2D constraint of arc center to line?

Joe Dunfee

This may not be possible with the set of constraints we have available, but I realize it may be more likely I have not found a way to do it. 

I am trying to draw a steel angle profile, and use 2D constraints with it.  What is giving me problems, is the radius at the end of each leg.  I want to constrain the arc to stay coincident with the outer straight lines.  Sometimes they just behave OK, but other times they misbehave.

I suppose it would be possible to create a construction line, but I don't want that line to show up in drawings.  Of course I know I can put the construction line on the defpoints layer, but I would really prefer that it is not visible at all.

See the picture attached.  Note that the angle works find if D4 is changed to be 3".  I have also attached the v14 dwg.

Thanks for the advice,
-Joe

David Waight

Joe,

I have had a play around with your design after redrawing it in millimetres (sorry it's a European thing!) and I have come up with the attached.

The three things I have done to your original, is add a geometric constraint from the centre of the radius to the entity d1 and the other vertical of d4 (entity is important as opposed to endpoint).

I have done away completely with the radius geometric constraint.

And finally added a dimensional constraint to the other straight part of each leg.

Please have a play with the drawing and see if it works for you, I have tried various combinations of measurements and it seems to work.

During creation of this design I found frequently the dimensional constraints stopped working and I was unable to save the drawing.

I then made a regular habit of pressing Regen All frequently and this seemed to help.

Like you I am really struggling with 2D constraints in Bricscad and they are something I use every day in my line of work, so everything I can learn seems to help.

I will be sending a report request this week to find out if 2D Constraints in Bricscad will be suitable in my profession as I am coming across a lot of difficulties when the designs start to get complicated.

Hope this helps,

Regards,

David Waight

Angle-ParametricV2.dwg

David Waight

 Joe,

Apologies, I mean't dimensional constraint on the radius and NOT geometric.

Sorry,

David Waight

Roy Klein Gebbinck

Adding a 'coincident' constraint between the centers of the arcs and the outside of the flanges is not effective. In the image the center of the topmost arc is already on the leftmost vertical line. Adding such a constraint does not change the geometry because for this constraint the line is considered infinite. But other than that David is right: you need more dimensional constraints to get predictable results. You need at least 6 dimensional constraints. And apart from the radial dimensional constraint you can also remove the 'equal' constraint between the arcs.

Joe Dunfee

Thank you for the reply David,

Your sample works for me.  But, I cannot re-create it. There is a coincident constraint which keeps the radius centered on the line. I attempted that constraint before, but I cannot figure out how to select the center of the radius.  I tried starting the coincident constraint process, and then the Entity option, but that still does not permit me to constrain the radius center to the line.

One funny thing on your sample, was that all the dimension text had text overrides.  So to edit it at all, I had to change all the dimension text to show the default value with <> before I could get it to work.

-Joe

Joe Dunfee

Roy, Thanks for the reply. You were creating your message while I making my last reply, so I didn't see yours.

I cannot follow your sequence, other than you saying that  constraining the arc center to the line is not effective.  However, my own tests on David's sample is that it works.  Still, I know that parametric constraints can be hard to predict, even when they are working according to design.

I wonder if there are other issues at play here.  For example, I had learned recently that polylines have problems with constraints, even thought he manual shows that they should work.  It is important to keep these discussions on the forum, or we tend to end up spending hours trying to make something work that is not workable.  The polyline is not an issue with my angle, because I had already exploded the polylines.

-Joe

David Waight

Joe,

Sorry about the dimension text overrides (whatever they are!), I haven't been using Bricscad for long and not used Autocad at all so I have obviously done something wrong with my setup!

I have hopefully attached a small video which shows how I attached the arc/circle to an entity.

If it doesn't make sense, I will try and write it down instead!

Regards,

David Waight

Constrain_to_entity.swf

Joe Dunfee

Thank you very  much for the video David.  It allowed me to constrain a circle to a line.  In some other CAD programs they are visible. I didn't realize you could select a center point, even if that center point is invisible. 

I attempted do re-create your own angle drawing, but it wouldn't behave and when I tried to save it, I got an error message that kept coming back.  So, I had to abandon it.  Peraps there are some bugs I am running into.

-Joe

Roy Klein Gebbinck

@ Joe:
To clarify: I am not saying that David's example is not working. But it is my opinion that constraining the centers is superfluous. The 6 dimensional constraints should already be enough. But, having said that, I do run into some strange (buggy) behavior. So perhaps you do need these constraints to work around some issues.

Roy Klein Gebbinck

FWIW: Angle-Parametric_V3.dwg

Angle-Parametric_V3.dwg