Surfacing Best Practices

8/10/2019 Surfacing Best Practices

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AITHcrew Automotive Interior & Exterior Trim Design

Shivkumar Gummadi Page 1

Unit: 13

Surfacing Best Practices

82: Surfacing Best Practices Unit Introduction

83: Surfacing Considerations

84: Avoiding Problems in Surfacing

85: How to Fix Imperfect Faces and Bodies

86: Finding the Balance


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82: Surfacing Best Practices Unit Introduction

Introduction

In this unit ...

Learn to choose and create appropriate features.

Comprehend how to avoid common surfacing problems and how to improve the quality ofimperfect surfaces and bodies.

Find the balance between speed and quality.


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Overview: Because there are hundreds of features, each containing multiple options, deciding on

which one to use for your application can be daunting. Within surface features alone, there aredozens to choose from. Then, to take the issue a bit further, within a single surface feature there

are multiple options available. Demystifying some of these functions can help you complete yourwork without the software hindering your progress. The following sections explain surface featuresand options we recommend.

Surfacing using Sections and Guides:Probably the two most useful features in surfacing are Multi-Sections Surfaceand Sweep. Even

if you don't create many surface objects, it's a good idea to learn these features because mostsurface models contain them.

Multi-Sections SurfaceThe Multi-Sections Surfacefeature, one of the most versatile commands in CATIA, enables youto create some of the most complex surfaces, and is a standard method when producing a surface

with smooth transitions between section strings. Unlike other CAD systems, the Multi-SectionsSurface feature produces surfaces through sections, or uses a combination of sections andguides.

Here are some advantages to using the Multi-Sections Surface feature:- You can define various cross-sections that do not need to be evenly spaced.- You can use the operation on a blend surface.- You can create a surface that is tangent to an existing surface.


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02: Section String for Multi-Sections Surface

03: Multi-Sections Surface with Tangency Constraint


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- You can use the operation when you do not have a smooth transition between your sectionstrings. Like the sections, you can also add tangent constraints to the start and end guide string.

04: Multi-Sections Surface with Guide Strings

- In conjunction with the re-limitation options, you can extend the surface to the ends of the guidesor spine without defining a section at either end.

When using Multi-Sections Surface, you cannot create a curvature constraint between a sectionand an existing surface. Nor can you can add tangent constraints to intermediate sections.

Sweep: The Sweep feature is extremely useful when creating a surface that is controlled using

one section and a guide. Options available to you vary, depending on which workbenches youhave available. With the Wireframe and Surface Design workbench, the standard Explicit sweep isavailable. This feature allows you to sweep one section along one or two guides. In addition, youhave extra control over the orientation of the section as you sweep it along.

05: Sweep

One disadvantage is that you can only define one section. If more than one section is required,then use the Multi-Sections Surface or Adaptive Sweep feature (which is only available in theGenerative Shape Design workbench).

If you have the Generative Shape Design workbench, the Sweepcommand really comes into itsown. The extra features allow you to create linear, conical, and circular sweeps along guides,


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without defining the section strings. After you define the guides, you can modify the valuesexplicitly or control them by using a law. The guides themselves don't need to be linear, but canvary in shape.

06: Conic Sweep with Three Guide Curves

Adaptive Sweep: Adaptive Sweep is only available with the Generative Shape Designworkbench. This function allows you to sweep a profile along a guide and gives you control overthe profile dimensions as you sweep it. The aim of the feature is to stop you from having to createmultiple sections with different dimensions. While the intention is good, it proves difficult to predictthe position of the computed generated sketches. To make this command work, you need to beextremely careful in how you constrain the original section sketch. The extra sections createdalong the guide keep the original constraints and often become out of place. If you have difficultygetting the sections to perform correctly, it may be best to create the various sections and useMulti-Sections Surface to create the surface.

Spine Curves:A commonality to most surfaces created through sections and guides is the use of

a spine curve. A spine controls the orientation of the surface as you create it. Using a spine

creates the surface normal to the spine.

08: Sweep with Spine

Using Surfaces with Solid Bodies: One common question asked after you create a surface is,"How do you turn it into a solid or use it to modify the shape of an existing solid?" The followingsections describe the surface-based features, which control the shape of a solid or create a soliditself. You use the following features regularly after you create your required surface and you mustbe in the Part Designworkbench to access them.

Split and Sew: Split and Sew are similar features in that they both style an existing solid to takethe form of a surface. The difference is that Split always removes material while Sew can addmaterial between an existing solid and the surface.


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09: Splitting a Surface

ThickSurface: The ThickSurface feature allows you to give a surface a constant thickness, thus

creating a solid that follows the form of the surface.

10: Thickening a Surface

Close:

The Close surface allows you to create a solid body from a surface that has only a planar openingor is totally closed by surfaces. You can create an entire product representation in surfaces, jointhem together, and then close them to produce a solid.

11: Close Surface

Tips

Multi-Sections Surface was formerly named Loft.


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84: Avoiding Problems in Surfacing

Overview:

When working with surfaces, you can prevent many problems by carefully planning your model.Start with a good foundation using the correct curves and modeling techniques. Try to avoidcreating a complex model when a simpler design will do. Surface features are essential forcreating complex models, but they also invite trouble.

Closing Points and Coupling

It is extremely important that the Closing Point in each section is in the equivalent area to

prevent the solid from twisting.

01: Incorrect Closing Point 02: Correct Closing Point


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On open sections, the arrows in each section should point in the same direction to avoid thesame twisting problem.

