Surface edge moving?

[MArruda]

Hello all,

I'm trying to model something to put out to my CNC. The ouput needs to be slightly undersized from the original, but the curvature of the top of the unit has to be the same.

Here's how I did it-
1. Make point cloud of original on CNC.
2- Import point cloud into sketchup, edit a couple points, triangulate and save out to .stl
3- Make outline profile in Corel Draw to exact dimensions necessary and ouput as .dxf
4- Import the .dxf and .stl models into Rhino, placing the model over the outline, centered.
5- Drape surface over triangulated surface.
6- Extrude curves of outline up above model and use boolean difference to trim the model to the boundary of the outline necessary
7- Edit control points to smooth model as necessary
8-?

8 is where I get lost. Since the surface was removed outside the boundary of the outline, the "edges" of the model are above the z0 plane of the model. So, I have the ouline at z0, and exactly perpendiculat to that, above it, is the edge of the model. The edge is not flat, and I need the height to stay where it is, so I can't just move the model down.

Is there any way to morph the edge of the surface so that it comes down to the outline curve? I know it would change the contour of the top surface some, but as long as the height and outline are where they need to be, the contour could be a little off. Editing the control points to bring it down is frivolous- there's too many, not in line with the ouline, and it's just wrecking the curvature of the model.

Any ideas?

Thanks,
Michael

[chmillman]

Well, I'm still not really sure what you're trying to do here - you have a "scan" of an original object, you make a mesh from it and then...

You say it needs to be slightly smaller than the original - it appears that you want to create an outline smaller than the original extents and trim the excess of the model off with that outline. The problem is then that the edges are no longer at Z0 (I assume the original edges were) as the trimming cuts the edges back from the original and so they have a certain Z height.

What I might do is the following - depending on how coarse or fine your mesh is:

Don't do the drape. Work instead on the mesh itself. Use Rhino's MeshTrim to trim off the edges with the curve. Turn on control points for the trimmed mesh, select all the controlpoints on the outer edge and then use the command SetPt to set all the Z levels of those points to 0. The result will be that the edge of your mesh object is now at Z0 as you want, but depending on the mesh, the transitions may not be that nice...

Otherwise, it's very difficult to get an arbitrary freeform trimmed NURBS surface to have a trim border that passes through specific points - only untrimmed surfaces can be made to do that. That's not a Rhino limitation, it's a NURBS one.

Other tactics you can try: trim the surface back further than you want by a certain amount and then useBlendSrf to blend between your trimmed surface and the curve where you want it to finish at 0 (you may need to create a dummy surface to blend to). Or, try using Patch to patch through your original scan points and the outer edge curve - it may get close enough to 0 at the edge to be satisfactory.

--ch

[MArruda]

Thanks for the reply!

I don't think I'm going to be able to do what I want to do- the curvature has to remain smooth, so joining the edge of the mesh won't work. It would give me an odd profile where it edited tha twouldn't match up well to the rest of the model.

The reason I'm draping asurface is because the trianglated surface is too polygonal. with draping, I make the surface smooth and then after editing, rebuild a mesh from the nurbs surface with much better triangulation to output as an STL for cut3D to make a tool path from.

-Michael

[chmillman]

Well, as I said earlier - you might try running a Patch surface through the points plus the trim curve at Z0 - after first eliminating the points outside the trim curve. You may need to play with the point settings and stiffness to get it to pass through the trim curve within tolerance. Not having seen the point cloud or object, it's hard to tell what will work - there are no hard and fast rules on this stuff. --ch