internal surface quality

8/17/2019 Internal Surface Quality

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Internal Surface QualitySurface Tessellation For Visualization and AuditTo perform many of the visualization functions in EVALVIEWER, UG, SURFSEG etc., themath surfaces need to be tessellated (defined as a network of polygons that closely representthe math, a triangular mesh. After performing this conversion, interactive display speeds arefaster and a variety of evaluation functions are enabled.The tessellation should be finer for surfaces that have more shape and change of curvature. Anexample of this is shown on a quarter panel below.

The misinterpretation of surface quality can sometimes be a result of improper tessellation. If asurface or boundary condition between surfaces is in question, an extra fine tessellation may benecessary in order to make a valid judgement of the surface quality.

Guidelines for Tessellation in Surfseg and Evalviewer

Desired Analysis Tessellation Value

Exterior and High Gloss Finishes(interiors) 0.02 mm

Interior Trim and GrainTextured Parts 0.05 mm

Die Pull/Draft Angle Check or Interference/Clearance Check 0.15 mm

Medium Mesh - Tessellation

Fine Mesh - Tessellatio


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Surface Analysis ToolsEvery surface should be evaluated during and/or after it is created.There are many diagnostic tools available to interrogate the internal and adjacent surface quality,

but no one particular analysis can be relied upon to show every flaw.Therefore, it is recommended that a combination of these tools be used to conduct a thoroughsurface review.

The following are some suggested tools to use:• Fixed or moving light sources tracking highlights• Face or Surface Color Curvature Maps

Ø GaussianØ Max & Min CurvatureØ Mean or average CurvatureØ SectionalØ U&V directionØ Slope/Draft

• Reflection Lines , Photo Horizon, or Texture Maps• Section Analysis with Curvature Combs• Pole Network or Isoparms• Minimum Radius Measurement and Offset Surface

Surface Analysis StepsSome basic steps can be taken to evaluate the internal quality of a surface.

• Shade the surface and look for any unusual bumps, ripples, twists or inconsistenthighlights. By moving the light source or by rotating the part, many initialimperfections can be identified.

• Create or Run a dynamic section with curvature combs through the surface in the Uand V direction of the surface and look for any differing comb displays, multiple

peaks, or unwanted inflections. Other section creation methods, such as, radials or perpendicular to a curve, can be helpful in identifying problem areas aroundopenings.

• Look at the Pole Network or Isoparms of the surface to determine if the surfacestructure is consistent and proportional.

• Use the U and V Direction surface analysis tool to identify or support any problemsthat the U and V section cutting tools found.

• Use surface analysis tools such as Gaussian, Minimum and Maximum Radius, andMean/Average to further investigate or help confirm any problems that were found.

• Use minimum radius check to identify any small radii that may cause possiblesurface offset problems.

• Use Slope or Draft check to analyze surface for die lock conditions.


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Internal Surface Analysis – A-Pillar- Exterior

The following images illustrate several different types of surface analysis methods which allsupport the findings of an internal surface quality issue. The adjacent surfaces leading into thisarea are of acceptable quality. The issue at hand is the inconsistent flow or transition of thehighlight from the lower portion of the A-Pillar to the area above the top of the mirror patch.Analyzing the highlight on the shaded model doesn’t show the surface problem as well as theother analysis displays.

Surface IssueArea

Wavy ReflectionLine display

Wavy ColorReflectionLine display

Wavy SimulatedHorizon display

Maximum Radiusdisplay

Gaussian Radiusdisplay

Inconsistent Radius of Curvature Color Maps support Reflection Line displays above

HighlightTransition


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Addressing Surface CorrectionsWhen problems are identified within a surface, a determination needs to be made as to thescope of the surface rework in terms of max / min limits.Also, considerations need to be made pertaining to the amount of time needed to make thecorrection and whether the correction will be value added.

Localized changes can generally be resolved easily, but there are cases where a subtle changecan propagate through an entire panel and a majority of its surfaces.Generally when a primary surface change is made, it will drive several surface changes.The location of a surface imperfection should also be considered. If the problem is in a lowvisibility area, the surface correction may not be made, or more attention may be spent oncorrecting surfaces located in higher visibility areas.


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Spline Curves for Surface DevelopmentThe basis for quality surface development starts with good curve development. Here are some general rules for Spline curves:

1) Splines used as build curves for surfaces should be developed as single segment curvesto help reduce surface complexity. Using a combination of different degree curves withdifferent segmentation will generally result in a higher amount of complexity in a surface.As a result, any curve projected onto that surface will result in a curve with morecomplexity (more segmentation). There will be cases where curve segmentation cannot

be minimized and a system derived curve must be used.

2) Splines created as 3 to 5 degree curves will generally be flexible enough to capture mostshapes for surface development.

