Shortcuts

AutoCAD UNI Shortcut : UNION : Unions two solid or two region objects

If you need to merge two or more 3D objects in AutoCAD, this guide explains in clear, step‑by‑step detail how to use the UNION command (often referred to as the union shortcut), why it sometimes fails, useful alternatives, and practical tips to get reliable results.

What is the union shortcut?

The UNION command in AutoCAD is a Boolean operation that merges two or more 3D solids or regions into a single solid/region. You can run it by typing UNION at the command line (AutoCAD may accept a short unique prefix such as UNI if no other command shares that prefix), or by using a customized command alias.

Key points:

  • Works for 3D solids and regions (not directly for 2D polylines).
  • Produces a single object from selected objects when they are touching or intersecting (see troubleshooting if it doesn’t).
  • Used together with other Boolean commands such as SUBTRACT and INTERSECT.

Quick explanation of related terms

  • 3D solid: A solid (volume) object created by commands like BOX, EXTRUDE, REVOLVE, etc.
  • Region: A planar area created from closed 2D curves using REGION; regions can be used in Boolean operations.
  • Boolean operations: UNION, SUBTRACT, INTERSECT — operations that combine solids/regions logically.

How to use UNION — Step by step

  1. Prepare the objects:

    • Make sure the objects are 3D solids or regions. If you have closed 2D polylines, convert them with: type REGION → select the closed curves → Enter.
    • Ensure the solids/regions are coplanar (for regions) or properly positioned in 3D (for solids).

  2. Activate the command:

    • Type UNION at the command line and press Enter.
    • (If you set an alias like UNI to map to UNION in your acad.pgp, you can type that instead.)

  3. Select objects:

    • When prompted (Select solids), click the solids/regions you want to merge or window-select them.
    • Press Enter when selection is complete.

  4. Review result:

    • AutoCAD will merge overlapping or touching solids into a single solid. Use a shaded visual style (e.g., 2D Wireframe -> Realistic or Shaded) to confirm visually.
    • Check the resulting object by selecting it — the Properties palette should show a single object.

Example: Create two overlapping boxes (use BOX), move them so they intersect, type UNION, select both boxes, press Enter → they become one combined solid.

When UNION doesn’t work — common causes and fixes

  • Objects are not the right type:

    • Problem: You tried to union 2D polylines or curves.
    • Fix: Convert closed curves to REGION (type REGION and select the curves) or extrude them into solids using EXTRUDE.

  • Objects do not touch or intersect:

    • Problem: The solids are separated by a gap.
    • Fix: Move or align objects so they touch or overlap. Use ALIGN, MOVE, or snap coordinates.

  • Objects are part of a block or xref:

    • Problem: Objects inside blocks or external references cannot be directly unioned.
    • Fix: Explode the block (EXPLODE) or edit the block definition, or bind the xref. Convert block geometry to editable solids.

  • Objects are on a locked or frozen layer:

    • Problem: Selection fails or is ignored.
    • Fix: Unlock/unfreeze the layer first.

  • Objects are not planar/coplanar (for regions):

    • Problem: Regions must lie in the same plane.
    • Fix: Ensure the curves are coplanar, or use solids instead.

  • Corrupted geometry or thin/invalid solids:

    • Problem: Solids with invalid faces or extremely thin geometry can prevent Boolean ops.
    • Fix: Use AUDIT/RECOVER, recreate the geometry, or simplify shapes. Use INTERFERE to test overlaps and MASSPROP to check mass properties.

  • Command alias conflict or typo:

    • Problem: Short alias typed maps to a different command (e.g., U is UNDO by default).
    • Fix: Type the full command UNION, or create a custom alias in acad.pgp (see Tips).

Error messages you may see and what they mean:

  • “No solids found” — you selected objects that are not solids/regions.
  • “Objects must intersect or be tangent” — objects are not touching; move them or adjust.

Alternatives to UNION (when union is not the right tool)

  • JOIN: Joins 2D lines, arcs, and polylines into a single polyline (not for 3D solids).
  • PEDIT: Edit polylines — convert single segment polylines to joined polylines.
  • REGION + UNION: Convert closed 2D curves to regions (REGION) then union regions.
  • EXTRUDE + UNION: If you need volumes from 2D shapes, extrude them into solids then union.
  • SUBTRACT / INTERSECT: Other Boolean operations used for cutting or keeping overlap volume.
  • GROUP or BLOCK: If you only need to manage multiple objects together without merging geometry, use GROUP or create a BLOCK.
  • MODELSPACE commands / 3D modeling tools: For complex merges, consider using commands like SOLIDEDIT or using the 3D tools in an advanced modeling workflow.

Practical tips and best practices

  • Always work in a 3D visual style (Shaded/Realistic) while performing Boolean ops to catch visual problems.
  • Use UCS and ORTHO to align objects precisely before union.
  • If you perform many union operations, consider saving a backup or using WORKSPACE versions so you can rollback with UNDO.
  • To create a convenient shortcut, edit acad.pgp to map an unused alias (for example UNI) to UNION; then reload the profile.
  • Use INTERFERE (or the 3D toolset) to check whether two solids actually intersect before attempting union.
  • If union produces unexpected topology, try unioning two objects at a time rather than many at once.
  • Run AUDIT and OVERKILL (for 2D cleanup) to remove duplicated or bad geometry before creating regions or extrusions.

FAQ

Can UNION merge non-touching solids into one object?

No—UNION typically requires solids to intersect or be tangent. If solids are separated by a gap they will not merge. Move or align them to touch, or create an overlapping region and then apply UNION.

How do I create a keyboard shortcut or alias for UNION?

Open your acad.pgp file (Customize User Interface or edit the PGP directly), add a line like:
UNI, *UNION
Save and reload, then type UNI to run UNION. Be careful not to override commonly used aliases.

Can I union objects that are inside a block?

Not directly. You must explode the block or edit the block definition so the geometry becomes editable solids/regions. After conversion, you can use UNION.

How do I combine 2D polylines into one object?

Use PEDIT or JOIN for polylines. To perform Boolean union on their filled area you must convert closed polylines to REGION (type REGION), then use UNION on the resulting regions.

How can I undo a UNION operation?

Simply use UNDO (type U or press Ctrl+Z) immediately after the operation. If you saved and closed the drawing, you cannot undo past the save point—use earlier backups.

Does UNION preserve materials or layer properties?

After a union, the resulting object typically inherits properties (like layer) from the first selected object. Material assignments and some properties may be lost or need reapplication—check the Properties palette and reassign materials if required.

Why does UNION change the object’s face normals or appearance?

Boolean operations can change geometry topology, which may alter face orientations or normals. If shading looks wrong, try flipping faces, recalculating normals, or use SOLIDEDIT tools to correct the geometry.

Is there a limit to how many solids I can union at once?

No strict small-number limit, but unioning many complex solids at once can be slow, increase model complexity, and produce errors. Best practice: union step-by-step (two at a time) and save frequently.

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