Tips on kinematic synthesis in 3D using sketch/constraints or other tools?

[erik3000]

This is a somewhat non-specific question, I'm looking for discussion on different methods for doing kinematic layouts (linkage design) in 3D, since generally everyone will develop their own unique variation of techniques.

If you are experienced with kinematics and specifically using sketch tool in NX, you are aware of the general technique of sketching multiple positions of a mechanism graphically, then using the sketch constraints to relate link lengths, angles, position, etc., to solve the parameters and define the linkage.

This is a powerful method that allows you to prescribe very exact motion specific input, but is limited to planar linkages... Since there is no '3D sketch' tool, for any non-planar motion it requires either 2D approximation, quite cumbersome trial-and-error adjustment, or -- the most useful method I have found -- to use spheres, intersection planes, and points/lines to 'constrain' the non-planar elements of the linkage relative to elements of a 2D sketch. Again, this still requires entering and exiting the sketch to adjust it, and then checking output and manually changing parameters of the non-planar features to alter it.

I have considered a method of using assembly constraints (like slider joints), to create 'phantom' links and joints that would allow solution of some variables of a non-planar portion of a linkage based on a 2D sketch it is linked to. But still, there is no way to "back-solve" sketch parameters based on any constraints in 3D (apart from using expressions based on formulas, at which point you may as well solve it analytically).

There must be better procedures out there. Anyone here knowledgeable on this subject that can share your method? I have not used many other CAD tools, perhaps there are some that do have a true 3D sketch and constraints?

 

[crecore]

Here's a technique that I came up with that might be able to be applied. I use what I call a drive sketch and a driven sketch. The drive sketch is created using input parameters (user expressions). The results are then associatively measured to create new expressions. Equations are written using the drive sketch results and tied to the driven sketch. The driven sketch shows the results that best fit the parameters that I laid out using the equations (rounding, even numbers, prime, etc). It takes some work but if you have a scalable design once it is set up you can use it as a template.

NX 1847
TC 11.5