3) It is a good practice to develop surface curves as a family of lines having the same

characteristics and proportions (see below). For example, all U-direction curves would be 3 degree single segment curves and all V-direction curves would be 5 degree singlesegment curves. Also, the curvature combs should have the same shape or at leasttransition from one shape to another along the U or V-direction of the surface.

Family of Lines – SimilarPro ressive Curvature Combs

Door Surface

V-direction

U-direction


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Curve ContinuityThe following images show curvature combs, which help detect the curve relationships betweentwo curves. The most desirable condition is a curvature continuous relationship. In some cases,tangency will only be attainable and is also acceptable.

Continuity Conditions between Curve Endpoints

Continuity Conditions – Bridging between Two Curves

Most DesirableDesirable

Non-Positional Positional (C0) Tangent (C1)

Tangent (C1)

Curvature (C2) Curvature (C3)

Positional (C0)

More DesirableDesirable

Curvature (C2) Curvature (C3)


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Adjacent Surface ContinuityThere are four levels of continuity between patches of surface

1) Curvature (G2 and G3) – Most desirable2) Tangent (G1) – Acceptable in some cases (low visibility areas)3) Positional (G0) – Acceptable for quick surface or early design intent.4) Non-Positional - Undesirable

Examples of Tangent and Curvature continuity are show below.Using reflection lines and sections with curvature combs can be helpful in identifying continuityconditions. Surface boundaries can also be checked using angle and distance deviation checkingtools.

The continuity constraints applied to the mating surfaces depend on the design intent, butgenerally curvature continuity is most desirable.However, imposing surface boundary constraints to obtain tangency or curvature continuity atthe expense of internal surface quality is ill advised.

Curvature (G2)“S-shaped”

Tangent (G1)“Z-shaped”

Surface Boundary “Noise” caused byConstraints from ad acent surfaces


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Surface PatchesExcessive numbers of surface patches must be avoided. If not, it invites increased problems withcurvature continuity, adjacent surface checks, gaps, highlight separation and possible oscillations

between surface patches and within their interior.

Cutting a clay model is a valuable design verification tool to help identify possiblesurface imperfections, but it may not expose subtle surface quality problems thatbecome major problems at the DIE MODEL cutting stage.

Multiplesurfaces

Colored Reflection Lines showinginconsistent flow between surfaces

Shaded Model showing surface patch break-up

Gaussian Analysis showing radius ofcurvature changes within the panel

Simulated Horizon showing reflection breaks between surface patches


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Secondary Fillet Surface Quality and ContinuitySecondary surfaces should be curvature continuous, but where it makes sense, tangentcontinuous fillets are acceptable.The following math data conditions lend themselves to creating tangent continuous fillets insteadof curvature continuous fillets:

1) The Size or Width of the Fillet from Tangent line to Tangent lineGenerally very small fillets only need to be tangent continuous

2) The Direction of the Primary SurfacesPrimary surfaces that curve the same direction as the fillet being created canalleviate the need for a curvature continuous fillet.

3) The Lead-in of the Fillet

Having a longer lead-in on a tangent fillet helps soften the surface transition andthe highlight .

4) The Area of VisibilityIf the fillet surface is going to be in a low visibility area it wouldn’t be necessaryto spend additional time and effort to make it curvature continuous.

Section Curves withcurvature combs show thecontinuity between thePrimary and Fillet surfaces.(G2 continuity)

Colored RefectionLines

Colored Reflection Linesare used to observe anysubtle or major surface

breaks along adjacent

surface boundaries.

A Gaussian Color map isdisplayed to observe theinterior surface shape andalso the surface flowacross the fillet tan ent


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Gaussian Analysis


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Boundary Curves for Tangency and Curvature

When building a filletwith curvaturecontinuity, expandingor offsetting the tangentcurves usually isnecessary to attaincurvature continuityand to hold the filletsha e.

Yellow Curves forTangent Continuity

Reflection linesshowing the results ofusing the yellow

boundary curves for building the filletsurface.

Reflection linesshowing the results ofusing the red curvesfor building acurvature continuousfillet surface andmatching the tangent

fillet sha e.

Curvature

Red Curves forCurvature Continuity

Tangency


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High Visibility Areas Curvature Continuity must be obtained in High Visibility Areas.To attain curvature continuity, expanding the Tangent lines (or Curvature lines) may be requiredin order to allow for more lead-in to produce the desired transition characteristics.Areas of Low Visibility (i.e. inside fascia openings, lower knee bolsters or underside surfaces)need only be tangent continuous . In the illustration below, the section curve shows theexpanded tangent lines and the fillet shape used to obtain curvature continuity.

The following images show Curvature Continuity over the Wheel Opening

A

A

Section A-A

Curvature Fillet

Shaded Image of aQuarter Panel in aHigh Visibility Area

Reflection lines showingthe smooth transition ofthe CurvatureContinuous FilletSurface.S-shaped reflectionlines are generally moredesirable in these areas.

Inflection Pt.

Inflection Pt .

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