Mesh Shell, Plane Cut and Other Mesh Edits

Overview#

Once a mesh is clean, you often need to modify it as a mesh: hollow it for printing, cut it in half to fit a build plate, merge two meshes, or extract section lines to model over. These tools work directly on triangles and do not require conversion to BRep, so they are quick, though less precise than solid features.

Where to find it#

Design workspace > Mesh tab:

• Modify > Plane Cut
• Modify > Shell (hollows a mesh)
• Modify > Combine and Separate
• Modify > Smooth
• Modify > Erase And Fill (Direct Edit only)
• Create > Create Mesh Section Sketch
• Create > Tessellate (converts a solid or surface body to a mesh body)
• Modify > Direct Edit and Convert Mesh
• Modify > Scale Mesh and Extrude Texture (Extrude Texture is only available in Direct Edit in Parametric mode or in Direct Modeling mode)

Step by step#

Plane Cut#

1. Choose Mesh > Modify > Plane Cut.
2. Select the mesh body.
3. Select the Cut Plane: an origin plane, a construction plane or a planar face. Position it with the distance, planar and rotation manipulators, or type X, Y and Z Distance and Angle values.
4. Choose the Type:
   • Trim: removes one side of the mesh and keeps the other.
   • Split Body: separates the mesh into two mesh bodies.
   • Split Faces: slices the faces but keeps a single mesh body.
5. For Trim and Split Body, choose the Fill Type: No Fill (leaves the mesh open), Uniform (closes with evenly distributed faces) or Minimal (closes with the fewest faces). With Trim, the Flip icon switches which side is removed.
6. Click OK. A Plane Cut feature is added to the timeline; right-click it and Edit Feature to change it.

Typical uses: splitting a large print into two pieces, removing a scan stand or base, cutting a flat bottom.

Shell (hollow)#

1. Choose Mesh > Modify > Shell.
2. Select the mesh body. A closed mesh is expected (general guidance); run Repair first.
3. Set Thickness, the wall thickness in design units. Autodesk's help documents only Thickness and Preview for mesh Shell; there is no documented direction or open-face option, so make drain holes separately (see below).
4. Use Preview to see the result live, and click OK.

Shell hollows the body, which saves resin or filament and weight, and Section Analysis lets you inspect the hollow interior. Add holes for resin drainage in SLA/DLP prints afterward.

Combine and Separate#

• Combine: choose a Target Body, Tool Bodies and an Operation: Join (combines and removes internal structures), Intersect (keeps overlapping areas), Cut (removes overlapping areas from the target) or Merge (combines and preserves internal structures). Optionally check New Component or Keep Tools. Both target and tools must be mesh bodies. Check the result with Repair.
• Separate: the Type is Mesh Shells, All Face Groups or Select Face Groups; Keep Body preserves the original. Useful for scans with floating islands or STLs containing multiple parts.

Smooth#

Smooths rough or creased regions of a body or face groups. The dialog has a single Smoothness value from 0 (minimal) to 1.00 (maximum). Use a low value to remove scan noise; too high flattens features.

Erase and Fill#

Enter Direct Edit, select faces (double-click a boundary face of a hole to select the whole boundary), and choose Uniform (regular faces, set by Density) or Minimal (fewest faces, often long thin triangles). Use it to remove a support stub or noise region, or to cleanly patch a hole you chose.

Mesh Section Sketch#

1. Choose Mesh > Create > Create Mesh Section Sketch.
2. Select the mesh body and a Section Plane; an orange line shows where the plane intersects the body. Adjust Distance and Angle with the manipulators or Numerical Inputs.
3. Click OK. A sketch containing a Mesh Section is created in the Sketches folder.
4. Edit the sketch and use Fit Curves To Mesh Section to generate sketch profiles. Create many sections at different offsets to guide a loft or an extrude.

Options reference#

Tips and pitfalls#

Tip: Plane Cut is a quick way to flatten the bottom of a scan to seat it on the build plate. Cut 1 or 2 mm above the base with Trim and the Minimal fill for a flat cap with few triangles.

Warning: Shell requires a watertight mesh. If it fails with an error, run Repair, check for self-intersections, and try again. Very high-resolution meshes may take a long time or run out of memory; reduce first.

