Ken Baker left a suggestion on the Topic Suggestion page. I prefer working on technical problems as opposed to writing about crap like clouds or whatever. So if you send me more suggestions, I”ll post less crap and more good stuff.
Here”s the issue. Ken has a 3D surface, and he wants to find the offset of the edges. You can kind of but not really do this with offsetting stuff in a sketch and projecting the sketch, the problem is when a line perpendicular to the surface at the edge is not parallel to the sketch projection direction, you will just get a bad approximation as the result. The picture to the right spells out the problem. The construction line that is perpendicular to the surface at the edge is not parallel to the projection direction, so projecting the rectangle onto the surface will not produce the same result as offsetting the edge of the surface in 3D. Trying to use a projection or a trim is trying to solve a 3D problem with 2D. Because of the symmetry of this simplified example, you might get away with an approximation, but you can do better.
The short answer is that SolidWorks doesn”t have the tool to do this directly. What you need on a conceptual level is a 3D sketch offset. This kind of thing has to be hard to do, because how do you describe the offset direction when the offset direction could be so variable. Anyway, since we don”t have a direct method, let”s work on a workaround.
To remove the simplification of the symmetrical surface, let”s work with my favorite, the dreaded modified egg carton. This was created by lofting 5 spline squiggles, and then drawing a spline-on-surface around the border to trim it so that it wraps around more than 180 degrees, doesn”t have symmetry or straight sides, and basically offers no shortcuts.
I have two possible solutions for this, both feel like workarounds, but are closer than the projected sketch approximation.
#1 – Ruled Surface offset
Because this is a 3D problem you Â have to treat it as a 3D problem. What kind of things can we offset? 2D sketches and surfaces. Ok, we can”t use a 2D sketch, so let”s try to do this by offsetting a surface. If we were able to offset a 3D sketch, the offset would work in a direction perpendicular to the edge, along (parallel to) the changing surface. I don”t know how to get where I need to take you using a geometrical proof style of argument, so I”ll just make the leap.
If you create a surface around the edge of the surface where the new surface is always perpendicular to the existing surface at the edge, you can then offset the perpendicular surface, and the intersection of the offset surface with the surface will give you the offset edge. It turns out that the Ruled surface can make a surface that is always perpendicular to a surface along a set of edges. I told you ruled surfaces were great construction geometry tools. Follow these steps:
1. Make a Ruled Surface around the edge of the surface – use the Normal To Surface option, and make the distance something that will work. If you have tight curvature on the original surface, the ruled surface has to be smaller.
2. Offset the new Ruled surface by the amount you want to offset the 3D sketch. Again, tight curvature will make this more likely to fail. Offsets follow rules. The original surface is blue, the Ruled surface is black, and the offset ruled surface is yellow.
5. Make a boundary surface from the loop around the original surface to the loop around the offset and trimmed red surface. Getting this to work will involve using connectors to keep the boundary surface straight. Connectors is a blog post topic all its own. The secret here is to make the connectors as short as possible. Go straight from one surface to the closest point on the edge of the other surface.
#2 Face Curves U-V
The second way may be less risky, but it will only be usable in certain situations. You can”t use this method on the part I just showed, but you could use it on the original part Ken sent. I”ll use it on the part I created, but without the spline-on-surface trim. Here”s why.
This method uses the U-V curves of a face. In order to for an offset to work, the surface must be the original 4-sided surface – it must be untrimmed. If you look at the picture to the left, notice that the blue and pink lines (created by the Face Curves feature) do not run parallel to the part edges.
If you look at the image to the right, the lines are parallel to the edges. The right is the underlying untrimmed version of what”s on the left. The U-V lines can be used as offsets because if you think of the entire length or width of the face as a distance of 1, you can offset an edge by a percentage, which you should be able to figure out as an equivalent distance. So let”s pull back 10% from each edge. Here”s how to do it:
1: Use the Face Curves feature while in a 3D sketch. You can do this outside of a 3D sketch, but it will be lots more work. Use the Position option and set the blue and pink positions to 10%. Click the green check to accept the curves in the 3D sketch.
2: Start the command again, but this time use 90% for both blue and pink. Click the green check again to accept the curves.
3: Next (still in the 3D sketch) use the Trim Sketch command to trim off the corners so you are left with a 3D rectangle on the surface where all of the sides of the rectangle are offset from the edges. Be careful, because Trim is buggy on 3D sketches. It will often clip off the part you want to keep. You may have to rotate the point of view to get it to select properly.
4. You can start the Surface Trim feature while still in the 3D sketch. Make sure the type is set to Intersection, and then discard the outside of the surface.
5. Loft or boundary the outer edges of the surfaces to get the result.