How to Inspect STL and 3MF Cross-Sections Before Printing

A 3D model can look correct from the outside while still containing geometry that deserves a closer look. A cross-section lets you move a visual cut through an STL or 3MF model, hiding one side so you can inspect the interior from different angles before you spend time slicing or printing.

Open a model locally in the STL Viewer or 3MF Viewer, then use Inspect cross-section below the viewer controls. No file is uploaded just to view a cross-section.

What a cross-section helps you inspect

A cross-section is particularly useful when an external view cannot tell you how a model is arranged inside. Move the cut slowly across the model and rotate the view to look for features such as:

• unexpected internal shells, cavities or overlapping parts;
• holes, channels, mounting features or pockets in the expected place;
• internal supports or ribs that are easy to miss from the outside;
• large areas that appear unexpectedly thin or crowded; and
• multiple 3MF build items that overlap or sit in an unexpected position.

This visual check is useful before a more specific analysis. If a region needs closer thickness review, use the STL Wall Thickness Checker or 3MF Wall Thickness Checker.

How to inspect a model cross-section

1. Open the STL or 3MF file in the matching local viewer.
2. Rotate the outside first so you understand the model orientation.
3. Choose Inspect cross-section.
4. Select X, Y or Z depending on the direction you want the cut to travel.
5. Move the cut-position slider gradually from one side of the displayed model to the other, rotating and zooming as needed.
6. Leave the cut-plane guide on when it helps you understand the current cut, or turn it off for a clearer view of the exposed geometry.
7. Follow up with the relevant checker if you find something to confirm.

Choose the right cut axis

The axis describes the direction in which the visual cut travels across the currently displayed model. An X cut moves left to right, a Y cut moves bottom to top, and a Z cut moves front to back. Try more than one axis for a complex model: a hidden channel may be obvious in one direction and invisible in another.

The percentage shown by the viewer is a position across the displayed bounds, not a guaranteed design dimension. For a specific feature size, use point-to-point measurement and confirm the intended STL unit. For overall printer fit, use the STL Dimensions Checker or 3MF Dimensions Checker.

Cross-sections do not replace validation

The cross-section viewer hides part of the displayed mesh. It does not create a watertight cap at the cut, repair a model, calculate a reliable wall thickness, or prove that a physical print will succeed. Mesh resolution can also make curved surfaces and small features look less exact than the source CAD design.

After visual inspection, run STL Print Preflight or 3MF Print Preflight for mesh, size, fit and other file-level checks. Then, after slicing, use the G-code Preflight Checker to review the resulting toolpath separately.

Model cross-sections versus slicer layer preview

A model cross-section helps you understand the source geometry before slicing. It is useful when deciding whether the file contains the shapes you expect or when choosing what to inspect next. A slicer layer preview is different: it shows the toolpath generated using a particular printer, material, orientation, support strategy and slicer profile.

Use both at their proper stages. Inspect the model first, validate the file, then inspect the final G-code before starting the print.