Wednesday, 5 November 2014

The Ten Golden Rules of Surface Modeling

Clay Model Analogy
Create Larger Surfaces First.
Just like clay modeling, where you want to work with your largest tools first, build your CAD model by creating the largest surfaces first.

Analyze your design concept and determine the largest features and surfaces and try to create those first.
Leave the smaller details and features for last.


Take the example of a hair dryer shown in figure.
There are three distinct parts to it.
The top one is the blower, the second is the handle and the third one in the middle is the joint between the two parts.
It is very much possible to create them in the order : blower-joint-handle.
In this case, it is recommended to create the blower and the handle first, since it establishes the relative position and angle between the two.
The smaller detail like the joint can then follow.
To understand this rule even better, read a detailed tutorial on creating the hair-dryer Here.


Use Birails for Effect Maximum
Try to visualize a given surface as two cross-sections and two guide-curves called birails.
A BlueSurf can then be fitted using just these four curves.
The birail method allows you to quickly get to the overall form of your design with less of curve creation.

See the bottle in figure and its four curves.



Similarly, a door knob with complex looking organic shapes can also be quickly created using just four curves.
Detailed tutorial on creating the bottle is
Here and for the door-knob its Here.


Hybrid Modeling - Best of Both the Worlds
If you peep a little deeper into the history of CAD, it were surfaces that were invented first, then came along the solids.
In Solid Edge, both the Surfacing and the Features toolbars sit side-by-side in the same part environment.
Tools from both toolbars can be used in conjunction to create a single parts.
This type of modeling practice is called as Hybrid modeling.

Use hybrid modeling to your advantage.


The mouse shown in the top figure is an example of hybrid modeling at work.
Curves were used to to create the orange surface.
Curves were also used to create a base protrusion.
Using the Replace Face  
replface   tool from the surfacing toolbar, the top face of the Solid protrusion is replaced by the surface to give the overall form of the mouse.
By combining a solid and a surface together, you can form a hybrid model to obtain your desired shape without needing to build it with surfaces only.
A heavily illustrated tutorial on creating the mouse



Think Global - Act Local
Needless to say, curves play an important role in the final appearance of a surface.
So think global (about the form of the final surface) and act local (pay attention to the curves).
When tweaking curves, it is often required to move the control points.
When doing this, the endpoints of the curve may go haywire, affecting the desired shape of the curve.
To keep the end points in place, you may apply the Lock constraint to the endpoint of the curve.


Another powerful feature that Solid Edge provides is the Local Edit option available on the Ribbon bar when you select the curve (see figure)
By turning this option ON, only that part of the curve lying between the two control points adjacent to the control point being edited is affected.
The beauty of this option is that the endpoints stay in place even when you are editing the second control point from either end on the curve.



Start with Analytic Elements
Not just because Solid Edge cannot create a closed curve with the Curve
curvtool   tool, you must first start with an analytic element like a circle, an ellipse or a rectangle.
Further using the Convert to Curve
convcurv   tool, you have a curve for further tweaking.


Take the example of the hair-dryer. The front end opening of the hair-dryer is a closed curve.
See figure.


Here, first an ellipse (an analytic curve) is first created.
Then, using the Convert to Curve tool, it is converted to a curve where the shape remains the same (that of an ellipse).
By simply dragging with mouse, the top and bottom parts of the ellipse are flattened to get the desired shape.



Garbage In - Garbage Out
When creating curves, it is always recommended to use lesser number of control points on the curve.
Not any more with Solid Edge.
Start out with a curve having as many control points as you wish.
You may also use the Add and remove points
addpoint   option on the ribbon bar when a curve is selected.
Tweak the curve to your heart's desire.


Once you are satisfied with the shape of the curve, right click on the selected curve and select Simplify from the menu.
Adjust the tolerance value in the dialog box and keep a watch on the current number of edit points and control vertices.
Click OK when done.
Now you have a much lighter curve.
The Tolerance value determines how much detail and accuracy a curve and a surface will have.
This way you can have more control points to begin with and still end-up in a much lighter (non-heavy) curve with the desired shape.
Also, this will insure a smooth and geometrically clean surface.


Vertex Mapping - Avoid Heavy Curves
When building surfaces from curves, Solid Edge employs a mechanism called Vertex Mapping.
In this, a vertex or point from one curve is matched (mapped) with a vertex or point from the other curve and a surface is laid out.
Here, take care that there is no mismatch between the number of vertices on one curve and the number of vertices on the other curve.
That is, do not use heavy curves along side normal curves when building a surface.
Heavy curves produce large, hard to update files and often do not render quality surfaces that simplified curves do.


Vertex mapping is only useful for controlling or eliminating twists and discontinuities in a surface, since Solid Edge takes care of the mapping stuff internally.
Take a look at the two sides of the same BlueSurf, spanning two open curves.
Observe carefully, the zebra strip (Inspect > Zebra Strips) surface analysis on one side (upper figure).
The curve on this side of the BlueSurf is more kinky or appears cracked without the zebra strip.
From the lower figure its clear that the curve on the other side of the BlueSurf has lesser number of control points and hence appears smoother - such surfaces are always desirable.


Doesn't Work This Way ? Then Try That Way
Creating BlueSurfs and you have a set of curves for the cross-section and and another set of curves for the guide-curves.
There are no hard-and-fast rules as to which should be the guide curves and which are the cross-sections.
The surface may fail in one way, do not panic, if it doesn't work this way, then you can always try it that way.
Here is how...


Say you picked the cross-sections and guide-curves as shown in the top-side figure and things didn't work your way.
Swap partners and it will work.
Try picking the guide-curves and cross-sections as shown in the bottom figure.
Chances are you may get the desired result.



Handling Cusps - The Zebra Strip Tool is Your Friend
Surfaces are always made up of groups of four edges, mathematically.
So, creating surfaces (Blue or Bounded) not made up of four i.e. even number of edges, result in a concentration of control points coincident to each other, as the fourth edge is brought to a zero at that point.
These concentration points are called cusps.


Given a free ticket to the African safari, which zebra would you choose to fondle and caress.
The one with a smoother and even skin shown in top figure ( four curves ),
Or the species with a coarser skin shown in bottom figure ( three curves ).
To save time and money travelling across continents, I would prefer selecting Inspect > Zebra Strip.



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