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support controlling ratios of sketch dimensions/measures
kyle_altendorf
Member Posts: 60 ✭
long ago i got used to being able to control the ratios of two dimensions in a sketch. #d1 = 1.3 * #d2 or whatever it was back in pro/e. it would be great to be able to do that in onshape.
i have looked at other discussions on the topic and agree that in various cases it is quite a good option to pre-calculate dimensions in variables in the feature tree. this is not the case when you know only the ratio you want but the actual dimension values are going to be controlled indirectly via other sketch constraints and dimensions. this is a common need when using sketches for skeleton layouts where you need to enforce, for example, the ratios of mechanism elements while retaining other non-trivial constraints that affect the exact values of the dimensions.
thanks for your consideration.
cheers,
-kyle
Comments
The important thing to understand is that you can create new variables on the fly while in sketcher, and you can modify their values from the side bar while in the sketcher or whenever. This ends up being more powerful, but you do need to come up with variable names, and each dimension doesn't automatically get a locally scoped d1, d2, d3… name.
Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.
does that allow me to measure something in the sketch that is driven by non-trivial geometric calculations made by the sketch solver and store that in a variable?
i very much like variables when i know values and want to reuse that same value in multiple places. in the cases i'm discussing, i don't know the value. just how two of the values should be related to each other. the actual value needs to be figured out by the sketch solver. otherwise, i would just be recreating (probably a simplified form of) the math the sketch solver is doing. then i would additionally have to remake the geometric equations anytime i changed the sketch design.
Short answer, no.
Longer answer, you can create variables from measurements and then use them in subsequent sketches, but you can't use driven dimensions within the same sketch to drive other dimensions.
That said, the "non-trivial" might depend on your point of view. I've done a lot of things in sketchers in various CAD programs where I use the equals constraint to my advantage. For instance, you can easily make something a 1/3 of another distance with a few construction lines and some constraints. I do a lot of geometric constructions.
Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.
yes, i absolutely have done that equality of 3 segments things. very limited though and that itself can't be parametrized on existing variables or configurations.
:]another workaround that can at least be parametrized is this similar-triangles approach where i can sketch a right triangle with the sides having the ratio i want (7/9) and then make similar triangles (parallel hypotenuse) that can then scale freely in terms of value but their sides retain the desired ratio.
this still only works for ratios rather than the full features of equations.
anyways, i think the request has been made more clear, thanks. i imagine there are various forms this could be implemented in, not necessarily exactly how pro/e was doing it 20 years ago.
:]I created this improvement request a while ago. I think it would help, at least where there are ratios involved:
https://forum.onshape.com/discussion/19025/ratio-dimensions-in-sketcher-and-elsewhere
Simon Gatrall | Staff Mechanical Engineer | Carbon, Inc.
Thanks for the link, I starred it. That does indeed cover the more common use cases.