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Freeform surface modelling is a technique for engineering freeform surfaces with a CAD or CAID system.

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1. Resolution: Levels and Models (covered in ANSI A250.8 (SDI 100) 'Recommended Specifications for Standard Steel Doors and Frames') are SDI term aid in the selection and specification process for the steel doors and frames.Level - describes the degree of durability needed for a give commercial application and mainly designate the thickness of the steel face sheet.
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The technology has encompassed two main fields. Either creating aesthetic surfaces (class A surfaces) that also perform a function; for example, car bodies and consumer product outer forms, or technical surfaces for components such as gas turbine blades and other fluid dynamic engineering components.

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CAD software packages use two basic methods for the creation of surfaces. The first begins with construction curves (splines) from which the 3D surface is then swept (section along guide rail) or meshed (lofted) through.

The second method is direct creation of the surface with manipulation of the surface poles/control points.

From these initially created surfaces, other surfaces are constructed using either derived methods such as offset or angled extensions from surfaces; or via bridging and blending between groups of surfaces.

Surfaces[edit]

Freeform surface, or freeform surfacing, is used in CAD and other computer graphics software to describe the skin of a 3D geometric element. Freeform surfaces do not have rigid radial dimensions, unlike regular surfaces such as planes, cylinders and conic surfaces. They are used to describe forms such as turbine blades, car bodies and boat hulls. Initially developed for the automotive and aerospace industries, freeform surfacing is now widely used in all engineering design disciplines from consumer goods products to ships. Most systems today use nonuniform rational B-spline (NURBS) mathematics^[1] to describe the surface forms; however, there are other methods such as Gordon surfaces or Coons surfaces .

The forms of freeform surfaces (and curves) are not stored or defined in CAD software in terms of polynomial equations, but by their poles, degree, and number of patches (segments with spline curves). The degree of a surface determines its mathematical properties, and can be seen as representing the shape by a polynomial with variables to the power of the degree value. For example, a surface with a degree of 1 would be a flat cross section surface. A surface with degree 2 would be curved in one direction, while a degree 3 surface could (but does not necessarily) change once from concave to convex curvature. Some CAD systems use the term order instead of degree. The order of a polynomial is one greater than the degree, and gives the number of coefficients rather than the greatest exponent.

The poles (sometimes known as control points) of a surface define its shape. The natural surface edges are defined by the positions of the first and last poles. (Note that a surface can have trimmed boundaries.) The intermediate poles act like magnets drawing the surface in their direction. The surface does not, however, go through these points. The second and third poles as well as defining shape, respectively determine the start and tangent angles and the curvature. In a single patch surface (Bézier surface), there is one more pole than the degree values of the surface.Surface patches can be merged into a single NURBS surface; at these points are knot lines. The number of knots will determine the influence of the poles on either side and how smooth the transition is. The smoothness between patches, known as continuity, is often referred to in terms of a C value:

• C0: just touching, could have a nick
• C1: tangent, but could have sudden change in curvature
• C2: the patches are curvature continuous to one another

Two more important aspects are the U and V parameters. These are values on the surface ranging from 0 to 1, used in the mathematical definition of the surface and for defining paths on the surface: for example, a trimmed boundary edge. Note that they are not proportionally spaced along the surface. A curve of constant U or constant V is known as an isoperimetric curve, or U (V) line. In CAD systems, surfaces are often displayed with their poles of constant U or constant V values connected together by lines; these are known as control polygons.

Modelling[edit]

When defining a form, an important factor is the continuity between surfaces - how smoothly they connect to one another.

One example of where surfacing excels is automotive body panels. Just blending two curved areas of the panel with different radii of curvature together, maintaining tangential continuity (meaning that the blended surface doesn't change direction suddenly, but smoothly) won't be enough. They need to have a continuous rate of curvature change between the two sections, or else their reflections will appear disconnected.

The continuity is defined using the terms

• G0 – position (touching)
• G1 – tangent (angle)
• G2 – curvature (radius)
• G3 – acceleration (rate of change of curvature)

To achieve a high quality NURBS or Bézier surface, degrees of 5 or greater are generally used.

History of terms[edit]

The term lofting originally came from the shipbuilding industry where loftsmen worked on 'barn loft' type structures to create the keel and bulkhead forms out of wood. This was then passed on to the aircraft then automotive industries who also required streamline shapes.

The term spline also has nautical origins coming from East Anglian dialect word for a thin long strip of wood (probably from old English and Germanic word splint).

Freeform surface modelling software[edit]

• Alias (StudioTools)
• Blender Free 3D Modelling Software from Blender Foundation
• CATIA (FreeStyle)
• Fusion 360 (Autodesk)
• PTC Creo Parametric, formerly ProEngineer ISDX

See also[edit]

References[edit]

1. ^Metzger, Michael; Eismann, Sabine. 'Freeform Surface Modeling'(PDF). hp.com. Retrieved April 15, 2017.

Levels and Models (covered in ANSI A250.8 (SDI 100) 'Recommended Specifications for Standard Steel Doors and Frames') are SDI term aid in the selection and specification process for the steel doors and frames.

Level - describes the degree of durability needed for a give commercial application and mainly designate the thickness of the steel face sheet.

• Face (face sheet) is defined as the surface of door exposed to view when closed
  
• Doors are available with the following steel face sheets
• 20 gage = 0.035' (0.81mm)
• 18 gage = 0.042' (1.06mm)
• 16 gage = 0.053' (1.34mm)
• 14 gage = 0.067' (1.70mm)

1. Level 1 - designates a Light Commercial door opening
• 20 gage doors
• 18 gage frame minimum
2. Level 2 - designates a Heavy Duty Commercial door opening
• 18 gage doors
• 16 gage frame minimum
3. Level 3 - designates a Extra Heavy Duty Commercial door opening
• 16 gage doors
• 14 gage frame minimum
4. Level 4 - designates a maximum Duty Commercial door opening
• 14 gage doors
• 14 gage frame minimum

Model - describes the vertical door edge treatment

1. Model 1 – Full Flush Edge
• Full height vertical seam is permitted on door edges
2. Model 2 – Seamless Edge (options as follows)
• vertical seam edge filled, dressed smooth
• Intermittently welded seams, edge filled, dressed smooth

• Continuously welded seam dressed smooth

Examples of Level and Model designations:

1. Level 1, Model 1 designation = L20-Series door in a DW16-Series frame
2. Level 2, Model 1 designation = L18-Series door in a F16-Series frame
3. Level 3, Model 2 designation = B16-Series door in a F14-Series frame
4. Level 4, model 2 designation = LW16-Series door in a F14-Series frame

To determine which Steelcraft door/frame construction or SDI level and model designation to specify, refer to the section and usage guides in the notes below.

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