The SLICE command in AutoCAD: keep one half or both, and why a mesh will not cut

You need to split a solid to show its inside, trim a roof corner, or break a large block into two separate parts to move them apart. The SLICE command in AutoCAD does it: select the solid, define the cutting plane, then keep one half or both.

This article tries everything on the same 1000 × 1000 × 1000 box and reads each part's volume after every cut, to show exactly what the command does. It also covers two points other tutorials skip: how SECTION differs from SLICE, and why a mesh will not cut. For 3D modelling taught properly, the Institute runs an AutoCAD course for construction.

Where to find SLICE

The Autodesk documentation sums it up: "Creates new 3D solids and surfaces by slicing, or dividing, existing objects."

Step by step: halve a box on the YZ plane

Example: cut the box through its middle along X, with a plane parallel to YZ through 500,0,0. The middle column quotes the 2027 prompts verbatim.

Before the cut there is one solid of 1 000 000 000. Afterwards: two solids of 500 000 000 each. The box is split in two with nothing lost; the halves add up exactly to the original volume.

At step 4, instead of Enter, pick a point on one side and AutoCAD keeps that half and discards the other. The <Both> default only applies when you press Enter.

The cutting plane options

The prompt shows <3points> in angle brackets, so pressing Enter at the plane question goes straight to the three-point method. XY, YZ and ZX follow the current UCS: rotate the UCS and the plane turns with it. See our article on the UCS.

Cut a 3D solid through 3 points, keeping one side

The default 3points option cuts on any inclined plane. Example: the plane through the box corners 1000,0,0, 0,1000,0 and 0,0,1000. At the last step, pick 0,0,0 to keep the side containing the origin.

The result is one solid of 166 666 667, exactly 1/6 of the box: a three-sided corner pyramid. The remaining 5/6 is discarded.

This is a quick way to cut a 3D solid at an angle, such as a beam end or a sloping roof, without building a separate solid to subtract. Any three points that are not in a straight line define the plane, so vertices, midpoints and snapped points on the model all work.

Keeping both halves, then separating them

Keeping both halves is handy for showing the inside: cut in two, then move one half aside. The halves are two independent solids, so each can be selected, moved, rotated or recoloured on its own. After SLICE with <Both>, a MOVE of one half reveals the cut face straight away. For a presentation view of a wall or slab build-up, slice through the element, move the front half forward by a round distance such as 1000, and the layers show clearly without any extra drawing.

Autodesk notes: "Sliced objects retain the layer and color properties of the original objects, however the resulting solid or surface objects do not retain a history of the original objects." The halves keep the original layer and colour but lose the modelling history, so change the original dimensions before cutting, not after.

How the SECTION command differs from SLICE

The section command sounds similar but does something quite different. Same box, run SECTION on an XY plane through 0,0,500:

SECTION only extracts the slice: a flat shape exactly matching the cross-section, leaving the solid alone. That region serves for area take-offs, hatching materials, or building a 2D section drawing. Because the solid stays whole, you can run SECTION at several heights, one region per level, and keep the 3D model intact for later changes. Regions are covered in the REGION command.

People searching section plane autocad usually mean a third command, SECTIONPLANE, which places a section object in the model. To split a solid for real, use SLICE; for just the flat shape of the cut, use SECTION.

Mesh to solid: why a mesh will not cut

Select a mesh (made with the MESH command) and run SLICE, and the 2027 release does not cut; it shows a "Mesh - Convert to 3D Solids or Surfaces?" box saying SLICE is not supported for mesh objects, with three choices:

The choice matters more than people think. The same 1000 × 1000 × 1000 mesh box, turned mesh to solid with CONVTOSOLID:

Smooth conversion rounds the edges, so the solid shrinks. For building elements that must keep their size, set SMOOTHMESHCONVERT to 2 first, or choose the faceted line in the box. To convert up front, before cutting:

Converting first also lets you check the solid's size before any cut, which is much easier than discovering a shrunken part after it has been sliced and moved.

The box also has an "Always filter mesh objects from the selection set" tick; as its name says, ticking it drops meshes silently from then on, so if SLICE seems to ignore a solid, check that box.

Which command for which job

The first three all use a flat plane to cut; the fourth uses a second solid as the cutting tool, which suits openings in walls and slabs better than a plane ever could. Choosing the right one first saves undoing and redrawing later.

Three things often repeated that are not true

XCLIP is also sometimes offered as a way to cut 3D solids. It only clips the display of blocks and external references; see the XCLIP command.

Frequently asked questions

What is the SLICE command in AutoCAD?

It cuts 3D solids, alias SL. Select the solid, define the plane, keep one half or both.

How do I cut a 3D solid and keep both halves?

At Specify a point on desired side or [keep Both sides] <Both>:, press Enter.

Why won't SLICE cut my object?

It is most likely a mesh. Convert it to a solid first, preferably faceted to keep its size.

How does SECTION differ from SLICE?

SECTION leaves the solid whole and creates a region of the cut; SLICE cuts the solid into new solids.

Can the two halves be pulled apart?

Yes. They are independent solids; use MOVE on one of them.

Why did my solid shrink after converting from a mesh?

Smooth conversion rounds the edges. Set SMOOTHMESHCONVERT to 2 for a faceted conversion that keeps the exact size.