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when surfaces meet solids
Midrange MCAD applications
benefit as surfaces and solids converge.
By Don
LaCourse

src=”http://www.cadalyst.com/images/interface/corner_ul.gif” width=5> src=”http://www.cadalyst.com/images/interface/spacer.gif” width=1> src=”http://www.cadalyst.com/images/interface/corner_ur.gif” width=5> class=box>don.lacourse@cadalyst.com“> src=”http://www.cadalyst.com/Library/authorbios/Don_LaCourse.gif” width=90 align=right>Don LaCourse has spent the last 24 years in product design, mold design, and CAD/CAM operations. He is a Cadalyst contributing editor and president of The Solid Modeling ExChange, a design, applications support, and online help documentation firm in Cookeville, Tennessee. This article originally appeared in the January 2003 issue of CADALYST Magazine. src=”http://www.cadalyst.com/images/interface/corner_ll.gif” width=5> src=”http://www.cadalyst.com/images/interface/spacer.gif” width=1> src=”http://www.cadalyst.com/images/interface/corner_lr.gif” width=5> src=”http://www.cadalyst.com/images/interface/corner_ul.gif” width=5> src=”http://www.cadalyst.com/images/interface/spacer.gif” width=1> src=”http://www.cadalyst.com/images/interface/corner_ur.gif” width=5> In this article Thanks to the following companies for contributing time, material, and images to this column:     href=”http://www.autodesk.com/” target=_blank>Autodesk (Inventor)     href=”http://www.solid-edge.com/” target=_blank>EDS PLM Solutions (Solid Edge)     href=”http://www.ptc.com/” target=_blank>PTC (Pro/ENGINEER)     href=”http://www.solidworks.com/” target=_blank>SolidWorks (SolidWorks)     href=”http://www.think3.com/” target=_blank>think3 Corp. (thinkdesign)     href=”http://www.vx.com/” target=_blank>VX Corp. (VX CAD/CAM) See Also     class=box_white href=”http://www.cadalyst.com/solutions/model/0103model/glossary.htm”>Glossary of Solid and     href=”http://www.cadalyst.com/solutions/model/0103model/glossary.htm”>Surfacing Terms src=”http://www.cadalyst.com/images/interface/corner_ll.gif” width=5> src=”http://www.cadalyst.com/images/interface/spacer.gif” width=1> src=”http://www.cadalyst.com/images/interface/corner_lr.gif” width=5>

With little fanfare, EDS PLM Solutions and the
folks down in Huntsville, Alabama, added basic surface modeling functionality to
Solid Edge v12 earlier this year. Though meant only to provide users with basic
tools needed to import and repair 3D models that are less than perfect solids,
this addition is a significant development. When viewed from the broader MCAD
industry perspective, it signals one more nail in the coffin of the solids-only
midrange MCAD application. SolidWorks Corp. with its flagship product broke
similar ground in recent releases.

These MCAD developers recognize the important role that surface modeling can
play in building complex solid models. They are joining the ranks of veteran
surface/solid applications such as VX CAD/CAM from VX Corp. and thinkdesign from
think3. Surfaces and solids are finally converging in midrange MCAD
applications, to the benefit of all users.

IN THE BEGINNING
When solid modeling made its debut
in the mid-1980s, MCAD vendors employed two distinct database structures (CSG
and B-rep). CSG (constructive solid geometry) used mathematical Boolean
operations (union, subtraction, and intersection) to implicitly combine simple
solid shapes to form more complex shapes. B-rep (boundary representation)
explicitly positioned and related surface elements to form an airtight boundary
that enclosed the volume defined by the part.

src=”http://www.cadalyst.com/solutions/model/0103model/fig1.jpg” width=300 border=1> Figure 1. Imported surface data displays problem areas on several surfaces. Solid Edge v12’s new interrogation tools highlight problem areas of the surface model, in this case, nonstitched edges. Using Solid Edge’s new tools for trimming, extending, and creating surfaces from bounded edges, we fixed the surfaces and added missing ones, then stitched the surfaces together to form a solid body.

Both methods had
their uses and merits. B-rep was predominantly used by the 3D surface modelers
of the day. Early solid modelers employed CSG.

SHARED ROOTS
Surface and solid modelers today share
the same roots within their database structures. CSG was fine for the early
solid modelers, but soon went by the wayside as users demanded the ability to
model more complex shapes that CSG alone could not handle. In stepped hybrid
modelers that took advantage of the strengths of both CSG and B-rep.

As MCAD developers migrated toward their second generation of modelers,
programmers discovered how to employ CSG-like operations using B-rep. You still
find Boolean operations in today’s solid modelers, even though virtually all of
today’s MCAD applications (surface and solids) use B-rep as their sole database
structure.

