Notes

The NUMMRG command does not change a model's geometry, only the topology.

After issuing the command, the area and volume sizes (ASUM and VSUM) may give slightly different results. In order to obtain the same results as before, use /FACET, /NORMAL, and ASUM / VSUM.

The merge operation is useful for tying separate but coincident parts of a model together. If not all items are to be checked for merging, use the select commands (NSEL, ESEL, etc.) to select items. Only selected items are included in the merge operation for nodes, keypoints, and elements.

By default, the merge operation retains the lowest numbered coincident item. Higher numbered coincident items are deleted. Set Switch to HIGH to retain the highest numbered coincident item after the merging operation. Applicable related items are also checked for deleted item numbers and if found, are replaced with the retained item number. For example, if nodes are merged, element connectivities (except superelements), mesh item range associativity, coupled degrees of freedom, constraint equations, master degrees of freedom, degree of freedom constraints, nodal-force loads, nodal surface loads, and nodal body force loads are checked. Merging material numbers (NUMMRG,ALL or NUMMRG,MAT) does not update the material number referenced:

When merging tapered beam or pipe sections, the program first uses the associated end sections for merging. If the merge is successful, the program replaces the tapered section database with the end section data.

If a unique load is defined among merged nodes, the value is kept and applied to the retained node. If loads are not unique (not recommended), only the value on the lowest node (or highest if Switch = HIGH) is kept (except for "force" loads for which the values are summed if they are not defined via tabular boundary conditions).

After a NUMMRG,NODE command executes, some nodes may be attached to more than one solid entity. As a result, subsequent attempts to transfer solid model loads to the elements may not be successful. Issue NUMMRG,KP to correct this problem. Do NOT issue VCLEAR before issuing NUMMRG,KP.

For NUMMRG,ELEM, elements must be identical in all aspects, including the direction of the element coordinate system.

For certain solid and shell elements, the program interprets coincident faces as internal and eliminate them. To prevent this from occurring, shrink the entities by a very small factor to delineate coincident items (/SHRINK, 0.0001) and no internal nodes, lines, areas or elements will be eliminated.

When working with solid models, you may have better success with the gluing operations (AGLUE, LGLUE, VGLUE). Please read the following information when attempting to merge solid model entities.

Gluing Operations vs. Merging Operations

Adjacent, touching regions can be joined by gluing them (AGLUE, LGLUE, VGLUE) or by merging coincident keypoints (NUMMRG,KP, which also causes merging of identical lines, areas, and volumes). In many situations, either approach will work just fine. Some factors, however, may lead to a preference for one method over the other.

Geometric Configuration

Gluing is possible regardless of the initial alignment or offset of the input entities. Keypoint merging is possible only if each keypoint on one side of the face to be joined is matched by a coincident keypoint on the other side. This is commonly the case after a symmetry reflection (ARSYM or VSYMM) or a copy (AGEN or VGEN), especially for a model built entirely in Mechanical APDL rather than imported from a CAD system. When the geometry is extremely precise, and the configuration is correct for keypoint merging, NUMMRG is more efficient and robust than AGLUE or VGLUE.

Model Accuracy

As with all boolean operations, gluing requires that the input entities meet the current boolean tolerance (BTOL). Otherwise, AGLUE or VGLUE may fail. In such cases, relaxing the tolerance may allow the glue to complete. An advantage of gluing is that it is unlikely to degrade the accuracy of a geometric model. Keypoint merging can operate on almost any combination of entities (although you may have to override the default tolerances on NUMMRG). However, it can also introduce or increase accuracy flaws, making later boolean operations less likely to succeed. If the input tolerances are too large, NUMMRG can collapse out small lines, areas, or volumes you intended to keep, possibly rendering the model unusable.

Mesh Status

As with all boolean operations, gluing requires that the input entities be unmeshed. Keypoint merging is effective for meshed models under the right conditions. More information on keypoint merging follows.

Merging Solid Model Entities:

When merging solid model entities (Label = KP or ALL), keypoint locations are used as the basis for merging. Once keypoints are merged, any higher order solid model entities (lines, areas, and volumes), regardless of their select status or attachment to the merged keypoints, are considered for merging.

Keypoints that are attached to lines will be merged only if:

The TOLER field is a consideration tolerance. If a keypoint is within TOLER of another keypoint, then those two keypoints are candidates to be merged. If, when "moving" the higher numbered keypoint, the distance exceeds the internal relative solid model tolerance, or the global solid model tolerance (GTOLER) if specified, the keypoints will not be merged. Lines, areas, and volumes are considered for merging in a similar manner.

The internal relative solid model tolerance should be overridden by the global solid model tolerance (GTOLER) only when absolutely necessary. GTOLER is an absolute tolerance; if specified, relative lengths of lines in the model will no longer be considered in the merge operation. If GTOLER is too large, you can "merge-out" portions of your model accidently, effectively defeaturing the model. If using GTOLER, it is good practice so first save the database before issuing NUMMRG (as undesired merges of solid model entities could occur).

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