Technical LibraryArticle: JMAG A to Z

Issue 6 Understanding Geometry Modeling from A to Z

Have you mastered JMAG?
JMAG continues to evolve with each passing day. There may be functions in JMAG that even those who have already been using it will learn for the first time, as well as some useful procedures that are not well known yet. Why don't we aim at making operations more efficient by becoming familiar with new functions and operations that we don't know about?
In this series, I would like to introduce "Things that we should know" in JMAG, as well as some advantageous applications that you can try.

Overview
For a simulation which involves an electromagnetic field analysis, it is necessary to create data that represents the geometry of the target model. JMAG can load external CAD file, and can create geometry data for simulations using JMAGDesigner.
This report focuses on "Geometry Creation" and introduces the functions to create geometry data using the Geometry Editor bundled in JMAGDesigner.

Creating 2D geometry using Geometry Editor
The flow to create 2D geometry
The flow to create 2D geometry data using the Geometry Editor in JMAGDesigner (hereinafter referred to as "Geometry Editor") is as follows:
 Create a sketch
 Create a basic geometry
 Adjust the basic geometry (Break at Intersection /Trim)
 Add constraints
 Create regions
 Add region feature
In this section, we will describe about the functions and operations that are used in the above mentioned each step.

Creating a Sketch
The tree that is displayed in model manager is the data structure of the model (Fig.1). The assembly is the top of the hierarchy of the data structure. When creating a 2D geometry using Geometry Editor, you need to add a sketch to the assembly. A 2D sketch is inserted by clicking [Edit Sketch] on the tool bar. And the 2D sketch has a hierarchic structure that includes basic geometries, constraints, regions, and features under it.
Fig.1 2D geometry data displayed in the model manager.

Creating Basic Geometries
In order to draft geometry of the model for an analysis, locate basic geometries on the inserted sketch. The "basic geometry" is fundamental elements (straight lines, arc lines, spline curves) to represent the shape of the geometry. Basic geometry is sometimes called an "entity" in other CAD system.

Adjusting Basic Geometries
1. Break at Intersection
In order to use the created shape for an analysis, you need to create a region enclosed by basic geometries. Basic geometry constructing a region cannot be intersected. If the basic geometries intersect each other, you need to divide these basic geometries using [Break at Intersection] to create vertices at the intersected points (Fig. 2).
Fig. 2 Break at Intersection
2. Sketch Trim
When drafting a shape using basic geometries, a lot of overwrapped basic geometry will be appeared. The "Trim" function can resolve this situation.
The operation to trim basic geometries is as follows:
 Click [Sketch Trim] on the tool bar
 Click any basic geometries (straight line, circle, arc, spline)
The basic geometry is cut at the closest intersection(s), and a vertex (vertices) is created at the intersected point (Fig. 3).
Fig. 3 Trim

Adding Constraints
In order to control dimensions of the shape you have created, "constraints" need to be added to the geometry. The "constraints" define a relationship between basic geometries. In other words, it is a material to represent the "intention" of the author of the drawing.
To understand the notion of constraints, consider the way of drafting by hand on a drawing board and corresponds the way of the constraints.
When drafting on the Geometry Editor, adding a "fixation" constraint is necessary. And for parallel lines with the base line, a "parallel" constraint and a "distance" constraint are added.
For the line angled with the base line, an "angle" constraint is added. When a distance constraint or an angle constraint is added, a dimension line will appear on the basic geometries (Fig. 4).
Fig. 4 Constraints

Creating a Region
Conditions for analysis can be set on regions, edges and vertices. The edges and vertices must be contained in a region. So it is necessary to create regions to use the created geometry for analysis.
To create a region, it is necessary that basic geometries are not intersecting each other, and basic geometries are enclosed to construct the region (Fig.5). If there are intersected entities, use [Break at Intersection]. When DXF or IGES (2D entity) file is loaded into Geometry Editor, the loaded basic geometries might not be fulfill preconditions to construct regions. To resolve this problem, see "Create 2D geometry by reading external files".
The operation to create a region is as follows:
 Select target basic geometries.
 Select a menu [Insert][Basic Shape][Create Region]
When the selected basic geometries fulfill the precondition to create a region, the part enclosed by the selected basic geometry is highlighted in yellow. Click the [OK] button to determine the region. If the part enclosed by the selected basic geometries is not highlighted in yellow, the basic geometries do not fulfill the precondition to create a region. If so, check the basic geometry encloses the region correctly.
Also analysis cannot be run with duplicated regions. To resolve the duplicated regions, it is necessary to use [Break at Intersection] to divide the basic geometries which will be contained in a region in advance.
Fig.5 Region not closed (left) and region closed (right)