Warning: After Shell, thin features may become thinner than the wall thickness and collapse or produce holes. Use Preview and Section Analysis to check thin sections.

Tip: For SLA/DLP prints, put at least two 3 to 5 mm holes through the shell to drain uncured resin, ideally at the lowest and highest points. Add them with a cylinder converted to a mesh (Tessellate) and Mesh Combine (Cut). The 3 to 5 mm size is general guidance.

Tip: For precision parts you will print, a better workflow can be to convert to BRep and use the Solid Shell, which can be parametrically adjusted (general guidance). The mesh shell is the fast option when a mesh is all you have.

Tip: Mesh Section Sketches are the foundation of the reverse engineering workflow: they give you the actual contour to trace.

Warning: Combine does not guarantee a manifold mesh where two meshes overlap. Always inspect after a Combine and run Repair.

Plane Cut in practice#

Plane Cut is as much a preparation tool as an editing tool. The most common jobs and their settings:

Worked numbers: a 180 mm tall figurine must be printed on a 150 mm tall printer. Split at 90 mm with Split Body and Uniform fill, giving two 90 mm bodies, each with a flat cap. To make registration pins, model a 6 mm cylinder as a Solid body, convert it to a mesh with Tessellate (Mesh Combine works on mesh bodies), and use Mesh Combine to subtract the pin hole (Cut) from one half and Join the pin to the other, leaving about 0.2 mm clearance (general guidance) if you design the hole slightly larger than the pin. Check each half with Repair Quick Analysis afterward.

Mesh Combine, step by step#

1. Choose Mesh > Modify > Combine.
2. Select the target body and the tool bodies.
3. Choose the operation: Join, Intersect, Cut or Merge.
4. Optionally check Keep Tools or New Component.
5. Click OK, then run Repair and look at the Quick Analysis result.

Combine does not create a guaranteed clean mesh where two meshes overlap deeply, so before combining, make sure both meshes are closed and the overlap is clean.

Section sketches as measurement tools#

A Mesh Section Sketch gives you the real outline of the mesh at a plane. Place planes at fixed spacing, such as every 10 mm on a 100 mm part, to get 10 sections. You can then measure a hole's diameter by fitting a circle to the polyline, and loft between sections to rebuild a freeform. Use Fit Curves To Mesh Section, or fit simple curves (circles, lines, three-point arcs) over the section, to get usable profiles.

Preparing a hollow print: a full recipe#

1. Run Repair until Quick Analysis shows closed, oriented, positive volume.
2. Reduce to a reasonable count (a hollow shell on a multi-million triangle mesh is slow).
3. Apply Shell at the wall thickness for your process (general guidance: about 1.2 mm on FDM with a 0.4 mm nozzle gives three perimeters; 2 mm on resin is common).
4. Inspect with Section Analysis for wall thickness along thin features.
5. Add drain holes: for resin, two holes of 3 to 5 mm at opposite ends (one low, one high) so resin flows out and air flows in. Make them with a Solid cylinder converted by Tessellate, and Mesh Combine (Cut).
6. Export as 3MF or STL.

Common failures and fixes#

Practice exercise#

Prepare a mesh for printing and record the numbers.

1. Insert a closed mesh figurine and run Repair until Quick Analysis shows no warnings.
2. Use Plane Cut with Trim and Minimal fill on a plane 2 mm above its lowest point. Check that the base is flat with Section Analysis.
3. Use Shell with a 2 mm thickness and inspect the interior with Section Analysis. Record the smallest wall you can find in the thin features.
4. Make two 4 mm cylinders in Solid, convert them with Tessellate, place one at the lowest point and one at the highest point of the hollow, and subtract them with Mesh Combine (Cut) as drain holes.
5. Use Plane Cut with Split Body and Uniform fill on the XZ plane to produce two halves.
6. Right-click each body in the browser and choose Save As Mesh (3MF, STL or OBJ), then reopen one in the slicer and confirm it is watertight.

Expected result: two hollow, flat-bottomed halves with drain holes, ready for printing. If the slicer warns about open edges, run Repair on the half and check the fill type used on the cut.

Related#

• Mesh repair, reduce and remesh
• Converting a mesh to BRep
• Reverse engineering workflow