Classification of Class A, B and C Surface in Surface Modelling in Cad Tools like Catia, NX/UG

Classification of Class A, B and C Surface in Surface Modelling in Cad Tools

Surface Finish is most important parameter when it comes to high end automotive and Aerospace and Defence Industries the reason behind that is if we have good surface finish we can reduce air drag and surface friction which effects so many parameters like fuel economy, endurance, life cycle time e.t.c. 

Following are the some of the major parameters which can help while designing the aircraft outer surface or high end automotive.

1. The distance between each point of the edges of two neighbouring patches must be with in

    The following limits:

  • For class A: no more than 0.01 mm.

  • For class B: no more than 0.02 mm.

  • For class C: no more than 0.05 mm.

2. The angle between the tangents to the surface on the edges of two neighbouring patches    must

     Comply with the following limits:

   • For class A: no more than 6’ (0.1◦).

   • For class B: no more than 12’ (0.2◦).

   • For class C: no more than 30’ (0.5◦).

Curvature continuity

The control parameter is the patch curvature along its contour.

• Class A surfaces must have coincident curvature at least every 100 mm of

   Contour of two neighbouring patches.

• Class B and C have no applicable rule.

Points of maximum curvature or inflection are only allowed along patch contours of Class A

Surfaces.

Completion surfaces

As we have seen, they do not belong to the aesthetic surface (even if part of them is visible

And influences the aesthetic evaluation of the body), but are added to represent the part as it

Can be produced.

• For class C surfaces: completion surfaces are not required; gaps are represented by double

   Lines drafted onto the aesthetic surface.

• For class B surfaces: only rounded contours are represented on the surface

   That is always fixed to the body.

• For class surfaces: completion surfaces must be designed in all detail.

   Shape tolerance

The shape tolerances are measured by comparison of a surface patch with the

Counterpart on the aesthetic surface.

• For class A surfaces: no more than ±0.5 mm on body shell surfaces (or large

   Surfaces) and no more than ±0.2 mm for interior trimming (or small surfaces).

• For class B surfaces: no more than ±1.0 mm on body shell surfaces (or large

   Surfaces) and no more than ±0.5 mm for interior trimming (or small surfaces).

• For class C surfaces: shape tolerance is not applied.