Adding a region feature
1. Copy
If all shapes are created with only basic geometries, it is hard to adjust the shape. Finding the regularity of the shape (symmetry or periodicity) and using the "copy" feature for source regions is recommended because modifications in the source regions are also made in the destination regions. It is helpful to reduce the work load of modifications and to avoid operation mistakes.
In order to utilize the symmetry of the shape, apply "mirror copy" feature (Fig.6).
In order to utilize the periodicity of the shape, apply "linear pattern" feature or "radial pattern" feature (Figures 7 and 8).
Added features appear under [2D Sketch][Feature] in the tree of the model manager. The setting can be changed by rightclicking the feature and selecting [Edit] on the tree item.
Fig.6 Region Mirror Pattern
Fig. 7 Region Linear Pattern
Fig.8 Region Radial Pattern
2. Fillet / Chamfer
Shapes like fillets and chamfers frequently appear in the shape of actual products. If you draft the fillet and chamfer shape directly using basic geometry on the sketch, it is hard to change and remove the fillet / chamfer shape.
Using the fillet / chamfer feature for a region, makes it possible to change the properties and remove it from the region easily.
But a fillet / chamfer shape which steps over different regions cannot be created by the region feature. In such case, it is necessary to create a fillet /chamfer shape directly as basic geometry using [Insert][Basic Shape][Create Fillet] (or [Create Chamfer]).

Creating a 2D geometry by reading external files
The flow to read DXF / IGES 2D format files
When reading DXF / IGES (2D Entity) format file in the Geometry Editor, the read entities are treated as basic geometries. And it is necessary to create regions on these basic geometries using the Geometry Editor. When creating a region, all the edges of basic geometries must be connected at each vertex and form a closed loop.
If the geometry does not fulfill the precondition, it is possible to adjust the geometry on the Geometry Editor (described later). But setting sufficient constraints and congruencies on the original CAD system is helpful to decrease the work load to fix the geometry. And increasing the number of columns of significant figures when outputting DXF / IGES files sometimes make it easier to create a region.
The steps to read DXF / IGES (2D Entity) format and create regions using Geometry Editor is described as below:
 Open a file.
 Adjust the basic geometries.
 Highlighting free vertexes
 Deleting duplicate vertices
 Connecting two basic geometries
 Create a region.
In this section, we will introduce the functions to highlight free vertices, delete duplicate vertices and connect two basic geometries. These functions are useful to adjust basic geometries.

Highlighting Free Vertices
A free vertex refers to a vertex that does not connect to two or more edges (Fig.9). Highlighting free vertices is useful to detect geometries which should be adjusted when loading CAD files like IGES format.
The step to highlight free vertices is below:
 Select menu [View][Visibility][Highlight Fee Vertex]
Lines and arcs which has free vertex cannot be loaded in JMAGDesigner because the lines and edges are not a part of regions.
Fig.9 Free Vertices

Deleting Duplicate Vertices
When reading DXF files or IGES files, regions sometimes cannot be created despite nothing seems to be wrong with the geometry's appearance. The cause of this problem is the existence of duplicated nodes at the cross point between entities. In order to resolve the problem, use "Delete duplicate vertices", then vertices which can be treated as same vertex is merged to a single vertex (Fig.10).
Basic geometry may be deleted or modified after deleting duplications. Therefore, constraints that are no longer valid may be deleted after deleting duplications
Fig.10 Deleting the duplicate vertices

Connect Two Basic Shapes
When reading DXF file or IGES files, entities which should be connected to each other might be disconnected by a small distance. To fix this kind of geometry, use [Connect Two Basic Shapes] function to connect basic geometries at some distance.
This function connects two basic geometries by extending one or both basic geometries. And also a new vertex is created using the break at intersection function at same time (Fig.11). The basic shapes that can be extended to connect with the other basic shapes are arcs and lines. This function can be used for arcs and splines, but cannot be extended or be contracted in the size of radius.
Regions with the two basic shapes are connected using this function after the regions are automatically deleted.
Fig.11 Connect two basic geometries

