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"Class A quality" surface

Some NX features claim to create "Class A quality" surface. In the official documentation very little information can be found on "Class A quality" surface. Some information from other software documentation can be found on web.
Q1: What does it mean exactly (not roughly "better quality") when a feature claims to create "Class A quality" surface"?
Q2: Where can I find more information on "Class A quality" surface, especially guidelines for constructing it with common NX features?

Re: "Class A quality" surface


Ciao @surfactant,

in some threads as

@StevenVickersgave you some very effective and good tips.


When I did my first works of this kind, the best definition of class A surfaces was given to me by a customer: it must be 'bella' (beautiful in english Smiley Happy). 

More than a mathematical definition a class A surface is defined whit what you must NOT to do, never use three sides surface, for example.

Some customers require specifications such as never using Blend command for radii higher than 5 mm, an so on.

I design small parts, I don't work in the automotive industries, I try to work on the details. Below only the red one is a blend.




Re: "Class A quality" surface


Hi @Cesare Thanks for the very helpful guidance!

Re: "Class A quality" surface

Siemens Phenom Siemens Phenom
Siemens Phenom

Class A, as Cesare states, are surfaces that look good.

Mathematically it is generally seen as a collection of surfaces were the reflection lines are clean, usually this means curvature continuous (G2) or higher continuity (G3) between surfaces and no odd indentations/bulges. But it is more an aesthetic evaluation.

Main commands to get to know are the surface analysis ones such as reflection lines and highlight lines.


Steve V

Re: "Class A quality" surface


Hi @StevenVickers Thanks for the reply!

Re: "Class A quality" surface

Siemens Legend Siemens Legend
Siemens Legend
Another, maybe fading feature is that designer of those surfaces must be able to prove that the surfaces are as good as intended, before going into production. When you are cutting a tool, and find that there are bumps or other defects...
The traditional way of proving this is to look at the pole structure of the surfaces. A skilled surfacer can then decide .
The logical foundation for this is that the surfaces must be as clean and simple as possible, a surface of 20x20 patches is very difficult/ impossible to prove the quality using this method, compared to a single patch surface.
surfaces with high numbers of patches should probably never be considered class A, because NX had to create all these patches to fit the tolerances, which in turn probably means that the curves either are bad , or the shape too complex.
in this, there might be bumps which are difficult to spot before production. This shape should have been created in separate surfaces instead of a single.
Someone mentioned "Class A as informed modeling", - Knowing the result that just got created. -How many patches at what degree did i get ? How are the patch seams ?


Re: "Class A quality" surface


Hi @1u7obd Thanks for your comments!

Re: "Class A quality" surface

Siemens Genius Siemens Genius
Siemens Genius

Opinions will be divided on this so below is just that. An opinion.


this does not represent any official Siemens view and is purely my own.


Why is it called "A" = The visible product exterior

The other side is "B"


Before folks start throwing rocks: Below are some of my personal definitions.


General A-Class

Sufficiently good and acceptable visible exterior.

If needed: has the required required tangency and possibly  curvature qualities.

Maybe: C2 and certainly higher continuity isnt appropriate.

Maybe: reasonable reflective quality.

Easy (easier) to NX create without too much option trial and erroring..

Surface order:  Low (typically degree 3, maybe higher), Can have  knots.



Some of the NX surface quality tools aren't appropriate


"True" A-Class   for the main = big, primary surfaces:    typical: automotive exterior

Low polynomial surfaces (degree ~typically 6 to 8) The user is constantly trying to get the lowest order.

No (= zero) knot lines

Has the required: very high quality  G0,G1,G2,frequently G3 continuity to adjacent surfaces.

Good reflective qualities, means: unable to detect the surface boundary on a reflective plot.

Surfaces typically  have single convexity and not "S" multi convexity type.



Much more labour intensive to create.

Mr User has to be aware of every pole, every surface order.

Many of the NX analysis tools are used extensively to give surface feedback.

Only a few NX features are suited (and using only some of the options) 

Secondary surfaces: eg: joining the big primaries:  Some blending features may not be approprate.

Results in: Very good Follow on NX modelling behaviour


For both the above:

Good curvature is better than bad flow continuity

Good tangency is better than poor curvature continuity

some NX features (although extremely useful) I do not consider A-Class. eg: Fill surface.


Your question: Where can you find more info?: I'd expect training to deliver on that one.

- Neil