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

Comments

• NeilCooke Moderator, Onshape Employees Posts: 5,912

  August 2018

  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);
      });
  

  
  

  
  

  Senior Director, Technical Services, EMEA

  3
• billy2 Member, OS Professional, Mentor, Developers, User Group Leader Posts: 2,115 PRO

  August 2018

  @NeilCooke this ones a keeper
  
  This one should be published, good job!
  
  
  
  

  1
• NeilCooke Moderator, Onshape Employees Posts: 5,912

  August 2018

  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 })});
      });

  
  
  

  Senior Director, Technical Services, EMEA

  2
• EvanReese Member, Mentor Posts: 2,619 PRO

  November 2019

  @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"?

  Evan Reese
  The Onsherpa | Reach peak Onshape productivity
  www.theonsherpa.com

  0
• NeilCooke Moderator, Onshape Employees Posts: 5,912

  November 2019

  @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.
  

  Senior Director, Technical Services, EMEA

  0
• EvanReese Member, Mentor Posts: 2,619 PRO

  November 2019

  @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
  

  Evan Reese
  The Onsherpa | Reach peak Onshape productivity
  www.theonsherpa.com

  1