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# Sketch constraints - any guidelines?

Valued Contributor

Usually there are a number of ways sketch elements can be constrained, and I'm curious if there are any rules or guidelines for this (i.e are certain constraints considered better to use, if given a choice, because maybe they're easier to solve?).  I assume fewer are better, but maybe that depends on which are being used as well.

Thanks for any thoughts on this. Just curious.

10 REPLIES

# Re: Sketch constraints - any guidelines?

Gears Phenom
Geometric first, then dimensional. That's about the only rule of thumb I have.
-Dave
NX 11 | Teamcenter 11 | Windows 8.1

# Re: Sketch constraints - any guidelines?

Siemens Phenom

If the sketch is a first feature, you can use geometric as well as dimensional constraints, no doubt. If I want to align a line to datum axis, I can use collinear constraint or create dimension with a zero value if position of the line going to change at some later stage. If the sketch is created after some features, existing datums/faces/edges can be used with geometric constraints which can avoid the clutter of dimensions to some extent. I would say it will depend upon the design intent required and individual user approach.

# Re: Sketch constraints - any guidelines?

Valued Contributor
Thanks for the responses. I guess I knew to do geometric constraints first, then dimensions, as that will reduce the number of dimensions needed. But my question was mainly about geometric constraints. Often there are several ways a profile can be constrained, and maybe it doesn't really matter in the end which constraints are used.

But just to illustrate, yesterday I had a horizontal line I wanted to make collinear with the x-axis. Because I had draw it horizontal, a horizontal constraint had already been applied. So to make it collinear I only needed to align it with the origin. But alternatively, I could have used a collinear constraint with the x-axis. That would have only required one constraint rather than two. So would that be better (i.e. easier for the constraint solver to handle)?

Another common situation is making a 4 sided shape rectangular. There I could:
a) make opposite sides parallel, and one corner perpendicular
b) make 3 corners perpendicular
c) make opposite sides equal and one corner perpendicular
d) make opposite corners perpendicular and one side parallel
Does the constraint solver handle all these equally well?

Needless to say, this is all very minor, but just something I've been wondering about for a while. So thought I would throw it out there. Thanks!

# Re: Sketch constraints - any guidelines?

Siemens Phenom

Hi,

• Everybody agrees on the rule to first create geometric constraints and then dimensional constraints.
• Limit the amount of external references. If possible dimension to the internal origin and axes. This makes your sketch less vulnerable for external changes.
• In general H and V constraints and alignments are OK, but if a part of a sketch might have to rotate later then it is easier to have applied perpendicular constraints and some parallel constraints. Deleting H and V related constraints and the re-constraining before rotating is a lot of extra work.
• I prefer perpendicular over parallel were possible. This is just because in the case of perpendicular the curves are typically closer together (or even touching). This makes it easier to understand the sketch.
• Do not use fix. Use the associative origin of the sketch feature and dimension to the internal origin and axes.
• Keep it simple. Sometimes multiple smaller sketches are easier to manage than a big one. Update is also faster. So are these curves really all interacting with eachother or is there are hierarchy?
• Use the correct tool to constrain. The preferred tools are:
1. Infer during creation
2. Drag and drop
3. Select objects and use the context toolbar
4. The constraint dialog. I often get comments/PR's about the constrain creation because user use the wrong tool. The constraint command is only there to allow you to constrain multiple objects at the same time. If you want to make many vertices coincident or many lines parallel to another line, then this is the tool to use. In normal workflows you do not need this dialog.
• Let the autodimensions do their work. You can turn them off, but it will lead to sketches folding into themselves when you are constraining. These autodimensions point you to locations where you may need to add geometric constraints.
• Let the autodimensions do their work :-). Curves do not move when you do not look at them. Autodimensions are constraints. For the solver autodimensions are constant length constraints. So it is not bad to leave an autodimension around. If you do not enter the sketch again the dimension will never change. Your sketch is fully defined and will behave predictably even with some autodimensions in it.
• Be an expert in "Applied Laziness". I would even go as far as stating that constraints and dimensions are not needed at all for a sketch, if you are not changing the curves. If the curves are correct and you have no expressions you want to drive, why bother? Who will change the curves? NX will not change them. Just like AutoCad never changed your drawing after you went home.
I realize this is something to get used to... But you will.

I know it is work, but I wish you have some fun while using NX!

Regards, **bleep**

# Re: Sketch constraints - any guidelines?

Gears Phenom

My only issue with autodimensions, is that a user can still drag the curves, and change the auto dimension.  We have seen it happen unintentionally.  I'd suggest converting them to driving, if this is a concern.

I'm old school, and don't like being told what to do, so I turn auto dimenions off

-Dave
NX 11 | Teamcenter 11 | Windows 8.1

# Re: Sketch constraints - any guidelines?

Siemens Phenom

Dave, I know that many long time users do not like the auto dimension. The one nice thing in NX 11 is that there is a toggle to hide them from the display. They are still there and you can show and hide them. It does help the use of auto dimension.

I am not sure if I can change your mind about them, but I just wanted to make you aware of this.

Thanks,

Scott

# Re: Sketch constraints - any guidelines?

Valued Contributor

**bleep**  Thank you for that list.  Very nice and useful!

A lot of what you mentioned I already do.  The one exception is autodimensioning.  I don't use it because I would rather decide myself where to place dimensions.  To me, it's part of establishing design intent.  I know I can replace them, but that creates extra work.  Also, I don't like the way they clutter the sketch.  But if there's a toggle that will allow me to easily switch them on an off, I may revisit that.

With regard to "limit the amount of external references", one suggestion I saw (which I now do) is when starting a new sketch, change the selection scope to "Within Active Sketch Only".  Why that isn't the default, I don't know.

With regard to "Use the correct tool to contrain", could use explain "Drag and drop"?  That's one I've never used.

Thanks again.

# Re: Sketch constraints - any guidelines?

Siemens Phenom

Hi,

This small video shows what I meant by constraint creation through drag and drop:

(view in My Videos)

Selection scope in sketcher is a complicated topic. It divides our user-base in two parts:

• One part (the longtime users) want to be able to select anything they want, even when it makes selecting what they want to select harder most of the time.
• The others (mostly new users) wonder why everything highlights all the time.

We are constantly trying to find the right balance between respect for both new and existing users, functionality and usability.

Regards, **bleep**

# Re: Sketch constraints - any guidelines?

Valued Contributor
Very nice! I didn't know you could do that, and I will certainly start using it. Thanks for the video!