What is the Modeling Plane, why do components sometimes appear below the surface, and how do I fix negative Z values?

The Modeling Plane is the flat reference surface from which all 3D component heights are measured. Think of it as the floor of your model: everything built above it has positive Z values, and anything below it has negative Z values. Understanding it helps you troubleshoot models that sink, trigger depth warnings or give unexpected toolpath behavior.

What the Modeling Plane Is

The Modeling Plane is not the same as your material surface. It is the base plane of the 3D composite model. In Material Setup, the Modeling Plane Z field reports the height of that plane once you have positioned the model in the material with Gap Above Model or Gap Below Model. It is for information only, results from those gap settings, and cannot be edited directly.

Components are assembled relative to this plane. The first component's combine mode only decides how it combines with the modeling plane. A Zero Plane component (Model > Create Component > Add Zero Plane; in Aspire you can also hold Shift while clicking the Flat shape tool) is a flat component the size of your job with a height of zero.

Why Components Go Below Zero

Negative Z values can appear when:

• An imported STL model has data below the import origin, such as the undesired thickness of a 3D scan.
• You sculpt a model, or combine it with a deep Subtract component, and part of the result ends up below the plane.

Negative Z shows up when you hover over the 3D model and the Z readout at the bottom of the screen is a minus number. It can also cause the Material Setup warning that the model thickness exceeds the material thickness.

When Negative Z Is a Problem

• The model is positioned at the bottom of the material block, so going below zero means cutting through the bottom of the material.
• The composite model is thicker than your material, which triggers a Material Setup warning.
• You plan to export the model as an STL or as 3D clipart and need a clean base.

If the model sits in the middle or top of the material block, small negative values may not affect the cut at all.

Three Ways to Fix Negative Z

Method 1: Replace Below (Aspire)

Select the component(s) and open Model > Replace Below. It trims or flattens the bottom of a component. Tick Lower Component by Value and set it to 0 to remove everything below the Zero Plane. You can also set Replace Value Below and Replace Value With (Transparent removes those areas, Plane fills them with a plane at that height). If you select several components or a Level, they are baked into one component first.

Method 2: Zero Plane with the Merge Combine Mode

1. Add a Zero Plane component (Model > Create Component > Add Zero Plane).
2. Open its Component Properties and set its Combine Mode to Merge.
3. Place it above the component with the problem in the Component Tree.
4. Merge keeps the higher of the two, so anything below zero is replaced by the flat plane at zero.

This adds a component to your tree but is reliable in both products.

Method 3: Slice the Model

Slice Model divides the composite model into Z-slices that each become a component. If the negative data is real geometry you want to trim, slice at the height you need and delete the bottom slice. Hide any components you do not want included first, and remember the original components stay in the tree alongside the new slices.

Bonus: Cutting Planes (Zero Plane as a Cutter)

You can use a Zero Plane to cap the top of a model. Give the Zero Plane a Base Height equal to the height you want to cut at, set its Combine Mode to Low, and place it above the other components. Low keeps the lower of the overlapping heights, so everything above the plane's height is cut off. This is useful for slicing the top off a tall component or making a flat-topped relief.

Viewing Negative Z

Hover over the 3D model. The Z value in the status bar at the bottom of the screen is the model height at that point.

Common Mistakes

• Using Add on a Zero Plane instead of Merge. This can inflate the model rather than floor it.
• Using Low on a Zero Plane at height zero and expecting it to remove only the negative parts. Low keeps the lowest values, so it flattens everything above zero and keeps the negative areas.
• Assuming the Modeling Plane Z must read zero. It follows your gap settings.