9 Replies Latest reply on Feb 2, 2011 10:21 AM by John Burrill

# How to constrain this sketch?

Hi Guys

It must be simple but I´m fighting with this sketch for enough time so I decided to ask for help.   I need to constrain the sketch that created the surface trim3 on the attached file,  in order to make possible to keep those shapes even if I reduce the part size on any dimensions (lenght or widht).

I hope my question is clear and that somebody figures out a good way.

Victor

• ###### Re: How to constrain this sketch?

how to constrain a sketch depends on how you want it to respond to the changes you expect. try  to define that and it usually leads to a contraint/dimensioning scheme.

also, if the "swoops" are to be mirrors of the ones on the opposite edge, use that in your sketching.

my 2 cents

• ###### Re: How to constrain this sketch?

There are two steps to fully defining a sketch.

1)     Each element must be fully defined (coloured black).

2)     The collection of elements must be constrained against translation and rotation in document space.

• ###### Re: How to constrain this sketch?

I would tackle this by breaking it down into a few simpler sketches and then a couple of circular patterns.

And you could use equations to drive the locations of the construction lines in relation to the size of the part.

Something like this ...

• ###### Re: How to constrain this sketch?

Here's an alternative method.

• ###### Re: How to constrain this sketch?

Hi guys, thanks a lot for your efforts, I tried both solutions and what happens is that the offset that defines the distance between the 2 lines that form the "loops" get distorted when I reduce the external widht.  I can´t understand why the points in contact with the outer border that supose to be separated by 6mm "slide" over the border going closer and loosing the offset on the lines, so damaging the shape and overlaping the lines that should keep parallel all the time.

If I try to put a dimension on the ends of the loops to make them keep the 6mm distance between them, I get an over defining warning.

Here is what I´m talking about.

• ###### Re: How to constrain this sketch?

Hello Victor,

I know it might sound silly but, Have you tried Fully Defined Sketch option ?

This might help you to give the dimensions from one particular origin in the x & y Directions.

Cheers !

Raghu.

• ###### Re: How to constrain this sketch?

Hello Victor,

I know it might sound silly but, Have you tried Fully Define Sketch option ?

You will get this option on the right mouse button click when you are in sketch mode.

This might help you to give the dimensions from one particular origin in the x & y Directions.

Cheers !

Raghu.

• ###### Re: How to constrain this sketch?

Raghu no possibility is silly when we have a problem in SW :-)  is curious, I tried also with the fully define sketch option, but the points keep sliding when I change the lenght or widht, and is strange because inside the sketch all the lines are black, besides when that happen I simplyi open the sketch and freely slide the point where its supposed to be, and it goes fine and the model is ok again.  I´m missing something for sure.

• ###### Re: How to constrain this sketch?

My first instinct would be to approach this with a much simpler sketch and rely on offset surface and patterns to create your geometry.  Sketch geometry is hard to constrain compared with feature geometry-splines in particular.

Also take into account that you can dimension  spline handles to control the weight and tangent angle of the spline points.  This gives you control over how the splines update.

I went ahead and modelled up an example (some variation from your original, mind you). I worked this on the fly so it's got a few more features than a well-thought-out design would.  But you can change the overall dimensions and it won't blow up. Have a look.  (It's in Solidworks 2010 format).