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Modelling Auto parts

Modelling Auto parts

Modelling Auto parts

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?

RE: Modelling Auto parts

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 ofa part of a surface that is permanently covered by another part.

BIW is stuff like the body side etc..

RE: Modelling Auto parts

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.

RE: Modelling Auto parts

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.


Greg Locock

RE: Modelling Auto parts

A support to GregLocock & gdodd, 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.


RE: Modelling Auto parts

Thank you for clearing up what BIW and the different Class surfaces.

I understand that 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.

Would this affect the Class A surfacing? Our company is
planning on doing interior trim design for GM.  Would we need UG/Shape Studio? Or cubic surfaces would be enough.

RE: Modelling Auto parts

Hello friend

A_class surface means - it is not just seen surface and unseen

In normal no technical words, A_class surface means

It is smooth looking reflective surface with no distortion  of light
highlites, which moves in a smooth uniform designer intended

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 curveture continuity this  does  qualify as A_class surface.

( ps:  sine curve is good example for curveture continuity. but you can not call it a A_class surface )

reason is very simple  the real requirement of asthetic and good looking and designer intended shape is not there.

i am yet to complete this topic

see you some other time.


RE: Modelling Auto parts

Hear is a further understanding of Class-A surfacing based on experiences with two automotive companies and whites goods manufacturers. They independently have the same definition for the classification.
  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 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 etc etc). It is a requirement where aesthetics has a significant contribution. 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.

RE: Modelling Auto parts

BIW - Some surfaces are Class A, i.e. bodyside, roof, sill applique.

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 colour so imperfections were easily visible.
I am very interested to find out where this term originated.

And finally, the only reason why Ford is switching to Catia is simply that GM have got there tongues somewhere near EDS's butt and catia is the obvious (ind.Standard) choice.
Plus, they are not switching straight away, prob around V11 I-deas.

RE: Modelling Auto parts


Regarding your query on 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, I repeat 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 modelling of the trim, generic engg softwares will do, but if you intend to go down right from styling, you would need Studio, etc.

GregLocock your comments are required.....

RE: Modelling Auto parts

IHO, Catia V4 has added a tool called Blendsurf 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-Wavefront Studio Tools (AutoStudio) 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.

RE: Modelling Auto parts

I got a situation where I need to fix
imported data (IGS to UG)for a center console wherein
the front surface was really bad.
I got to smoothen it out in UG (took a long time)
but the adjoining surfaces is still off a bit
when looking at it using reflection.

The styling guy said that it would do for now and
they would wait for the first shots from the mold
to see if we still need to tweak it.

It is very hard to make the curvature continuous surfaces
just by using UGNX.  I tried all the sweeps, swept, blends,
bridge, etc, smoothen constructions curves, make tangent, etc. but still the adjoining surfaces is still off.
The only way I could fix that is to recreate the adjoing surfaces too.  All surfaces that should be tangent has to be recreated which is a pain in the ...

RE: Modelling Auto parts

Hello friends ,


It is true i agree with you, it is tough to make good curvature continuos  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.

For your case of centre console ,even a smooth second order continuity surface is good enough. What your friend says is true
wait for the molded component.

Good luck friend


RE: Modelling Auto parts

In Europe an  '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 - ie 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.

best wishes


RE: Modelling Auto parts

Hi friends,
 A-class surface is nothing but the smooth outer surface of a body or part which is visble. The surface should have point, tangent and curvature continuties.
for example: if u put light on a surface nd if the reflection is perfect then, we can say its a A-calss. surface.

RE: Modelling Auto parts

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", ie 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 yougently 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. This is not an advert for ICEMsurf - I'm just a user. Hope you found that fun!!!

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