Author Topic: Tips for creating complex surfaces  (Read 750 times)

Dr PR

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Tips for creating complex surfaces
« on: January 21, 2016, 10:45:36 AM »
I needed to make a complex shaped cushion for a model of a MK 23 Mod 0 Target Designation Transmitter (TDT, the first picture). This took a couple of attempts to get a satisfactory result. I thought I would post some images and descriptions of the efforts as guides to others needing to create such complex shapes.

My first attempt used a lot of curve lines (Surface lines 1.jpg) to define the shape. Then I used these curves as template curves for the "Draw/Surface Connect" function. Each line consisted of two or three arcs with straight segments between them. The ends of these curves terminated on the flat back plane of the cushion.

I had created some less complex surfaces this way in the past so I thought it might work. The "surface shape 1.jpg" image shows the results. This image was generated after I had spent an hour or two tweaking the curves and generating new surfaces trying to smooth the wrinkles. It was a pretty wrinkled cushion, and not quite what I had in mind.

To make it smoother I would need to add a lot more intermediate template lines and use more planes per line. But DesignCAD allows only a pretty small number of total points in a grid, so this might not work.

I decided to start over with a more tedious process that I have used several times in the past with success. I started with the initial cross section lines that I used to create the first set of curves. I made planes of the X, Y and Z cross sections and extruded them. Then I used these to find the common volume for all three. "Solids/Solid Intersect" will do this. "Surface shape 2.jpg" shows the result. This is the basic shape that I wanted, but with sharp edges.

CONTINUED IN NEXT POST

Phil
« Last Edit: January 21, 2016, 10:47:52 AM by Dr PR »
DesignCAD user since 1987

Dr PR

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Re: Tips for creating complex surfaces
« Reply #1 on: January 21, 2016, 11:33:33 AM »
CONTINUED

Unfortunately, the "Edit/Trim-Extend/Fillet Edge" function won't work on complex edges like this. So I had to use some other method to "fillet" the edges. I had to create a series of arcs along the edges of the solid that could be used as template curves for the "Draw/Surface Connect" function. This is easier said than done.

First I traced the edges of the work solid. You can see these blue curves in "surface lines 2.jpg. I used the "Draw/Tickmark" function to generate many evenly spaced marks along each curve. Then I created a circular plane with a 0.25" radius and set two handles, one at the center and the second at a point on a line passing through the center and perpendicular to the plane.

I copied the circular plane at each tickmark, with the center handle on the edge curve at a tickmark and the second handle on the curve at the next tickmark. This placed the circular plane copies about perpendicular to the curve at each point (close enough for modeling government work). Then I used "Draw/Lines/Surface Intersection" to generate lines 0.25" long on the surface of the work solid that intersected at the blue curve. You can see these in the "surface lines 2.jpg" image.

Next I created arcs with a 0.25" radius that were tangent to these intersecting lines. Unfortunately, DesignCAD doesn't have an "Arc Tangent to 2 Lines" function. To create the arcs I used "Draw/Circles/Circle Tangent to 2 Lines) to create circles tangent to the pairs of lines. Then I used "Draw/Lines/Perpendicular to a Line" to create perpendiculars from each circle center to the two tangent lines. Then I used "Draw/Arcs/Arc (center, begin, end)" to draw an arc with the center at the center of the circle and ends at the ends of the perpendiculars. After creating about 230 arcs this way I deleted all of the 500+ circles and perpendiculars. The resulting (red) arcs can be seen in "surface lines 3.jpg."

I cleaned up the intersections where the curved edges come together by generating template lines common to both surfaces. I used these arcs as template lines for "Surface Connect" to produce the blue grids shown in the "surface lines 4.jpg" picture.

The two green surfaces between the blue grids and the back plane were easy. I just drew perpendicular lines from the end of each arc closest to the back plane of the cushion. First I drew a large rectangular plane for the back surface. Then I started the "Draw/Lines/Line" (V) function and gravity snapped to the end of the arc. Then I pressed F7 to execute "Point/Plane Snap" to set the second point of the line on the plane. This created a perpendicular from the end of the arc to the back plane. After drawing all of these perpendicular lines I used "Surface Connect" to create the green surfaces. I did have to draw separate polygons at the bottom sides where the blue grids curve around and back to the back surface.

The front red surfaces were drawn one polygon at a time. Each of the three and four sided polygons was hand drawn with the "Draw/Planes/Plane" (P) function. Then the back plane for the cushion was drawn with the plane function. This completed all of the surfaces. All template lines and arcs were then moved to a separate junk layer for safe keeping in case I screwed something up and had to try again.

I should say that I had to experiment a bit with the hand drawn polygons (red) on the vertical surface. If you put too many triangles together with a single common apex point - fan shaped - they don't render very well, and each polygon is visible in a different shade of gray when rendered. By alternating three and four sided polygons I got a better result, but you can still see some unevenness in the shaded surface.

All panes and grids were selected and then combined into a single solid with "Solids/Solid Define." The result can be seen in "surface shape 3.jpg." This is a lot smoother than the first attempt. After several days of drawing hundreds of template lines/arcs and circles and a few dozen grids I had the result I wanted.

CONTINUED IN NEXT POST

Phil

Phil
« Last Edit: January 21, 2016, 12:20:25 PM by Dr PR »
DesignCAD user since 1987

Dr PR

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Re: Tips for creating complex surfaces
« Reply #2 on: January 21, 2016, 11:46:17 AM »
CONTINUED

Sometimes the only way to generate a complex surface is to do it in pieces. The anchor (anchor.jpg) was created this way. Every curved edge is a separate radiused grid created as I described above. The other curved surfaces are also grids. All were made with "Surface Connect."

The boat hull (40 foot boat hull surface.jpg) is a much simpler example of a hull surface that was produced in several parts. In this case the sharp transition at the rear end of the skeg (the dark blue surface) just can't be created in a single grid surface. If you think my first cushion attempt in the first post of this series was wrinkled you should have seen the mess when I tried to figure out how to stretch a grid along the full hull length of this boat! Also, the sharp edge between the red transom and the green hull surface cannot be achieved with a single grid. I used eight separate grids to finish the hull surface.

For the surface of the Cleveland class cruiser (hull surface 2.jpg) I had to use many separate grids. I had the same problem with a sharp transition at the rear of the skeg to deal with as in the 40 foot boat. But in order to get smooth curves in some places I had to use large polygon counts. The entire hull surface was much too large for this and my attempted grids exceeded the maximum number of elements allowed. So I had to break some long continuous surfaces into several separate grids. This did allow me better control with high point counts where needed (bow and stern). I used much simpler low point count grids for the long smooth surfaces.

The main problem with multi-grid surfaces is getting the edges to align perfectly. You need all points of the two grids to line up (overlap)  along the edges of adjoining grids in order to have no gaps (or leaks for 3D printing). This can be very tricky in places where adjoining grids do not have same length edges. You can see this where the yellow and orange grids come together in the cruiser hull, and where the green surface joins the cyan and orange surfaces in the 40 foot boat hull. Also, getting the boat's red transom surface to join correctly with the green hull surface was tricky. For this I created the transom in two three sided parts using "Draw/Surface Patch."

Phil
« Last Edit: January 21, 2016, 12:09:40 PM by Dr PR »
DesignCAD user since 1987