What are the strengths and limitations of the 3D height-map system, and when should I use 3D vs. 2D toolpaths?

Overview

The 3D system is a pixel-based height map, not a true vector or solid 3D modeler. Understanding what that means tells you when 3D shines and when you should use 2D or 2.5D toolpaths instead. It is one of the most important concepts for 3D users.

How the Software Represents 3D Models

It does not work with true 3D meshes or solid geometry internally. Everything becomes a height map: a grid of pixels where each pixel stores a single Z height.

• White is the highest Z height and black is zero height (this is also how the grayscale component previews and bitmap-based components are read).
• Each pixel is one point on the composite model surface.
• The number of pixels is set by the job resolution and the job area.

When you import an STL file, the mesh is converted to this height map. In Aspire, Create Shape generates height-map data from your vector outline, and sculpting edits height-map values. The documentation puts it this way: Aspire only represents base-relief, so it cannot work with a completely 3D object. A model with a front and back must be oriented and sliced when it is imported.

What 3D Does Well

• Organic shapes: faces, animals, foliage, flowing curves, relief carvings, scrollwork and decorative panels, with gradual curved surfaces.
• Layered composition: building complex scenes from simple components that combine into one composite.
• Soft surface transitions: blending, merging and sculpting all work naturally on a height field.
• Clipart: V3M files from Vector Art 3D and Design and Make are made for this system and import as height-map components.

What 3D Does Poorly

• Straight edges and precise geometric shapes: in a height map, an edge is defined by the pixel grid. At a higher resolution the steps get smaller, but they are still there.
• Vertical walls: a height map stores one top-surface height per position, so a perfectly vertical wall cannot really be represented. This is one reason Add Draft to Model exists.
• Undercuts: a feature that needs cutting under another part of the model cannot be represented. On import, anything that is an undercut is discarded and a vertical wall is created down from the silhouette.
• Mechanical or CAD geometry: precise boxes with sharp corners, perfect cylinders and dimensioned holes look wrong, because the pixel approximation produces slightly bumpy edges.

The Right Tool for the Job: 3D vs. 2D

For many designs the answer is a combination of both: 3D for the sculpted relief elements and 2D or 2.5D for the crisp geometric elements.

Use 3D toolpaths for:

• Organic relief carvings (faces, animals, nature scenes).
• Dimensional signs with curved backgrounds and sculpted elements.
• Any model where the surface contours define the final appearance.

Use 2D or 2.5D toolpaths instead for:

• Text on a flat background. A V-carve generally gives sharper results than 3D finishing.
• Pockets with flat bottoms. A pocket toolpath with a depth is cleaner than a 3D model.
• Profile cutouts. A profile toolpath cuts clean vertical walls, while 3D finishing of steep walls looks rough.
• Geometric inlays. Use 2D profile and pocket toolpaths for a crisp fit.
• Mechanical parts with precise tolerances.

The combined approach:

1. Model the organic relief elements (background dome, sculptural figures, decorative accents).
2. Use 3D roughing and finishing toolpaths for those areas.
3. Use a separate V-carve toolpath for text and detailed vectors, with Project toolpath onto 3D Model ticked so it follows the 3D surface.
4. Use a profile toolpath for the final cutout of the piece.

This hybrid approach usually works better than doing everything in 3D.

Improving 3D Quality for Geometric Models

If you must machine something geometric, such as a logo with both an organic relief and crisp lettering, using the 3D system:

1. Use the resolution wisely. Set the job size to just fit the piece, and consider a higher resolution setting (Standard, High or Highest) if the part has small details.
2. Run a profile toolpath on a vector boundary after 3D finishing, to clean up crisp geometric edges.
3. Add Draft to Model (Aspire). Near-vertical walls with a slight draft angle machine much more cleanly than true verticals.
4. Project toolpath onto 3D Model. A V-carve or profile toolpath with this option follows the 3D surface contours while keeping the precision of the 2D vector.

Large Model Resolution Strategy

For very large panels with fine detail, a technique discussed in the software maker's user forum is to get a higher effective resolution by splitting the work:

1. Perfect the full-size design at whatever resolution you can.
2. Divide the design into sections.
3. For each section, create a new job sized to just that section at a higher resolution setting.
4. Bring the section component into the smaller job and refine it with sculpting and smoothing.
5. Machine each section separately.

Because the resolution is spread across the whole job, a smaller job gives more pixels per square inch for the same setting. Test the approach on a small piece first.