Moved from UX Thread. I was asking @goodduck how he creates his model skeletons ( for structure and stability) in Rhino. I had tried many different ways Boolean Operator, cut, trim and many others but basically gave up, thinking that it just wouldn't work with the objects I have to deal with on assignments. If you follow have followed that thread then you know there is his solution and my attempts to follow. His solution works! Some of the time, and most of the time, but here are some things I have found that occur when following his example. First off, he created his model in Rhino, which makes a big difference. Models that you gather from the net, or create in other software may or may not work. Rhino has its own definition of what an object mesh is. When you load a .obj mesh, Rhino says its a mesh but doesn't treat it as a Rhino mesh. Commands that work on RM (Rhino Mesh) do not work as they should on and OM (Object Mesh), that includes .obj, .fbx, .3ds or any other mesh not created inside of Rhino. So there are some things you just CAN NOT DO. On to what happens or what I have observed in the wee hours of this morning. Wanting to create a skeleton I followed @goodduck and knowing that a mesh cannot be trimmed if there are gaps or any part of what you are cutting cannot be within another part not selected as a cutter. Example, Want to create a frame to support the skin in the tail section of this HE-111. Position is EXTREMELY important in this case as it has a full interior, forward of where I placed this CP (cutting Plane). So I placed it just aft of the interior walls and just forward of the belly greebles. Below I added a second CP aft of the first and one vertically down the center line ( the vertical one failed completely as it touched too many things and I did not realize at this time I could have shortened it, as I thought it HAD to extend beyond the CM (cutting mesh). I removed the Vertical on and added a CP horizontally, shortening to not touch anything outside the CM. I used the function MESH TRIM to TRIM all of these CP to the inside skin of the tail section. First the Horizontal then the Verticals. ?? did not go well on first try and on subsequent tries. Several things happened. Forward CP did not do anything , second worked perfectly ? as it cut the CP and removed all the outer section with one click and the third cut the CP but left it attached to the inner section. As you can see here. This shows that it traced the inner surface of the skin but did not cut it. Using EXPLODE it separated it just fine as shown here. I then dealt with the verticals... I found out that if a just left-clicked the vertical CP it might work, but leave the outer parts still attached, or it might cut part and leave behind sections of the outer parts, or it just worked. After working on it for some time I found I had to hold down when left-clicking then it would cut and remove outer section. As shown here... Then I used Dup-Border to get an outline of the parts. That worked just fine. Trying to offset gave me problems as all would work except the aft vertical, it just would not offset a curve or it would only do a small section, fixed that eventually by changing the tolerance from .001 to .1 and then it worked, as seen here... So for at least I finally managed to get-er-done (hmm) just need to delete the outer curves an flatten them for a top view to print... first I have to figure out how to create the halfway slices so that they will mortice together. I think I will do another doing the same thing in Blender, as many many people use it and it is FREE for all.