Constraints not doing what I am expecting

Hi Adrian,

You've used 2D Constraints to define a 2d profile, which you've then revolved to generate a 3D solid. Unfortunately in BricsCAD the generated 3D solid has no connection to source 2D profile, they are two disparate objects with no history maintained between them.

For 3D objects you instead need to use 3D constraints. This is quite a different approach to applications like Solidworks, it takes a bit to get your head around.

Attach and example of pulley with 3D constraints and parameters applied. A few things to be aware of:

• Turn the BMBROWSER (Mechanical Browser) on to see the attached constraints and parameters.
• Use BMINSERT when inserting the pulley into other drawings (assembly's), this will expose the parameters and allow you to change them after insertion.
• Use BMMECH to initialise a 3D Component. This allows you to set key settings for your part, such as Name, and material type.
• BricsCAD is a direct modeller, not a history based modeller. When you make changes to parameters, these are applied relative to the current state of the component. It is easy to break components by driving them past the point where a feature would no longer exist. Care is required not to lose information from your model.

Controlling your 3D part to meet your expectations, and applying 3D constraints in some situations can be difficult (or next to impossible). Here I've found it useful to create helper objects to assist. You'll see that I've created a number of surfaces in the part, which I'm using to mimic the function of Solidworks planes. As these distract from the finished component, I've created a LISP routine that allows me to turn their display on and off. An alternative would be to put these on their own layer and freeze.

Regards,
Jason Bourhill
CAD Concepts

@akendall1966 said:
But, if I am following you correctly in general in a direct modeler when sketching a 2D profile and extruding or revolving to get a solid, there no benefit in investing a lot of effort constraining the 2D, as you have to reapply all those constraints as 3D constraints to the solid to 'fix' its geometry.

Yes that is correct. No relationship is maintained between the 2D profile and the generated 3D object, nor are any 2D constraints transferred to the 3D object.

It is rather labour intensive to apply 3D constraints, You also need a lot of them! This makes them rather difficult to create and maintain. I would hope in future versions that options become available to apply constraints automatically e.g. BricsCAD reacts to the action of you revolving a profile to automatically apply appropriate constraints.

Re-creating an object is fast if you retain the original profiles. With direct modelling it is often easier to throw away a 3D object, and re-create rather than edit. i.e. why worry about applying 3D constraints when it is so quick to simply re-create? Using a 2d parametric profile could be an aid in this situation to quickly generate similar parts. This video post illustrates this concept https://www.bricsys.com/tv/m/?new-in-v15---boundary-detection-m161. However this approach doesn't work with more complex components that require multiple modelling steps.

I assume you can either sketch/ extrude or construct from primitives choosing whatever is most efficient, as long as you then apply sufficient 3D constraints.

Yes, with direct modelling how you arrive at your destination has no bearing. This is a key advantage to this method. If 3D constraints are required, then I would apply on a finished model. If you apply to soon, then these can be lost by subsequent modelling actions.

Regards,
Jason Bourhill
CAD Concepts