If two separate entities have the same location, you can merge these entities together into a single entity via the NUMMRG command. The NUMMRG command does not changes model geometry. Only the topology is affected.

Many solid modeling operations create coincident keypoints, lines, and/or areas. You can use issue NUMMRG,KP to merge such coincident items. Keypoint locations are used as the basis for merging. Once coincident keypoints are merged, any higher order solid model entities (lines, areas, and volumes) attached to those keypoints are automatically considered for merging. The definition of coincident changes depending on the tolerances used. By default, merging of keypoints attached to lines is done when the distance between keypoints falls within the following criteria:

Criterion 1 above describes the consideration tolerance field (TOLER) on NUMMRG. TOLER is a consideration tolerance. If a keypoint is withinTOLER 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 (criterion 2 above), the keypoints are not merged. Lines, areas, and volumes are considered for merging in a similar manner. Criterion 2 describes the default internal relative solid model tolerance. The tolerance is a saftey measure to prevent the merge operation from eliminating tiny lines in the model. Both criteria must be satisfied for keypoints to be merged.

Figure 10.1: Default Merge Tolerances

The internal relative tolerance (criterion 2) can be overridden by an option to specify a global solid model tolerance (GTOLER) on NUMMRG. GTOLER is a global, absolute tolerance, rather than a relative tolerance. If GTOLER is used, the size of the lines attached to keypoints is no longer considered and it is fairly easy to defeature your model by using too large a value for GTOLER. You should save your database before attempting to merge, especially when using the GTOLER option.

The following example, which corresponds to Figure 10.2: NUMMRG Application Example, illustrates the use of NUMMRG to merge entities:

PCIRC,...	! Create a partial circle
RECTNG,...	! Create a rectangle
NUMMRG,KP	! Default merge tolerances used

Figure 10.2: NUMMRG Application Example

If you merge keypoints that are very nearly coincident, any very short line connecting those keypoints are deleted. If the keypoints are too far apart to be merged, issue the LCOMB command to eliminate the very short line. LCOMB generates a continuous (but not necessarily smooth) line. If the resulting line is kinked, it cannot be used as a drag path (ADRAG, VDRAG), nor can it be used in any Boolean operations.

As you build your model, you might, by deleting, clearing, merging, or performing other operations, create unused slots in the numbering sequence for various items. These slots remain empty for some items (such as elements) but are filled in for other items (such as keypoints) as new items are created. To save data storage space (by eliminating otherwise empty numbers) or to preserve desired sequencing (by forcing newly-created items to be assigned numbers greater than those of existing items), you can eliminate these gaps by "compressing" your numbering via the NUMCMP command.

The compression operation may be selectively re-executed for chosen item groups (elements, keypoints, etc.), or may be simultaneously applied to all valid items (using NUMCMP,ALL).

VMESH,...
VCLEAR,...    ! Node and element numbers are not reused.
...
! Change meshing controls, element attributes, etc.
...
VMESH,...     ! Node and element numbering will contain "gaps"
NUMCMP,NODE   ! Optional step - NUMCMP can free up some computer memory
NUMCMP,ELEM   ! by eliminating gaps in numbering sequences.

When creating new, automatically-numbered items, you might want to set the starting number of your new series of items higher than the greatest number used by existing items. Doing so ensures that the newly created entities are numbered consecutively. This prevents them from occupying gaps in the existing numbering sequence. Specifying a set of starting numbers is also useful if you are creating portions of your model independently of each nother and you want to avoid numbering conflicts when they are combined into one model. You can specify such user-defined starting numbers via NUMSTR the command. Issue this command for each separate class of items (nodes, elements, keypoints, etc.).

! Create one portion of model:
...
...
! Create a separate, distinctly-numbered portion of your model:
NUMSTR,KP,100
NUMSTR,LINE,100
NUMSTR,AREA,100
NUMSTR,VOLU,100

If you wish to combine two independently created portions of a model and want to prevent numbering conflicts, you can renumber all selected items by adding an offset value to their existing numbers via the NUMOFF command.

Execute the NUMOFF command for each item group (nodes, elements, keypoints, etc.) to be renumbered.

The CDWRITE command, which writes all selected model data to a text file, automatically puts a series of NUMOFF commands at the beginning of the file to push any existing data out of the way when the file is read. Gaps in numbering that may result from these NUMOFF commands can be removed via the NUMCMP command.

Solid model data (in IGES format) can be written to a text file in PREP7. You can also transfer solid model data from an externally-generated IGES file into the Mechanical APDL database. To write solid model data to a file, issue the IGESOUT command. To read IGES data from a file into PREP7 data, issue the IGESIN command.

New solid model entities created (in the AUX15 processor) by the IGESIN command are numbered automatically to avoid conflicts with other solid model entities that already exist in the database. If an IGES file is read into an empty database, solid modeling entity numbering does not necessarily start at 1 because intermediate entities might be created (and later deleted) in the translation process. See Importing Solid Models from IGES Files for more information about the IGES interface.