Howto set a point/ball into the center of mass?

wolfgang_maier

Hello,

howto set a point/ball into the center of mass of a solid.

regards

Wolfgang

NeilCooke

This will give you an approximate center:

FeatureScript 877;
import(path : "onshape/std/geometry.fs", version : "877.0");

annotation { "Feature Type Name" : "C of G" }
export const myFeature = defineFeature(function(context is Context, id is Id, definition is map)
    precondition
    {
        annotation { "Name" : "Select part", "Filter" : EntityType.BODY && BodyType.SOLID, "MaxNumberOfPicks" : 1 }
        definition.Part is Query;
    }
    {
        var point = evApproximateCentroid(context, { "entities" : definition.Part });
        var sketch = newSketchOnPlane(context, id, { "sketchPlane" : plane(point, vector(0, 0, 1)) });
        var pt = skPoint(sketch, "point1", { "position" : vector(0, 0) * meter });  
        skSolve(sketch);       
        debug(context, point);
    });

billy2

@NeilCooke this ones a keeper

This one should be published, good job!

NeilCooke

Thanks to @Jake_Rosenfeld for pointing out the error of my ways... this is a much simpler solution...

FeatureScript 877;
import(path : "onshape/std/geometry.fs", version : "877.0");

annotation { "Feature Type Name" : "C of G" }
export const myFeature = defineFeature(function(context is Context, id is Id, definition is map)
    precondition
    {
        annotation { "Name" : "Select part", "Filter" : EntityType.BODY && BodyType.SOLID, "MaxNumberOfPicks" : 1 }
        definition.Part is Query;
    }
    {
        opPoint(context, id + "point1", { "point" : evApproximateCentroid(context, { "entities" : definition.Part })});
    });

EvanReese

@NeilCooke
why does the documentation say "Warning: This is an approximate value and it is not recommended to use this for modeling purposes". Any insight into what makes it "approximate"?

NeilCooke

@Evan_Reese - that is all that is provided by Parasolid, so hence the warning. You would think a solid modeling kernel would give you a very precise value, but apparently not. If you look at mass properties, we show the +/- deviation that is returned from Parasolid. The centroid would have similar deviations based upon the model complexity.

EvanReese

@NeilCooke
Thanks for the answer. It definitely is strange to me, and now I have no idea how much to trust it. I guess I'll just avoid it for life-and-death designs  . I made a feature that places a mate connector there. It's public here: https://cad.onshape.com/documents/e5f0ac7e68b8ac1794dc8a8d/w/a05b9505cb476fcb7d78e6a5/e/ae4e26f2c8201d3259caab0c