Another important feature in Multi-Section Surfaces is coupling. Coupling defines how points inone section connect to points in the next section. The options include Ratio, Tangency,Tangencythen curvature and Vertices. If there are an uneven number of points in each

section, the surface becomes disjoined. Try to use the same number of points in each section, oruse the Curve Smooth feature as described below. Always remember that you can manuallyspecify the coupling, if required.

Using the Right Section Curves

If you start with the right set of section curves, you reduce the possibility of bad surfaces. How doyou create the right set of section curves? Reduce the complexity of the curves that form thebasis of the surface.

Free form features take on the characteristics of the underlying curves. Use a single curve for asection where possible. Multiple curves in a section string can create unwanted faces in the

resultant body. In the figure, a Multi-Section Surface feature was created using multiple curvesection strings and guide strings.

03: Multiple Face Surface 04: Single Face

The resultant surface has three faces with undesirable tangent edges between the faces.Although the section and guide strings are associative sketches, the result is not acceptable.Smooth the curves to produce one continuous section. This action results in a single face sheetbody.

Trim Solid Versus Final Look

Deciding whether to create a solid's final shape with one feature or multiple features is adilemma. If you try to add too much curvature into one operation, the resulting surface can havemany irregularities. Instead, create a base shape and use Split or Boolean operations to finishthe shape.

In the figure, a solid was created using different trim and Boolean operations.


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Overview:

There are methods you can use to repair parametrically created surface features, and techniquesyou can use so you do not create imperfect faces. In this course, we do not cover how to fixunparametric sheet or solid bodies. Imperfect faces / bodies are faces that contain unwanted dipsand blemishes, as well as faces that do not pass Connect Checker tests.

Checking for Negative Gaussian Radius

In order to check the Gaussian radius, you need to know what it is. Selecting Insert| Analysis|

Surfacic Curvature Analysis is one way you can check a face. Gaussian radius checks theradius at any given point that is determined by a group of section curves passing through thepoint. Each point on the surface has a normal (N). The section curves are created from theintersection of sectioning planes, which revolve around the point through the normal (N) and theface being analyzed. CATIA uses these section curves to analyze the curvature of the face ateach point. A negative Gaussian radius causes an opposite curvature in at least two sections(imagine a saddle). A positive Gaussian radius creates all curvature in the same direction.

01: Positive Gaussian Radius 02: Flat Spot and Negative Gaussian 03: Smooth Looking Surface 04: Analysis Showing Defects

If you have a flat spot, the Gaussian radius tends towards infinity. It also checks for dips orbumps in the face. Many times, a face appears to be smooth with no apparent defects. However,once you perform a face analysis, the Gaussian radius check shows the defects.


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It is easier to edit the curves if you create your sections by creating sketches and then joining thecurves. When you get a bump or dip in the face, check the curvature of the curves first, whichshould not be an issue if you used Join curves. Usually the curves will only cause localizeddeformities. Sometimes, the locations of your section strings cause the deformities. You mayhave a section string that is not aligned with other sections.

Connect Checker

If you discover problems in the surface, they are usually caused by problems in the constructioncurves. If you have a number of curves joined together to produce a section, Connect Checkerallows you to check

05: Connect Checker

the distance between the endpoints, as well as the tangencies and curvatures between thecurves.

After you identify the problem areas, you can modify the original sketch or wireframe geometry.

If you have the Generative Shape Designmodule, you can use the Curve Smoothfeature to

modify the curves automatically to make them tangent and curvature continuous.

With the surface analysis and Connect Checker, you should be able to pinpoint the problemareas and fix the underlying cause.


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Overview:

Whenever you are creating complex surfaces, you are inevitably facing a project deadline. If youhave no schedule to meet, you are one of the lucky few. Finding the balance between speed,complexity, size, and best features, while taking into account schedule time, file size, and thefunction of the part, can sometimes be a difficult task. Asking questions, such as "Do I need tocreate a surface with 50 sections when I can create a surface using four sections and still satisfythe design parameters?" is important.

Speed Versus Caution:

Each time you add a section to your surface, you need to create the curves and planes, anddetermine the curve parameters. Next, you must locate the section in the correct position, hopingyou sized it correctly. You may need to adjust each section string's location or size to get thesurface required. As you adjust the geometry, you need to check the validity of the faces. Notsurprisingly, these steps take time. Do not spend hours trying to make the face perfect if it doesnot need to be. If the function of the part does not depend on a perfect face, do not belabor it.

File Size Versus Multiple Features

With every feature you add to your model, you increase the file size of the model. Someoperations can increase the size of a file dramatically. For example, if you create a trimmedsurface and then use that to trim the main solid, it quickly increases the file size. Most often,these types of trim surfaces contain many wireframe features. You may need many features tobuild one trim surface. You must decide if you need all the surfaces in one trim surface or inmultiple trim surfaces.


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Surface Complexity

Surface complexity affects model size, update time, and manufacturing. If you truly need to keepsurfaces highly complex, you will have these obstacles. Most machined surfaces cannot even bemanufactured to the modeled solid. It is best if you can decrease model complexity withoutcompromising design intent.

Quite often, someone new to surface modeling creates the most complex surface possible, justbecause they can. You want to create the simplest model possible while maintaining designintent. You must remember that someone needs to machine the electronic model you aredesigning. The fact that you can create it on a screen does not mean that someone can actuallycreate it in reality. Even if it can be manufactured, how much time and money will it take? Simplesurfaces are the most desirable for model size and manufacturing.

When you try to put many different curvatures into a surface, you increase the complexity. Abetter method is to use multiple features to achieve the same topology. Your file size mayincrease, but the surface complexity remains small.

Surfacing Best Practices

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