How do I check a 3D model for carving problems before I make toolpaths (steep walls, tight gaps, separate islands)?

Why check first

The 3D View shows a perfect surface. The machine can only make what a round bit can touch from straight above. Walls steeper than the tool, gaps narrower than the tool, and pieces that are not joined to anything all look fine on screen and fail on the machine. Five checks catch most of it.

The checks

1. Thickness. Open Material Setup. Model Thickness must be less than the material thickness.
2. Wall shape and gap width. Use Create Cross Section Vector and click two points across a suspect area. You get a vector of the model's profile along that line. Look for near vertical walls and deep narrow slots, and use the Measure tool on the gaps. A gap narrower than the finishing bit cannot be cut to its full depth.
3. Steep walls. A ball nose leaves a wall no cleaner than its own shape allows, and a tapered ball nose is wider above the tip, so deep vertical walls are where rubbing and lost detail show up. If the design allows, in Aspire run Add Draft to Model (VCarve Pro does not have this tool). It builds a new baked component in which no wall is steeper than the angle you enter (the documentation's example adds 20 degrees). Compare it with the original before deciding.
4. Undercuts on imports. Undercuts cannot exist in a relief. On import they are replaced by a vertical wall dropped from the silhouette. Check Highlight Undercuts in the import form to see what will be lost.
5. Separate islands. Select every component and click Create Vector Boundary From Selected Components. If the result is a group of several closed vectors instead of one, the model has pieces that are not connected. Tie them together with a backing plane or a Base Height before cutting the part out.

Confirm with a preview

Calculate the finishing toolpath with the real bit and preview it. In VCarve Pro, judge the leftover material by eye in the preview. In Aspire you can go further: use Model, Create Component From, Create Component from Toolpath Preview while holding Ctrl. The new component is the difference between the model and the preview, which is the material the tool could not remove. Use it to draw boundary vectors for a second pass with a smaller tool only where it is needed.

Pro tips

• The official documentation says to judge detail in the preview and move to a smaller tool or stepover if needed. It calls 3D cutting a tradeoff between time and quality.
• If the tool does not cut fully down the outside edge of a raised model, increase Boundary Offset in the 3D toolpath.
• See the entry on minimum detail size versus bit geometry for the 2D version of this check.