Fully Constraining sketches in NX

[Aerohawk]

I saw this post thread561-216355 and heres my two cents about why I thought fully constraining a sketch was so important.

A unconstrained sketch is one which does not have a single solution for the given constraints/dimensions i.e a different shape could be expressed using the same set of constraints.

An example: rectangle with only one side dimensioned would mean that its other side could have any length (let alone the position of this rectangle in 2D sketching space or the angles of its sides). So you could never be sure what the exact shape is especially after moving/tweaking it around in the sketcher.

More so with the case of more complex shapes -> You make a rough sketch keep adding dimensions and you think you have enough but say a line you assumed is perpendicular is actually at angle of 89.9deg. you couldn't be sure unless you explicitly added a constraint/dimension for it. Also you couldn't be sure there weren't any other such errors if the sketch wasn't fully constraint because for that set of constraints you don't have one single geometry as a solution.

So now when your CAD modeling goes for machining. The CNC machine code is generated based on the solid model (the same one with that side off by 0.1 degs) and comes out with the same error.

No wonder companies make it policy to fully constrain sketches. And using Fixed / Constant length / Constant Angle constraints is like shooting your self in the head. Lets say you put a constant angle constraint on that line which was at 89.9degs just so you could satisfy your companies policy and produce a fully constraint sketch - well now your sketch even hides that there and error in there!

-Vik

 

[tsjaikdus]

I can't imagine what people think when they do not want to constrain their sketches. In my opinion constraints are what make up a sketch. Lines and arcs are nothing but a sheet of paper. Constraints are the scissors and glue. Now and then I hear rejoice from people nearby who managed to fully constrain their sketches. I mean is it really that hard? Therefore I also do not understand the auto dimensioning in NX 7.5. Auto dimensioning lacks the intelligence needed to know what's important. Auto dimensioning is the same as no dimension to me. Except for the fact the the bush of auto dimensions make selecting lines and arcs a real pain in the ass.

 

[tsjaikdus]

I can't imagine what people think when they do not want to constrain their sketches. In my opinion constraints are what make up a sketch. Lines and arcs are nothing but a sheet of paper. Constraints are the scissors and glue. Now and then I hear rejoice from people nearby who managed to fully constrain their sketches. I mean is it really that hard? What do you think you are drawing? Therefore I also do not understand the auto dimensioning in NX 7.5. Auto dimensioning lacks the intelligence needed to know what's important. Auto dimensioning is the same as no dimension to me. Except for the fact the the bush of auto dimensions make selecting lines and arcs a real pain in the ass.

 

[tqqlin]

Personally, I find that fully constraining a sketch can be sometimes tedious. I prefer to constarin the important things, Typically, I use sketchs as a starting point then add the shape features using master model from there. A well thought out design can be achieved using auto constraints such as vertical/horozontal automatic creation. It obviously varies by design. I also always design on a grid pattern. Sketching is a great tool, if used with common sense, over-thinking it only adds stress to your job. Just my two cents.

 

[DaSalo]

I agree that using fully constrained sketches is the best practice but also remember that not everyone is using the software the same way that you are or worried about working to the same level of precision. What Siemens has done is to allow the flexability to leave a sketch unconstrained so that the software is not arbitrarily forcing the user to work to a certain level of refinement if they don't need to. I am working on stuff that requires high precision. I always do fully constrain my sketches, but they don't always start out that way. Sometimes I will experiment with several different appraoches to a problem, not taking the time to fully develope each approach. Once I make a decision about the best sequence I'll go back and delete the sketches I don't need and fully constrain the ones I do. I really value the flexability to work fast and rough when it suites me. Many other pieces of software do not allow for this. I think the problem comes in when the users takes this advantage and uses it a pass to not do a good job. There is nothing the software can do about that. If someone is determined to make a bad model there is no stopping them.

NX 7.5.4, NX 8.0.1.5
Tecnomatix Quality 8.0.1.3
PC-DMIS 2011 MR1

 

[jnikolauk]

FWIW I always fully constain my sketches but often start pulling my hair out when it comes to constraining geometric patterns the constant toggleing between overconstained conflicting constrains and perfectly fine is often quite annoying.