Extend to intersect

Tom Foster
edited April 2019 in 3D Modeling

Best practice advice please, to achieve ...

In the attached dwg, the 76mm thick roof covering slab, white on Layer 0, sloping 12.5o diagonally, was created by DMEXTRUDE from the roof structure slab, cyan on Layer con-Solid, so it has square edges. One edge is pushed back to reveal the cyan slab below.

Now, how to extend the white slab to overhang the walls, on four edges? Slab edges to remain square. The extend to be such that the slab's top edge finishes 300mm horizontally outboard of the wall face.

Can see on two of the faces that I did it by COPY FACE (or was it EXTRACT FACE) as a Region>move 300mm outboard.
Then PUSHPULL, and to my amazement found that 3D Snap to intersection would snap the slab top edge to its intersection with the Region, as I pull it 'through' the Region.
Sort of. It was v tricky and only worked after many tries. And though it snapped, was not quite accurate - the Region is offset 300mm but the extend measures 299.998. So maybe it was just a lucky 'done by eye' operation.

Any better ways to do this?
How is 3D Snap to intersection supposed to work in this situation i.e. how exactly should I move the mouse, maybe UCS or something, to make it work reliably?


  • Hello Tom,
    since you likely used dmextrude without giving the z-axis as a direction, the extrusion of your roof covering has been done in normal direction to the face you selected, so the perimeter faces of the new solid are not parallel to the z-Axis, or your target regions.
    If you first slice the covering solid with the perimeter faces of your (vertical) walls, you'll get a geometry where you can pushpull all 3 perimeter faces at once to one of your regions, and the distance to the walls will be exact (use the zper snap to check the distance - I created a red line as an example measurement).
    If you want the roof perimeter to be inclined rather than vertical, I would dmmove instead of dmpushpull.

  • Thanks Knut for the DMMOVE suggestion because it enables the precision of anchoring the mouse to a base point, compared to PUSHPULL.

    Your last para is correct - inclined perimeter i.e. perp to the top/bottom faces - therefore in your first para, simple extrude was intentional.

    Still looking for insight as to how 3D Snap to intersection is supposed to work in this situation. It seems v tricky to achieve the snap (indicated by tooltip), much mouse movement to find the spot and then ultra-sensitive to losing it again. Some geometry-rules in play, which I'd like to understand and go straight to the spot.

  • Not sure if that helps in any way but ...

    a) there are 2 Intesection options in Snappiong
    I think one is for real intersections, while the other is also
    Intersections that happen only in View Projection (?)

    b) I currently have some snapping problems in v19 generally,
    with Faces in 3D (Both Windows and Mac)

  • a) is in 2D snaps toolbar - I'm interested in Snap to intersection in the 3D snaps toolbar.

    Strangely, the commands 2dintersection and 3dintersection both relate to the 2D snaps toolbar. The 3D snaps toolbar command is Zintersection but its tooltip says 3d intersection!

    Its Help description "Snaps to the intersections of linear entities, edges, and polar or entity snap tracking lines with faces." sounds potent - but I'm struggling to get that tooltip to appear!

  • Perhaps you can try and check v18 if you still have an installation.
    For me there is a lot of v19 tooltips not appearing, or if, being ignored
    issues since adaptive grid invention in general.
    But that is already in Support.

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