"Map sketch to face" - cylinder inertia attachment

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llll
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"Map sketch to face" - cylinder inertia attachment

Post by llll »

Trying to map circles to the cylinder face, for a pocket..

What are these inertia axes & how to apply them to a sketch?

Ty for the assist.

error: "InertialCS: inertia tensor has axis of symmetry. Second and third axes of inertia are undefined."
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Map sketch to cylinder.FCStd
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drmacro
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Re: "Map sketch to face" - cylinder inertia attachment

Post by drmacro »

Can't map a sketch to non-planer surfaced.

Use Part Project on surface or Curves Sketch on surface.
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drmacro
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Re: "Map sketch to face" - cylinder inertia attachment

Post by drmacro »

This file is pretty broken...unless, maybe you are using linkstage and the Pad features in a Part container are ok there..

At least that's what I see when I open in 0.20
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papyblaise
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Re: "Map sketch to face" - cylinder inertia attachment

Post by papyblaise »

whith Curves Wb > Sketch on surface :?:
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map on surface.JPG
map on surface.JPG (50.18 KiB) Viewed 945 times
Map sketch to cylinder2.FCStd
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bambuko
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Re: "Map sketch to face" - cylinder inertia attachment

Post by bambuko »

drmacro wrote: Sun Jun 05, 2022 3:21 pm This file is pretty broken...unless, maybe you are using linkstage
Yes, OP is using Linkstage3 (if it was Master it would ask to recalculate when I opened it in Link branch FreeCAD).
OP has been asked before to provide full version info when asking questions ... :roll:
(otherwise it confuses forum contributors and wastes their time on guessing).
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Re: "Map sketch to face" - cylinder inertia attachment

Post by edwilliams16 »

llll wrote: Sun Jun 05, 2022 1:06 pm
What are these inertia axes & how to apply them to a sketch?

error: "InertialCS: inertia tensor has axis of symmetry. Second and third axes of inertia are undefined."
It's a theorem in advanced mechanics that every body, whatever its shape has a set of orthogonal principal (or inertia) axes. These are the only fixed axes around which the body will continue to freely spin in the absence of external forces. For a rectangular block these axes are parallel to the edges.
If the the body in question is symmetric we have to modify the above. Take the earth, an oblate spheroid. It spins freely about its polar axis. However it could also freely spin about any equatorial axis through the center. These axes are all equivalent.
A sphere is even more symmetric - any axis through its center is a principal axis.

For a 2D object like a sketch, its normal is one principal axis. The other two lie in the plane and are well-defined unless the sketch is rotationally symmetric - like a circle around the origin. The principal axes are then ill-defined and FreeCAD complains.

If you experiment with a rectangular block, you'll find something interesting. You can easily spin it around its longest axis or about its shortest axis. However, it will be very difficult to spin it around its intermediate axis. It isn't stable. The slightest disturbance and it will start to tumble.
llll
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Re: "Map sketch to face" - cylinder inertia attachment

Post by llll »

This is a strange workflow to get bounding box dimensions into sketch. What's the better way to get face dimension?

@ChrisG
Can you speak to how to get better precision here when aligning to rays?

Note:LinkStage3 file
Attachments
FreeCADLink_WtjJDNM582.png
FreeCADLink_WtjJDNM582.png (24.33 KiB) Viewed 793 times
Map sketch to cylinder (close).FCStd
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chrisb
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Re: "Map sketch to face" - cylinder inertia attachment

Post by chrisb »

Moved from Help forum. The issue can be the same in master, but uploaded files may behave differently.
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freedman
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Re: "Map sketch to face" - cylinder inertia attachment

Post by freedman »

You can use "Regular Polygon" to make multiple faces on calculated angles. Here (pic) I made a sketch using regular polygon with 90 faces, then trimmed away the ones I didn't need. Padding then gives me faces to sketch on that are planar and focused on a common center.
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reg_poly.png
reg_poly.png (169.45 KiB) Viewed 558 times
reg_poly1.FCStd
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