Cutting an object with skew plane

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carlos420
Posts: 24
Joined: Fri Apr 27, 2012 6:31 pm

Cutting an object with skew plane

Post by carlos420 »

Hi.

Great project! I am running FreeCAD 0.12 rev5382 installed from packages on 64-bit Ubuntu (Pangolin).

I've been through a few tutorials and I do not see an answer to this yet. Let me give a fully constrained example to make this clear:

Step 1. I have a solid. Let's say it is a 1x1x1 cube made by starting with a square sketch with diagonal vertices (0,0)-->(1,1) and then padding that sketch 1 unit in the z-direction. So the cube has one one vertex at the origin, and the body-diagonal vertex is at (1,1,1).

Step 2. Now I want to cut the cube in half with a construction plane with surface normal (1,1,1) and passing through the point (0.5, 0.5, 0.5). This is a skew plane for construction only, so it must not become part of the physical geometry.

I can do step 1. I need help to do step 2. I need to do this in the GUI, and I will eventually need to script it.

UPDATE: I can solve the related but simpler problem of cutting a cube along a face-diagonal. This is easy, because the construction geometry can be sketched on a face of the cube.
What would really help is a "3D curve" or "space curve" not constrained to a face. How can I do this in the GUI? (I'm pretty sure I can script it.)

UPDATE: I solved this by creating a planar primitive (Part > Parametric > Create Primitive) with a manually specified axis and position, then sketching a rectangle on that plane and padding the rectangle to make a cutting tool. Finally, do a Boolean difference. This works pretty well, but I really need to parameterize the orientation of the cutting plane based on three verices which can move. It would be really nice if I could create a plane plane by specifying three vertices in the existing part. Is there a way to do this?

Thank you!
Last edited by carlos420 on Fri Apr 27, 2012 10:17 pm, edited 1 time in total.
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NormandC
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Location: Québec, Canada

Re: Cutting an object with skew plane

Post by NormandC »

Hi,

I confess having a hard time visualizing your cube and plane's placement. What is a "body-diagonal vertex"?

In any case, at the moment you cannot use a plane to cut a solid. So you need to create a second solid which you will cut from the first using a boolean cut in the Part workbench. As there is no workplane utility yet in the Part Design workbench, you can create your second sketch on a standard plane, then change its placement with the Edit --> Placement menu. Be aware that the sketch drives the base feature, if you attempt to change the placement of the feature itself, it will revert to its original position.

O/T: so you're on Precise already? I'm thinking of migrating this weekend. I need to so I can create a Precise branch in the FreeCAD PPA. :)
carlos420
Posts: 24
Joined: Fri Apr 27, 2012 6:31 pm

Re: Cutting an object with skew plane

Post by carlos420 »

normandc wrote:Hi,

I confess having a hard time visualizing your cube and plane's placement. What is a "body-diagonal vertex"?
Thanks for your reply!

It's not as bad as it sounds. Instead of looking at a side of the cube, look at it in a "pointy" direction, i.e. the way which would hurt the most if you head-butted it. :)
Now imagine "pushing" a plane past that pointy bit and into the solid. Body diagonal is just the a connecting two maximally distant vertices.

A cube has four body diagonals of equal length, D = SQRT(L^2 + L^2 + L^2) = L*SQRT(3). The simplest to think about connects the origin vertex to (L,L,L).

When I say "body-diagonal vertex, I'm being a bit imprecise. I really mean, the vertex, which together with the currently considered vertex, defines a line-segment which satisfies the definition of body-diagonal. This nomenclature comes from crystallography and point-group symmetry. I've never seen it used by CAD folks, and they always look at me funny, so I suppose I should stop. :)
normandc wrote: In any case, at the moment you cannot use a plane to cut a solid. So you need to create a second solid which you will cut from the first using a boolean cut in the Part workbench. As there is no workplane utility yet in the Part Design workbench, you can create your second sketch on a standard plane, then change its placement with the Edit --> Placement menu. Be aware that the sketch drives the base feature, if you attempt to change the placement of the feature itself, it will revert to its original position.
I figured out how to do it, but I did not know that last bit about being careful not to move the feature directly. Thanks!
normandc wrote:O/T: so you're on Precise already? I'm thinking of migrating this weekend. I need to so I can create a Precise branch in the FreeCAD PPA. :)
Yes. I have a clunker box that I can afford to burn. :) Just a day on, Precise is holding up nicely. I haven't noticed any big changes. There are some nice UI tweaks, not as many as I hoped. I'm still nto crazy about Unity, but it's getting better.

Thanks!
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