Author Topic: Extrude along two or three curves  (Read 234 times)

Dr PR

  • Hero Member
  • *****
  • Posts: 5379
Extrude along two or three curves
« on: October 20, 2016, 08:32:10 AM »
Sometimes it is desirable to create an extrusion that "corkscrews" along an the curved edge of an irregular surface.

1. Think of a long wet egg noodle that bends unpredictably. Now suppose you want to extrude a shape - say a rectangular plane - along an edge of the noodle with one surface of the rectangular extrusion in contact with one surface of the noodle.

If we could draw two curves, one along the edge of the noodle to define the extrusion path, and another along another edge of the noodle to define extrusion orientation. We then select the rectangle, setting two handles - one to follow the extrusion curve and another to define orientation. Then the program could make the extrusion in the desired direction with the desired orientations.

2. Now think of the intersection of two irregular grid surfaces where the angle of intersection between the two surfaces is not constant. Suppose you wanted to extrude a "L" shaped line along the intersection with one side of the "L" in contact with one grid surface and the other side of the "L" in contact with the other grid surface.

Draw three curves, one along the intersection of the two surfaces, a second along the edge of one of the surfaces, and the third curve along the edge of the second grid surface.

Set three points on the "L", one for the point to follow the curve, another at one end of the "L" to define one orientation for that end and a third at the other end of the "L" to define the direction for that end.

Now the program could extrude the "L" with a varying angle between the sides of the "L" so that each side of the "L" remained in contact with a surface of the grid.

****

Trying to do either of these tasks now with DesignCAD can be an all day adventure.

Phil
DesignCAD user since 1987

Dr PR

  • Hero Member
  • *****
  • Posts: 5379
Re: Extrude along two or three curves
« Reply #1 on: October 20, 2016, 12:24:10 PM »
Maybe I should say that extruding along wet noodles isn't just a theoretical exercise.

In construction where two metal plates come together at an angle it is common practice to weld or rivet an "L" bracket to each of them to strengthen the join.

If the plates are flat and they come together at right angles it is a simple matter to use an off the shelf "L" extrusion.

But in ship hull construction they rarely come together at right angles and the plates are almost always curved.

Ships are more or less "u" shaped from bow to stern, and the curvature of the decks, low in the center and high on the ends is called "sheer." In addition, weather decks are typically higher on the center line than at the sides of the hull, and this is called "camber." Combined, sheer and camber make the weather decks hyperbolic - there are no flat spots.

Hulls are pointed at the bow and rounded or squared off at the stern. They are widest amidships. This means the edges of the decks are curved.

The sides of the hull angle outward at the bow, making the angle between the ship's hull plating and the deck plating obtuse (greater than 90 degrees). Amidships the hull plating is often sloping inward, with the hull wider at the waterline than at the main deck level - this is called "tumblehome." So the angle of the decks and hull plating is acute (less than 90 degrees) here. The angle between the deck and hull plating is typically obtuse at the stern.

So the problem is to mate up a hyperbolic deck with curved edges with hull plating that is twisted as it wraps around the edges of the decks. This requires an "L" bracket that curves around the edge of the deck with a twist in it, and angles that go from obtuse to acute and back to obtuse again to match the angles between the hull plating and the deck. Like a twisted wet egg noodle.

Phil

I have attached an image illustrating this problem. The deck edge flange was created with "Draw/Surface Connect" after drawing many hundreds of cross section lines to serve as template lines for the "Surface Connect" function. It took the better part of two days.
« Last Edit: October 22, 2016, 08:45:38 PM by Dr PR »
DesignCAD user since 1987