as if I'm holding it in my hands. (So...I'm in the same situation as you).

[pcrawley · Accepted solution]

"Is this somewhat normal workflow?"  What is the question here? 

Is it "I constrained some parts to something I later changed, and it broke the constraints.  Is that normal?" - Yes, but there are ways to mitigate the problem. 

Or is the question "Do I have to individually fix each constraint?"  Yes, in this instance, you will have to fix the constraints.

If you post the model - or something similar if you can't share the problem model, I'm sure you'll receive some sound advice on "Proper Assembly Technique" which will help avoid situations like this in future.

As a general rule, if you imagine each face of a part has a number, and you constrain face 1 of a part to face 42 of another part, the constraint works.  If you later change the numbering of the faces (by deleting something, or adding more features), then the constraint that used to hold the faces together will fail.  (Ok, that is a bit simplistic, but it is useful to think this way if you want face-to-face constraints to hold up during changes)  

Constraining between planes/axes/points can be more reliable because adding/removing features doesn't change the number of the plane/axis/point.   There are plenty of techniques for deigning almost constraint-free, but it all depends on your application. 

[BDCollett]

It is a good argument for top-down modelling, thousands of constraints at the assembly level also slow things down and as you see can cause huge headaches if not controlled correctly. 

[BigPicture045]

Sounds like using work planes more for the constraints might help. Thanks.