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April 5, 2007 CLASS A
A Class surfacing and its importance: A class surfaces are those aesthetic/ free form surfaces, which are visible to us (interior/exterior), having an optimal aesthetic shape and high surface quality.
Mathematically class A surface are those surfaces which are curvature continuous while providing the simplest mathematical representation needed for the desired shape/form and does not have any undesirable waviness.
Curvature continuity: It is the continuity between the surfaces sharing the same boundary. Curvature continuity means that at each point of each surface along the common boundary has the same radius of curvature.
Why Class A is needed:
We all understand that today products are not only designed considering the functionality but special consideration are given to its form/aesthetics which can bring a desire in ones mind to own that product. Which is only possible with high-class finish and good forms. This is the reason why in design industries Class A surface are given more importance.
Understanding Class A surfaces:
1. The fillets – Generally for Class A, the requirement is curvature continuous and Uniform flow of flow lines from fillet to parent surface value of 0.005 or better (Position 0.001mm and tangency to about 0.016 degrees).
2. The flow of the highlight lines – The lines should form a uniform family of lines. Gradually widening or narrowing but in general never pinching in and out.
3. The control points should form a very ordered structure – again varying in Angle from one Row to the next in a gradual manner (this will yield the good Highlights required).
4. For a Class A model the fillet boundary should be edited and moved to form a Gentle line – and then re-matched into the base surface.
5. Matched iso-params in U & V direction are also a good representation of class A.
6. The degree (order) of the Bezier fillets should generally be about 6 (also for arc Radius direction) sometimes you may have to go higher.
7. Also you have to take care of Draft angle, symmetry, gaps and matching of surfaces Created with parent or reference surfaces.
8. Curvature cross-section needles across the part – we make sure the rate of Change of curvature (or the flow of the capping line across the top of the part) is Very gentle and well behaved.
The physical meaning:
Class A refers to those surfaces, which are CURVATURE continuous to each other at their respective boundaries. Curvature continuity means that at each “point” of each surface along the common boundary has the same radius of curvature.
This is different to surfaces having;
Tangent continuity – which is directional continuity without radius continuity – like fillets.
Point continuity – only touching without directional (tangent) or curvature equivalence.
In fact, tangent and point continuity is the entire basis most industries (aerospace, shipbuilding, BIW etc ). For these applications, there is generally no need for curvature.
By definition:
Class A surface refers to those surfaces which are VISIBLE and abide to the physical meaning, in a product. This classification is primarily used in the automotive and increasingly in consumer goods (toothbrushes, PalmPC’s, mobile phones, washing machines, toilet lids etc). It is a requirement where aesthetics has a significantcontribution. For this reason the exterior of automobiles are deemed Class-A. BIW is NOT Class-A. The exterior of you sexy toothbrush is Class-A, the interior with ribs and inserts etc is NOT Class-A.
What is Body_in_white?
What is class A surface?
Are the interior trim (A,B,C pillar, dash board, center console, handles) of a car using class A surface?
Anybody using the basic design bundle of UG for class A surfacing? UG\Shape Studio?
How does it compare with Catia?
A class A surface is anything that you the customer sees. i.e. exterior panels and interior surfaces.
A Class B surface is something that is not always visible i.e. the underside of a fascia that you would have to bend down to see.
A Class C surface is the back side of a part of a surface that is permanently covered by another part.
BIW is stuff like the body side etc..
Actually ‘body in white’ is the term used to describe the whole vehicle body after it has been welded/bolted together before it is painted or any parts are attached on the fit up line.
We also use it to mean after it has been painted – I always assumed that the white bit refers to primer. Next step is to fit the windscreen and backlight, when it becomes the glazed body in white, or BIW+G.
ANS: 4
BIW – Some surfaces are Class A, i.e. body side, roof, sill appliqué.
I heard some time ago from a old designer that the term BIW comes from when cars were built from wood, they were painted white as it gives the frame a uniform color so imperfections were easily visible.
BIW meaning Body In White is so called due to its appearance after the application of the primer to the entirely Body panel assembled vehicle just before going into the painting process.
Usually the primer is white or silver grey which gives the so called name.
ANS: 6
Catia is mostly used for BIW design (Ford switching to catia, and Toyota).  Is this because it
could easily create quintic surfaces? With UG with Design bundle only, most of the surfaces created are cubic.
ANS: 7
A class surface means – it is not just seen surface and unseen surface In normal no technical words,
A class surface means
It is smooth looking reflective surface with no distortion  of light highlights, which moves in a smooth uniform designer intended formations.
when you create – car body panel, due to their complex shapes it not possible to create the surface with one single face /patch so you make multiple face/patch ( surface is a group of face/patch added together.)
when these things are added, at the boundary of joining you need to have connectivity and continuation of minimum order two.
for example
In case one,  at the connecting boundary of two patches you have common boundary but it is sharp corner. this does not qualify as A class surface.
In case two –   at the connecting boundary of two patches have common boundary and no sharp corner – but you have tangent continuity, this also does not qualify as A class surface.
In case two –   at the connecting boundary of two patches have common boundary and no sharp corner –  you have tangent continuity and curvature continuity this  does  qualify as A class surface.
