Constraints between sketches

[pfaeff]

Hi there,

I was designing a very simple hinge for learning purposes:

In order for the hinge to move freely, I need a constraint that limits the distance from the moving part to the base.

(I added the dimension constraint for demonstration purposes only)

This distance is dependent on the side view though, which seems to correctly update according to the distance I set:

But from this sketch I am unable to add a distance constaint as everything is already constrained.

What would be the right way to solve this?

Thank you very much.

[Anonymous]

you can simply draw line align edge you want then add dimension after that you can remove lines

[pfaeff]

That's how I was trying to do it. Draw a line from the edge of the circle to the base and then set a dimension. But that didn't work, because that distance was already constrained. The software doesn't seem to recognize that there is still a degree of freedom in the other sketch or something.

[n8bot]

You cannot "back-drive" a projected sketch element. IE that purple line is not moving unless you break the link. You'll want to make this circle (side) sketch before the top sketch, and reference the side sketch to determine the location of the line in the top sketch.

[pfaeff]

Can I change the order of the sketches in the timeline after the fact? It would be strange to have to redraw the entire thing.

[n8bot]

At this point it is unlikely that you can simply drag the second sketch back. The first (top) sketch creates the geometry that is needed for the side sketch. You could start a new sketch that refines that geometry with the needed clearance, rather than trying to drive the original sketch with the side sketch.

[laughingcreek · Accepted solution]

Another chick and egg problem.  You can only change the order of the sketches if there aren't any features (bodies, etc) dependent on them.  The screen cat shows deleting the bodies and then restructuring the sketches in the time line.

This doesn't completely solve your problem of course.

This is a top down design.  Typically you want the most important criteria of a design to be in the first sketch.  In this case, you've chosen the clearance to be an important criteria, so the side sketch should come first.

On the side sketch, dimension some construction lines at each of the important parts, and project those into the next sketch.  the side view sketch will be used to drive all dimensions along the Y and X axis, and the top view will drive all dimensions along the x.

(Or you could use a bunch of math to create a formula based on what the height of the extrude is going to be,  but we use cad so we don't have to do that sort of thing.) 

[pfaeff]

Thank you. I learned quiet a bit from that. 

[SaeedHamza]

Hi,

Check this please, it might help

Regards

[SaeedHamza]

Sorry I forgot to attach the screencast 😛

[laughingcreek · Accepted solution]

@n8bot makes a good point.  Top down design approach works really well when you have a really good idea where your going with a part, and maybe just need work out a few small details.  We don't always have the luxury of already having a really good idea of how a part will end up.

To take n8's approach, you would create the top drawing, minus the second part that moves.  create the side view, with the clearance circle, then create a THIRD sketch from the top view, using geometry projected from both previous sketches to create the rectangle for the second extrude.

I'm sure someone will screen cast if that's not clear.

[laughingcreek]

Ok, one more approach.  just use one sketch, and take advantage of some of the options in the extrude dialog.

Every place you type in a number (either in a dim inn a sketch, or in the extrude dialog) can be a user parameter.  Go to user parameters under modify menu and define some. use them in the dims and extrude dialogs to get more control of the design

(sorry for the length, I didn't work out what I was going to do before starting.  had to back track some.  This demonstrate one of the nice things about parametric modeling though, being able to go back in time and change things.)