WHAT IS A SOLID?

src=”http://www.cadalyst.com/solutions/model/0103model/fig2.jpg” width=300 border=1> Figure 2. SolidWorks’ Surface Fill command quickly and easily fills in a complex area with a surface that is tangent to all adjacent existing faces.

In today’s
modelers, a solid is nothing more than a set of surfaces whose topology forms a
closed volume. Topology refers to the relationship of a part’s mating vertices,
edges, and faces. Solid modeling operations are governed by internal rules that
maintain valid topology. Valid topology means that all surfaces are present and
do not intersect each other and that multiple volumes do not share vertices,
edges, or faces, among other things.

Surface modeling is not governed by these rules. You are free to create any
number of surfaces, in any order. They may connect cleanly, or they may not.
It’s up to you to employ clean modeling techniques. This freedom attracts many
users and is what makes surface modeling so powerful. Once a surface model is
complete, you can use a simple “thickness” command to convert it to a solid.

TODAY’S HYBRID MODELERS

src=”http://www.cadalyst.com/solutions/model/0103model/fig3.jpg” width=200 border=1> Figure 3. Pro/ENGINEER’s Interactive Surface Design Extension (ISDX) converts a surface model to a solid.

Hybrid modeling
now refers to MCAD applications that employ both surface and solid modeling
operations. They combine the ease of use of solids with powerful surface
creation and editing tools-the best of both methods. In a hybrid modeler, you
can temporarily suspend valid topology rules of solid modeling under controlled
conditions.

You can apply traditional solid feature operations, such as holes, pockets,
protrusions, fillets, and chamfers, to open sets of surfaces. At the same time,
you can explode a solid to gain access to its underlying set of surfaces, then
apply surface modeling operations. You can add, remove, or replace surfaces, and
perform local surface modifications such as control point editing (tweaking).
When the required surface modeling is complete, sew the surfaces back into a
solid (provided they form a closed volume).

A CONVERGING INDUSTRY
Developers of traditional
solids-only applications add surface creation tools for a number of reasons, and
some vendors are further along than others. Other than the obvious need for
complex surface handling, the solids-only roots of many modelers cause problems
for

src=”http://www.cadalyst.com/solutions/model/0103model/fig4a.jpg” width=300 border=1> Figure 4. VX CAD/CAM v7 lets you work with surface or solid models or both within the same part or assembly. Solid feature tools can be applied to open surface sets such as those used to create the handle in this design. (top) VX CAD/CAM provides complex surface creation tools as well as control point editing (tweaking) and analysis and visualization tools. (right) A new Design Optimizer automatically adjusts driving dimensions to achieve a target parameter such as the volume in this bottle design. src=”http://www.cadalyst.com/solutions/model/0103model/fig4b.jpg” width=125 vspace=3 border=1>

users trying to
import and work with surface models or imperfect solid models. Neutral format
import and export translators such as IGES can produce topological errors, even
if the part began as a solid. Such errors can be caused by faulty translator
code, mixed interpretation of the standard, support for only a subset of the
standard, and the like.

To solve this problem, solids-only developers are starting to provide the
surface creation and editing tools needed to repair those less-than-perfect
models everyone seems to get once in a while. The hybrid solid/surface modelers
never faced this problem, but did need to provide better-than-average import and
export translators to handle the more complex surface models out there.

All users will benefit when the industry unites on support for both surfaces
and solids. The expanded design envelope that surfaces bring to users of
traditional solids-only applications is only one benefit. Import and export
translators should improve because developers will have to export the new
surface entities their programs can now create. With improved export, improved
import must follow.

WHAT SHOULD YOU BUY?
Even with the convergence of
surfaces and solids well underway, MCAD applications differ in their strengths
and weaknesses in each area. You can count on all applications to offer the
basics of feature-based solid modeling. They differ in their integration,
approach, and support for surfacing functionality. If you are wondering, “What
should I buy?,” you should ask some additional questions as well.

src=”http://www.cadalyst.com/solutions/model/0103model/fig5.jpg” width=300 border=1> Figure 5. Table Lighter created by Alessi Giovannoni Design using thinkdesign. This is another good example of the application of solid features on a free-form shape—see the thickness, ribs, holes, and bosses in the image on the left.

What will I use
the application for? What you plan to design plays a significant role in
deciding what products to look at. If you work with complex free-form shapes
that require attention to aesthetics and ergonomics, a solid modeling
application strong in surface creation and editing with tangency, curvature, and
visual controls will be very helpful. On the other hand, if you work with known
prismatic shapes with standard features such as holes, fillets, and pockets,
basic surface functions will be adequate.

Do I work with large assemblies? If so, an application with in-context
assembly design (the ability to edit a part within the context of the assembly)
and assembly management capabilities is a plus. Also look for an application
that allows surface models to be interchangeable with solid models within the
assembly. Find out what the restrictions are, if any.

