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Doctor Walt’s Tips and Tricks # 51
Converting Entity Types
This month let’s talk about the process of converting entities. While this may not seem as
glamorous as creating a dramatic solid model, it is nevertheless a critical component of
many design projects.
We typically convert entities for two major reasons:
1. A single source entity is replaced by two or more entities of the same type that define
the same shape.
2. A Single source entity is replaced by one or more entities of a different type that
define the same shape.
The first option is most often employed to
facilitate mapping of surfaces when creating a
transition.
Consider the geometry illustrated to the right.
(The source file for this is available as the file
“Transition1” available for download.
Let’s assume that we want to create four
transition surfaces from the lower square
profile to the top circle. I’ve shown one in
light purple in the illustration to the right.
Now the geometry we start with is composed of
the square, a circle, and a transition path that
contains two lines and a fillet.
The circle has an endpoint that aligns with one
corner of the square. (I’ve indicated this with a
point in the file.)
To simplify creation of the transition surface we
would want to replace the circle with four,
ninety degree arcs. Each arc would then map to
one of the sides of the square.
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To do this, simply click on the AUTOSEG
ENTITY USING SAME TYPE Icon.
Type 4 for the Number of segments and click on
the circle.
Hit the ENTER Key twice and then click on the
Yes Option on the Conversation Bar.
This gives you four arcs on top of the circle.
You can now delete the circle from the file.
Now we could quickly duplicate the two
lines and fillet that define the transition
path curve using the Xform Rotate Copy
Function to get a path at each corner.
Before we do that, though, let’s replace
the two lines and fillet with a single
spline. (We don’t need to do this, but it
simplifies the surface creation process.)
To do this we’ll use another powerful
entity conversion tool. Click on the
CREATE NURBS BY CHAINING
ENTITIES Icon.
Select the line, fillet, and line that define
the path and hit the ENTER Key.
This will create a single spline that
follows the same path as the three
original entities.
Note: Depending on your Configuration Settings, the original entities may be
deleted, remain, or you may be prompted for an action. To set the behavior, click on
TOOLS/OPTIONS/ENTITIES and select the behavior you desire in the Create
Options. Notice that there is also an option there or the AutoSegment function that
we used earlier.
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Now, Use the XFORM ROTATE COPY Function
to copy the single spline three times at 90 degrees.
Your screen should look like this:
You can then use the EDGE CURVE SURFACE Function to quickly create a transition
surface patch using a ninety degree arc, two splines, and a line.
Surface Modeling Versus Solid Lofting
In the above exercise we create an individual surface patch. You could use the Solid Loft
Function to create a solid directly from the square to the four arcs and use the spline as a
Guide Curve. (Note: Guide and Path Curves should be single splines so you would still
use the Chain to Nurbs Function to create the curve. In this Tips & Tricks we are more
concerned with converting entities not the relative merits of a pure solid modeling versus
a hybrid surface-solid modeling approach.
Before we move on to the second major area where we need to convert entities, let’s
summarize the key operations that we have used so far to facilitate surface creation.
1. Break an entity like a circle into multiple smaller arcs to provide edges for surface
patches.
2. Create a single spline from multiple entities to simplify surface creation or to generate
a path or guide curve for a lofting operation.
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Converting Entities to Support Shop Equipment
A second major reason for converting entities is to provide data in a format that a
particular shop machine can use. Customers typically need to do the following depending
on the process equipment being used: (In each case I show you what function you can
use to convert the entities.)
Convert all entities in a profile to line segments.
Convert polylines to lines and splines
Convert splines to arcs.
Convert Nurbs to BSplines
For this next section you can use file
“SampleProfile1.” Supplied for download
with this exercise. The profile is illustrated
to the right.
Convert all Entities to Line Segments
In this profile we need to create line
segments for the type 3 arc on the left and
the spline at the top.
To do this, click on the
AUTOSEGMENT USING LINES Icon.
You are prompted to specify a number of
segments. A larger number will give you
more line segments that are shorter and
that more closely follow the curve.
Select 8 for the number of segments for
each entity. Hit the ENTER Key twice
and click on the Yes Option on the
Conversation Bar.
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I’ve illustrated 8 segments in red
for the arc and spline to the left.
Notice that we might want to use
more segments to get a closer
approximation.
Convert Polylines to Lines and Splines
A typical polyline conversion situation occurs if you
want to use True Type Text in an extrusion.
Click on the CREATE NOTE BY TYPING Icon and
create the word “CAT” using text that is 0.5 high, Arial
Filled Font.
Now let’s say that you want to extrude this text.
Click on the GENERIC BURST Icon.
Select the text twice.
The first time you burst the text, it will convert into a mix of polylines and splines. (This
is similar to what you would get if you had clicked on the “Create as Geometry”
Option at the bottom of the Note Dialog Box.)
Use the Generic Burst Function a second time and the polylines are converted to lines.
(The splines remain splines.)
You can now extrude the letter profiles into solids. The faces of the solids can be used to
create NC toolpaths. Some engraving programs also require the splines and lines instead
of polylines as profiles.
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Convert Splines to Arcs.
You might have a shop machine that cannot accept splines as the defining curves for a
profile. You could convert them to lines using the AutoSegment Function we discussed
earlier. Or, you can take advantage of another tool that creates type 3 arcs to replace the
splines.
Using the geometry from the note
“CAT” that we just used, lets say that
you want to replace the splines on the
letter “C” with arcs,
Click on the AddIns Pulldown Menu
and then on the CDE Open Option.
Next, click on the Tools.cde.
A palette of additional tools will appear at the bottom left
of your screen.
Click on the CONVERT TO ARCS Icon.
A Dialog Box appears. Select the
Splines Option and type a tolerance.
Let’s use 0.1 this time. You can also
indicate whether you want to delete
the original entities or move them to
another level. Hit the ENTER Key.
Click on one of the splines in the letter “C” and hit the ENTER Key.
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The spline is replaced by arcs. (Note: This can also be useful if you need to add
reference dimensions for effective radius at points along a spline curve. You can
easily place radial dimensions on the replacement arcs that would not be possible
using the spline.)
Convert Nurbs to BSplines
I occasionally have a customer request a way to convert
a NURBS curve to a Bspline to satisfy import
requirements into a particular shop program. Note that in
the same Tools CDE Palette that we opened a moment
ago, there is a simple function called NURBS TO
BSPLINE that does just that.
I’ve indicated the Icon in the illustration to the right.
Summary
While converting entities to other types may not be as
dramatic as some of the other modeling exercises I bring
you, it is a critically important activity that you should master.
I’ve gone over some of the most common situations in this exercise. Use your creativity
to expand on what we have done.
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