How to design to center of gravity?

mike_phillips329

I'm new to Onshape but have over two decades of design work on other CAD systems like Creo Parametric. How can I create a mate, plane or some other "feature" at the center of gravity of my assembly. I'm sure Onshape has this capability as all other systems I've used can easily do this.

rick_randall

As far as I know, Onshape will give you the CG of an assembly and its location (x,y,z from the origin), but I don't think you can use the datum mark as a mate connector (or any other means of locating features). Maybe others know something that I don't.

MichaelPascoe

Here is a custom feature that will place a mate connector and a point at the center of gravity: Center of Gravity - Custom Feature

martin_kopplow

The issue with this custom feature is that it does only work in part studios. I guess what is neeeded here is a tool that puts some representation, say a mate connector, at the current CG of an assembly, so it can be used to calculate and maybe drive dimensions of counterweights, reposition components, position the assembly balanced on an axis, or the like.

MichaelPascoe

@martin_kopplow, yep, that would be an issue. Too bad we don't have custom features for assemblies. The best way I can imagine doing this with the current tools would be to edit part of the assembly in context, use the CG tool, then you will have the updated center of mass with a mate connector. Anytime you need to change the assembly, click the update context button again to update. This is a pretty limited workflow though.

martin_kopplow

@MichaelPascoe , well, that could at least be a workaround for smaller assemblies. I'd rather not try it on a helicopter first time, but might look into it with a smaller project. If there was a way to extract the coordinates from the CG dialog, we could put that to good use, probably in a more reliable manner, though. 

MichaelPascoe

@martin_kopplow
If you are going as complex as a helicopter, Find solution by @Konst_Sh may be of use to you. If you bring all your parts in context to a part studio, get the center of mass, then rotate what you need using variables to drive the movement, you could find the center of all of the center of masses for every combination of movement from the driving variables.

Konst_Sh

Worth to try if in-context bodies preserve density of originals. If yes then you can edit any assembly component in context, evaluate a center of gravity to every context body, and then calculate the overall center as linear combination of center vectors and masses divided by total mass. If this will not work, then it is probably the job for onshape api.

Jonathan_Hutchinson

This feels like a real bind for me too. I'm no power user but i though tit might be an obvious thing to need. I want to move some parts around and try different design solutions and see how it affects the COM, but can't just because it's an assembly? Huh?

The solution is make a mate and copy and paste each value of the calculated COM position into it. But these means also selecting the parts to measure three times repeated, as editing a mate will then close the Mass and Section properties dialog. Could it not just stay open more often?

mike_phillips329

Still no real answer to my question here.

I really find it hard to believe that a product as mature as Onshape does not provide this very basic capability OOTB (out of the box). I find it hard to believe that real paying customers have not begged for this functionality. I mean sure you can create a mate connector at the center and manually measure and calculate and update your location as the assembly changes but… YUCK! This tool needs the capability to create a "feature" at the CG of a part or assembly and automatically update this location as the design changes.

martin_kopplow

Ooops. I never even thought it couldn't. It is only I havend needed it this year. It was a basic thing of my previous CAD to place an origin (there were no mate connectors, but there could be more than one origin) at the overall GC of a design on request at any time. That is not only important for helicopters, but also for things that were to be picked up by cranes and such. You'd just select the whole assy, open the weigh calculation dialogue, there you'd hit the "Place origin at CG" button and it would even inform you about the design's intertia, while it was at it.