( sine curve is good example for curvature continuity. but you can not call it a A class surface )
reason is very simple  the real requirement of aesthetic and good looking and designer intended shape is not there.
ANS: 8
For obtaining Class-A surfaces, CATIA is more commonly used due to its inherent ability to model very high quality surfaces in general. But, any engineering software (CATIA, UG, IDEAS, Pro-E, etc) cannot develop a Class-A surface. This being due to engineering calculations involved in any surface generated by such softwares. For pure Class-A surfaces you would need styling softwares like Alias, Studio, etc.
The use of any software would depend on the level of expectations placed on you. If your projects need only the modeling of the trim, generic engg softwares will do, but if you intend to go down right from styling, you would need Studio, etc.
ANS: 9
IHO, Catia V4 has added a tool called Blend surf that is able to obtain virtual curvature continuity. Previously, even styling was comfortable with models- and hence tools- defining fillets with conics, and many OEMs still accept this for Class-A surfaces. Catia V5 has GUI interfaces to impose curvature continuity the same way that Alias-Wave front Studio Tools (Auto Studio) does. They are both based on piece-wise polynomial equations, for what its worth. While a conic fillet is not technically curvature continuous, there are many vehicles, including luxury models, that have utilized them for Class-A surfaces and downstream- parts. Considering the tolerances in creating molds and dies and then producing parts from them…. a sheet metal panel is not a math model.
ANS: 10
It is true that it is tough to make good curvature continuous  surface in UG, but not impossible.
Remember one thing A-class doesn’t mean just curvature continuity. and smooth reflections on CAD surface. it is lot more than that. Imagine. what happens to your A class surface in case pressed sheet metal body panel. and molded  plastic components.
They have to retain there intended smoothness and other characteristics to remain A class.  to achieve this lot of other things has to be taken care while designing A class surfaces.
For example :
1- Line features on body side external panel and feature on   hood panel which is very common, are to be designed  to avoid skidding while they are pressed. like wise
2 -Flange width and other things are to be taken care while designing fenders wheel arch area  for avoiding bulging effect and skidding effect.
3 – Fuel lid opening area, plunged flange for bulge effect.
4 – Panel stretching needs to be taken care. Lot many other things go in designing A class sheet metal panels for door , roof  etc.
5 – In case of plastic, sink marks  and other things.
ANS: 11
In Europe a ‘A’ class surface is generally taken to be the visible side of any component / assembly – a ‘B’ class surface generally relates to the opposite (or inside) face of an ‘A’ surface – i.e. the surface which defines the thickness of the part, and is where the mounting and reinforcing detail tends to be located.  ‘B’ class surfaces can also be referred to as ‘engineering surfaces.  I have not personally heard of any surface being referred to as a ‘C’ type.
Catia, while it is ok for surfacing tends to be more used for generating engineering surface detail and solid models  – software packages like ICEMSURF tend to be more used for generating visual quality surfaces.
ANS: 12
True A-class surfacing – especially on vehicle exteriors goes further than G2 or “curvature” continuity.
G3 is often sought on the more major block surfaces. G3 deals with curvature “acceleration”, i.e. the rate of change of curvature across a boundary.
G2 means as has been described before that the curvature value is the same across a boundary.
G3 means that the surface curvature leading to the boundary is changing shape at the same rate. Its like driving a car round a bend, you start off straight then gently add steering lock to the point where you need no more, then you gently wind off the steering until you’re straight again. If you look at the curve your car made, this would be G3.
A-Class and B-class would refer to surface quality required for the component which is different to A-side and B-side which refers to which is the visible/non visible part of a component.
ICEM surf is considered the best tool for speedy A-class surfacing due to the sophistication of its real-time diagnostics.
The consequence:
The consequence of these surfaces apart from visually and physically aesthetic shapes is the way they reflect the real world. What would one expect to see across the boundary of pairs of point continuity, tangent continuity and curvature continuity surfaces when reflecting a straight and dry tree stump in the desert????
vPoint Continuity (also known as G0 continuity) – will produce a reflection on one surface, then at the boundary disappear and re-appear at a location slightly different on the other surface. The same reflective phenomenon will show when there is a gap between the surfaces (the line markers on a road reflecting across the gap between the doors of a car).
vTangent Continuity (also known as G1 continuity) – will produce a reflection on one surface, then at the boundary have a kink and continue. Unlike Point continuity the reflection (repeat REFLECTION) is continuous but has a tangent discontinuity in it. In analogy, it is “like” a greater than symbol.
vCurvature Continuity (also known as G2 continuity, Alias can do G3!) – this will produce the unbroken and smooth reflection across the boundary.
vTo achieve the same Class ‘A’ surfaces that automotive manufacturers demand, consumer product manufacturers have availed themselves of the same advanced surface modeling tools. What is a Class ‘A’ surface? The simple answer is that it is a perfectly smooth surface with no anomalies, in which all adjoiningsurfaces have curvature continuity. This means that where two surfaces meet, the graduation of one into the other is achieved without discernible abrupt transitions. The techniques used to create Class ‘A’ surfaces typically reside in top level surface modeling software developed for the motor industry, rather than mid-range mechanical CAD packages that have evolved from 3D solid modeling for mechanical assemblies.