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I created a volume by intersecting a cylinder and a cube which were not perpendicular. I obtained a tube with an elliptic section. I want it now to be a true cylinder.

The longitudinal axis is global Y, and the section lives in the XZ plan.

enter image description here
(Initial situation: X=2*1.05=2.1, Z=1.875*1=1.875)

I was expecting it could be done by setting dimension X and scale X equal to dimension Z and scale Z. I first changed the scale Z value to 2, this changed the dimensions Z value:

enter image description here
(After equalizing scales: X=2*1.05=2.1, Z=2*1.067=2.134)

Then I tried then to to set dimensions Z value to the one for X, but actually it also changed the scale Z value:

enter image description here
(After equalizing dimensions: X=2*1.05=2.1, Z=1.969*1.05=2.067)


I've 3 questions that are linked and actually boil down to: How object size is managed?

  • How Blender did choose the new value of the scale on Z?
  • Is it possible to lock the scale while setting the dimension (I was expecting this was done using the padlock, no luck)?
  • What is the proper method to get the result I want?

I would appreciate the answer to the last question, and if possible to the two other.

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In short, you should only care about the dimensions, and not really about the scale. After you get the dimensions right (which would make your elliptical cylinder perfectly circular - with equal Z and X dims), you can Apply the Scale via the shortcut CtrlA > Scale.

Or, if you like menus better, through the Object > Apply > Scale menu.

This will reset the scale to 1.0 in all axes without affecting the dimensions.

Apply scale

Explanation for this behavior:

This behavior is due to the fact that an object is an instance of geometrical data, that has a location in 3D space, scale (multiplication of the original geometry's size) and rotation.

In other words, scale is a relative property, which is a multiplication of the "base" scale (of 1, i.e. 100%) - represented by the geometry being instanced. This is why you can have different copies (instances) of the same base geometry that each have a different location, rotation and scale, but all share the same set of vertices, edges and faces in the same basic arrangement in space.

To see this in action try creating instances (AltD) of an object instead of ordinary duplicates (ShiftD). When you go to edit mode and edit one instance, all other instances are updated.

Instances

So in summary, even if you have just one instance of a geometry (such as in your case), it can still have a scale that differs greatly from the base geometry.

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    $\begingroup$ Thanks for the good explanations. Actually my cylinder was already "circular" :) However I was pretty sure it was still an ellipse, and I compared with a duplicate rotated by 90°. This confirmed the radius was not constant. But I understand why. The section has an odd number of segments. The Z dimension is between two opposite vertexes, the X dimension is between two opposite faces. When they are equal, the vertexes of the latter faces are indeed further from the center :( $\endgroup$ – mins Nov 8 '15 at 18:28

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