Body in White (BIW) design using CATIA V5

A final design can be achieved using a variety of modeling approaches. A modeling approach is the modeling techniques utilized to produce a math model. This includes which features and/or what type of features are used in its creation. For instance, a solid modeling or surfaced based approach is a distinct method that can be used to create identical math models in appearance. Although internally specific circumstances may influence advantages and/or disadvantages one approach has over the other. A hybrid method is also available which allows the combination of surface and solid geometry together. Regardless of method, NX offers many feature commands that produce solids or surfaces. Several NX feature commands are universal since they can produce solid or sheet body geometry.

Many factors influence modeling approach. Major influences such as manufacturing requirements, downstream users, reference data, and ability to update are some factors which should always be considered. Other influences such as file size and feature count may also be important. In most cases, developing a clean, organized, parametric, and accurate model is most important.

Modeling methods typically include curve data as inputs for creation. Curves can be and are often the driving force behind a model. It is important to understand the multiple types of curves available in NX and the advantages/disadvantages of each.

Three forms of curves are available which include non-associative curves, associative curves, and sketch curves. Each form may visually appear identical within the graphics window, but react differently during development and modification of a model. Care should be taken as it is common for one type of curve form to be created unintentionally. This is common when the Settings pane of a dialog is not reviewed during a curve creation command. Design intent will determine which form of curves is best suited for a modeling scenario.

Non-associative curves are curves that have no parametric history. These curves are non-feature curves which do not display in the Part Navigator like other standard features. These types of curves can drive associative parametric features even though they are non-associative themselves.

Non-associative curves can be edited using edit functions found within the Edit > Curve menu on Top Border Bar. Double-clicking a curve will also allow modification.

Some popular functions that automatically create non-associative curves include Basics Curves, Ellipse, General Conic, Rectangle, and Extract among others. Other commands such as Line, Arc/Circle, Bridge Curve, among others can create non-associative curves if Associative is disabled within the Settings pane.

The disadvantage of non-associative curves is the level to which parametric modeling can be achieved. These types of curves are limited since no parametric intelligence is recorded. For example, if a line is drawn between two points, and the points are moved, the line does not update to the new location. The line would need to be manually modified to move the line to the new point locations. Non-associative curves are often used when working with legacy data.

Having no associativity can also be an advantage in some scenarios. Non-associative curves are sometimes used to reverse engineer models. Curves can be created quickly to match specific reference data. Once complete the reference data can be removed without affecting the created curves. Non-associative curves can then be converted to sketches if desired.

Associative Curves are curves that retain parametric history. These types of curves record and maintain creation inputs which update automatically. The level of parametric abilities is limitless. Each curve is considered a feature which is listed in the Part Navigator.

All aspects of associative curves can be edited. Some curve commands with associative ability include Line, Arc/Circle, Studio Spline, Helix, Offset, and Bridge Curve among many others.

The disadvantage of associative curves may depend on its use and purpose. If associative curves are used to create profile strings, a long listed history-tree may be produced. Many features will need to be managed. Additionally each feature's timestamp is preserved which means the creation order of each feature is recorded. This can make editing more cumbersome. For example if Line (2) is modified notice Line (1) does not update. This is because Line (1) was created first and therefore does not have an associative link to Line (2). Reordering features is often necessary in this type scenario.

Sketches from a parametric stand point have the most advantages. Sketches are placed on a desired plane or path. Sketches have similar abilities of associative curves plus more. Each curve element is consolidated into one sketch feature which avoids timestamp problems. A choice of two environments can be used to develop sketches which include many functions to trim, extend, copy, transform, and mirror sketch curves among many others. Dimensional and geometric constraints are created to control all aspects of curves in any desired tactic to capture true design intent. Auto-constraining and dimensioning are also available to create sketches quickly.

Sketches also have the advantage of how children features are linked to the curve data. Selection rules are available which allow specific parametric levels of associativity. (see Selection Intent within Lesson 4)

Disadvantages of sketches are relative to the user and/or design requirements. Many experienced users differ in opinion when describing any sketch disadvantages. One typical objection is scenarios where advanced parametric design is not required. For example, standard parts or models that are reversed engineered may never need modifications. This scenario may prove too much for sketches. In these types of circumstances, sketches may contain too much parametric ability that is not necessary. Some added time may be added in model creation. Another disadvantage may be that sketches are only 2D, although utilizing sketches and associative curves together can resolve many 3D curve creation requirements.

Deciding what curve type to use should be based on each individual design intent and requirements of a design. Overall sketches have the most parametric advantages; therefore sketches are typically the recommended choice of curve creation. Generally most design scenarios benefit from parametric techniques which heavily support the use of sketches.

Creating a BIW structure using a solid approach involves the creation of solid features in which the shape of one side of metal is developed. Typically a large solid is created to capture all required faces. Once the solid is complete, the Shell command is then applied to create the sheet metal thickness.

This modeling method works well with parts that are prismatic in shape. Structures that have complex bends, forms, or other advanced characteristics may require an alternate modeling technique. A general rule to follow is to add fillets before the Shell. This allows the Shell to create fillets on the opposite side of metal. Creating a single solid with one Shell feature likely will produce a clean, consistent model.

Disadvantages include the chance of creating unintentional knife edges or inconsistent metal thickness. If the pre-shelled solid contains corners that are not 90°, the final solid will maintain the same angle causing a knife edged corner. Additionally a fillet or other formed shape may be added after a Shell causing a non-uniform sheet metal thickness.

Structures with complex forms may benefit using a surface-based modeling technique. Creating individual surfaces of complex forms then combining them together can be a much more efficient approach. Common functions such as Variational Sweep, Emboss, Trim and Extend, and Law Extension among others can be used to create a surfaced based model. (These functions are described in more detail throughout this course.)

Once all sheet bodies are combined together to form a final surface, the Thicken command is applied to create the sheet metal thickness. This creates a solid body with uniform material thickness and no possibility of knife edges. Corner fillets and any finishing features can then be added to the solid.

Surfaced-based models can reduce the amount of features and geometry needed to develop a model. This reduces time and effort in the development and modification throughout the life cycle of a design.

Note: The term surface and sheet body are universal terms which refer to a body with zero thickness. A surface / sheet body can be made up of multiple faces or a single face. The following shows an example of both.

Hybrid modeling is a design approach consisting of combining the surface and solid-based modeling approach together. This typically involves developing a surface, and then using solid features to help shape areas that may have prismatic simplified shapes. For example a Floor Pan is mainly a large flat sheet metal panel with some complexity, but has many simple primitive shaped forms. The following image shows a simplified Floor Plan utilizing a hybrid modeling approach..

The Emboss Sheet command along with Boolean operations can add the solid shapes into the main surface.

NX offers a vast variety of command features in which many produce similar results. Two alike functions typically will have some aspect which will make it unique to one another. The ability to recognize and determine the best strategy for a design may take time to develop.