Do I frequently import models from neutral formats (IGES and STEP)? If
the answer is yes, then consider an application with strong support for
surfaces. For example, tool and die designers need to repeatedly import multiple
versions of customer parts that may come in many different formats. The CAD
application should not only support surfaces but also provide extensive topology
healing tools that help repair those occasional problematic parts.

src=”http://www.cadalyst.com/solutions/model/0103model/fig6.jpg” width=300 border=1> Figure 6. Advanced part creation using the unified shape technology in Autodesk Inventor 6 lets you mix solids and surfaces to create stylized, complex, and sculpted parts.

What
applications will I be required to interface with? Make a list of those
applications you send and receive part models from. You may afford to buy the
same applications. If those on the list are based on licensed geometry kernels
such as Parasolid or ACIS, look at applications that support that kernel to see
if they can handle your design needs. If those on the list are high-end systems
such as CATIA or Unigraphics, consider an application that is strong in surface
modeling capability. Chances are that you will receive complex surface models.

BRIGHT FUTURE
As surface and solid modeling
functions continue to converge, expect the line between traditional midrange
solids-only applications and hybrid modelers to blur considerably. Though hybrid
modelers have a leg up when it comes to developing the functionality and
integration between surfaces and solids, this gap will continue to close as
well.

From the user’s point of view, midrange applications can only get better.
Developers of hybrid modelers can’t afford to sit around and simply tout that
they offer the best of both breeds. This convergence will spur them to develop
more advanced surface functionality and possibly new areas to set their products
apart. Developers of solids-only applications will finally break free of those
bonds and look to expand their user bases into more complex applications. The
future of midrange MCAD application development looks bright indeed!

href=”http://www.cadalyst.com/solutions/model/0103model/index.htm#top”>^ TOP OF
PAGE

좋으신 말씀임다…

김기백님께서 먼저 답변하신대로, 인벤터는 solid modeling 세계과
surface modeling 세계를 자유롭게 드나드는 hybrid modeling을
추구하고 있습니다.

인벤터 6에서 처음 소개되거나 보강된 surface 기능들을 열거하자면,

3D 레일을 사용하는 로프트 (Loft with 3D rails)
3D 단면을 사용하는 로프트 (Loft 3D profiles)
Loft twist control: 인벤터6에서 보강됨
면 삭제 (Delete Face)
곡면 스티치 (니트 라고도 함) Stitch Surface (Knit)
면 대치 (Replace Face)
두껍게하기/간격띄우기 (Thicken/Offset)
엠보싱 (Emboss)
전사 (Decal)
승격 (Promote): 인벤터6에서 보강됨
3D 교차곡선 (3D Intersection Curve): 3D Sketch의 새 기능
2D 스플라인 조정: 보타이 핸들 등

위 기능에 대한 동영상 데모는
http://www.autodesk.co.kr/adsk/index/0,,2327204-1169528,00.html
를 보시면 됩니다.

인벤터에 아직 없는 기능은 Surface Trim/Extend, Fill Surface,
3D point들로 정의되는 3D spline curve 생성 등입니다.

대부분의 인벤터 사용자들은 surface modeling 이 거의 필요없는
기계 설계를 하고 있습니다. 설사 surface 기능이 필요해도
인벤터의 현재 기능으로만도 충분히 해결되는 경우가 대부분이라는
인벤터 뉴스그룹 (전세계 인벤터 사용자 토론 모임)에 올려진
경험담들을 읽은 바 있습니다.

그러나, free-form surface modeling 을 많이 해서 복잡한 곡면 생성을
원하시는 분들은 인벤터 6의 surface 기능이 아직 초보 단계라고
생각하실 수 있습니다.

MDT 에서 보여준 surface 기능을 인벤터가 아직 못 따라가는 것도
사실입니다. 그래서 일부 AIS (Autodesk Inventor Series) 사용자들은
free-form surface modeling은 MDT에서 작업을 하고, 모든 나머지는
인벤터를 사용한다는 AIS 사용자들의 경험담을 들은 바 있습니다.

Mold는 인벤터에서 파생부품 (derived part)을 사용해서 생성할 수는
있지만, 미흡한 점이 있는 게 사실입니다.

오토데스크가 인벤터 R1 출시한 지 이제 3년 조금 넘었습니다.
인벤터 초창기 때에는 산업기계 설계 분야에 중점을 두었지요.
그 분야는 복잡한 곡면을 거의 사용하지 않고, 수많은 부품들로
이루어진 조립품을 필요로 합니다.

하지만, 인벤터가 자리를 잡아가고 있고, 쓰이는 용도가 다양한
분야로 확장됨에 따라, 인벤터 개발팀에서는 surface와 mold 분야에도
성능을 높이기 위해 힘쓰고 있습니다. 자세한 내용은 회사 방침상
더이상의 계획은 말씀 못 드리지만, 기대하셔도 좋을 듯합니다.

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