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Modeling transition in AutoCAD

Creating a 3D model of a rectangle to round transition is easier than most think. Let’s try it.

Start by creating a rectangle of the desired size (use the ‘Line’ command), let’s use 12 x 5 for this example. To keep the math easy, use 0,0 for the first corner point. Then key-in the other corner as the size of the rectangle.

Select the ‘Rectangle’ command from the Draw menu.

Type the following coordinates at the command prompt:


‘Zoom Extents’ to display the rectangle.

Now for the round end of the transition! Actually we won’t draw it round, but will use a ‘Polygon’ command to make 16 segments around the circle. The number of segments should be a multiple of 4.

The radius will be specified as a Polar Coordinate to also tell AutoCAD the rotation of the polygon. We want a point at 0, 90, 180 and 270 degrees. To calculate the angle, divide 180 by the number of segments or sides. So a 12 sided polygon would use a rotation value of 15.

Pull down the Draw menu and select ‘Polygon’.

Enter number of sides: 16
Specify center of polygon or [Edge]: 6,2.5,10
Enter an option [Inscribed in circle/Circumscribed about circle]: C
Specify radius of circle: @3<11.25

This creates a polygon with a diameter of 6 at the center of the rectangle. The height of the transition is 10. Therefore the Z value for the center point is 10. If the round is to be offset from the center of the rectangle, add the offset values to the X and Y of the Polygon center location.

Time to create the bend lines! Make sure the ‘Osnap’ includes ‘Endpoint’. Then create lines connecting the points of the polygon to the corners of the rectangle. Look at my picture as a guide. You should have 20 bend lines.

Almost done, you still need a seam where the edges of the sheet metal will meet. If the transition is to be made in two pieces, there will be two seams. Create one more line from the polygon to the ‘Midpoint’ of the long edge of the rectangle. You may need to adjust your ‘Osnap’ for this.

From the View menu, choose ‘3D Views’, ‘SW Isometric’ to switch to an isometric view of the model. You should now have a 3D model of your Rectangle to Round transition.

But that is the easy part. What you really want is the flat pattern layout, right? Well I am saving that for next time.

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