Placement

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Overview

"Placement" is how FreeCAD specifies the location and attitude of an object in space. Placement can be specified in multiple forms.

Angle, Axis and Position

File:PlacementPropertiesv10.png
Properties affecting Placement

Placement = [Angle, Axis, Position]

Angle = r is a scalar indicating the amount of rotation of the object about Axis. Entered as degrees, but stored internally as radians.

Axis = (ax,ay,az) is a unit vector describing an axis of rotation (See Note about axis of rotation). Examples are:

   (1,0,0)       ==> about X axis
   (0,1,0)       ==> about Y axis
   (0,0,1)       ==> about Z axis
   (0.71,0.71,0) ==> about the line y=x
                                        

Position = (x,y,z) is a Vector describing the world coordinates of the reference point of the object.


Position and Yaw, Pitch and Roll

Placement as Yaw, Pitch and Roll

Placement = [Position, Yaw-Pitch-Roll]

Position = (x,y,z) is a Vector describing the world coordinates of the reference point of the object.

Yaw-Pitch-Roll = (y,p,r) is a tuple that specifies the attitude of the object. Values for y,p,r specify degrees of rotation about each of z,y,x. Sometimes referred to as "Euler angles". (http://en.wikipedia.org/wiki/Euler_angles)

Matrix

(see illustration of Python console) Placement = Matrix is a 4x4 affine transformation matrix (http://en.wikipedia.org/wiki/Affine_transformation) of the form:

  ((r11,r12,r13,t1),
   (r21,r22,r23,t2),
   (r31,r32,r33,t3),
   (0,0,0,1)) , with rij specifying rotation and ti specifying translation.


Placement Dialog

Placement Dialog

The Placement Dialog is invoked from the Edit menu. It is used to precisely rotate/translate objects. It is also used when we need to create a sketch on a "non standard" plane or change a sketch's orientation to a new plane.


The Apply incremental changes to object placement tick box is useful when translations/rotations are to be made relative the object's current position/attitude, rather than to the original position/attitude. .


Notes

  • The axis of rotation isn't really the x/y/z axis. It is a line parallel to the selected axis, but passing through the Position point.
  • The reference point of an object varies depending on the object. Some examples for common objects:
Object Reference Point
Part.Box left (minx), front (miny), bottom (minz) vertex
Part.Sphere centre of the sphere (ie centre of bounding box)
Part.Cylinder centre of the bottom face
Part.Cone centre of bottom face (or apex if bottom radius is 0)
Part.Torus centre of the bounding box
Features derived from Sketches the Feature inherits the Position of the underlying Sketch. Sketches always start with Position = (0,0,0).


Issues

  • As of version 0.13, update of Placement properties in the Data tab has been disabled for objects created with PartDesign. Placement functionality will eventually be handled in the Assembly workbench.
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