Creating a 3D geometry using Geometry Editor
The flow to create a 3D geometry
A tree displayed in model manager is a data structure of the model. An assembly is the top of the hierarchy of the data structure, and part is located under the assembly. A part has sketches, and the sketch does basic geometries, constraints, regions and features (Fig.12).
Also 2Dsketch can be copied to a part as a base of creating 3D geometry.
The steps to create 3D geometry using the Geometry Editor are mentioned as below:
 Create a 2D shape on the sketch
 Add features to the sketch and created solid.
 Extrude
 Copy
 Fillet / Chamfer
And also it is possible to project an outline of a solid face to sketch for a base of solid creation (convert sketch), or divide the contact area as a new face (imprint).
Fig.12 3D geometry data displayed in the model manager

Creating a 2D Geometry on the Sketch
In order to create a 3D shape using the Geometry Editor, first add a part to the assembly. Select [Edit Part] on the tool bar, then the new part is added.
After that, add a new sketch to create a base shape. It is necessary to select a base plane from [XY Plane], [YZ Plane], [ZX Plane] under [Part]. Otherwise, it is also possible to select an arbitrary surface as a base plane. It is helpful for easiness of operation to select a plane which shows the characteristic of the shape (Fig.13).
When creating 3D geometry, creating an over detailed shape on a sketch should be avoided because it is hard to adjust such detailed shape ahead. So the recommended way is to use feature (described later) and divide into several sketches. This is helpful to handle the 3D geometry easily.
Fig.13 Select basic plane

Adding Features to Solid
1. Extrude
A region in a sketch can be extruded in the Z or Zaxis with respect to the sketch face. Extruding is the most basic operation to create a solid.
And adding a skew to the solid is also possible.
2. Copy
Finding symmetry or periodicity properties and using copy make geometry creation easy.
In order to use symmetric property of a shape, use "mirror copy" feature. Clicking the "merge" check box enables you to merge a source solid and destination solids.
In order to use periodicity of shape, use "linear pattern" or "radial pattern". Clicking the "merge" check box also enables you to merge a source solid and destination solids.
3. Fillet / Chamfer
Of course, it is possible to create a solid with a fillet / chamfer by extruding a sketch on which a fillet / chamfer shape represented by only basic geometries directly. But adding the fillet / chamfer feature makes it easy to change the dimension parameter or to suppress the feature.
The steps to create fillet/chamfer are as follows:
 Click [Edge Fillet] or [Edge Chamfer] on the tool bar.
 Select an edge or face in the work plane.
 A translucent preview of the edge or face to fillet / chamfer appears in the work plane based on the settings that are specified (Fig.14).
 Click [OK]
Fig.14 Fillet feature (left) / Chamfer feature (right)

Converting the Outline of the Solid to the Sketch
When a SAT file that contains a 3D CAD model created with another CAD system is imported to Geometry Editor, the geometry of the model cannot be edited because the dimensions of the model are not imported. Click [Convert Sketch] button to project the outline of the selected solid on the selected plane as a sketch. The original solid can be created by adding features to a sketch, such as extruding the surface.

Imprinting
If solids are adjacent or contacting to each other, you might want to get the surface divided by the projection of edges of contact or closed region. The imprint feature is used to create a new solid face by projecting the area where parts have contact with or are adjacent to each other. For example, the contact area between Part A and Part B are imprinted as shown in the following figure. In this case, the shape of Part B is projected on the surface of Part A. The surface of Part A is divided by the projected shape, and two faces are created.
Fig.15 Before imprint Part A and B (Left) After imprint Part A(Right)

In Closing
This time, we introduced several functions related to geometry creation. Creating geometry data in a suitable way will save your time for analysis. Hopefully these functions are helpful for your job.
Next time we plan to introduce an A to Z for setting physical conditions of analysis. Be sure not to miss it.
(Motohito Hirose)

Contents
Issue 11 Electric Field Analysis from A to Z
Issue 10 Structural Analysis from A to Z
Issue 9 Understanding Thermal from A to Z
Issue 8 Understanding Conditions from A to Z
Issue 7 Understanding Conditions from A to Z
Issue 6 Understanding Geometry Modeling from A to Z
Issue 6 Understanding Geometry Modeling from A to Z
Issue 4 Understanding Meshes from A to Z
Issue 3 Shortening Calculation Time from A to Z
Issue 2 Evaluating Results and Viewing Models from A to Z
Issue 1 Running Multiple Case Calculations from A to Z

