Top-down modeling

A reader's question presents an interesting challenge

THE FABRICATOR® JULY 2009

July 1, 2009

By:

A reader has a question about modeling an assembly comprised of three plates and tubing. Columnist Gerald Davis provides guidance.

Ellipticalcut Precision

Figure 1How do you model the elliptical cut in the middle plate for the tubes?

This month's column is in response to a question from Mike B.: "I have three plates parallel to each other 12 inches apart. Plate 1 is 9 in. by 9 in., while Plate 2 and Plate 3 are 18 in. by 18 in. Connecting all three plates are four 1.5-in. pipe size tubes going from each corner of the outer plates.

"The question I have is how to put an elliptical hole in the middle plate where it intersects the tubes.The plate would be laser-cut on a 2-D machine. Adding to the problem is that I need half of the ellipse from the top side and the other half from the bottom side of the plate."

One solution to Mike's question is shown inFigure 1. We will go through the modeling process for this project right after this disclaimer: The 3-D CAD software I'm using may be different from yours, so you may need to adjust some of the terminology and options.

Our goal is to create drawings for parts that can be fabricated on a 2-D cutting machine, such as a laser, plasma, or waterjet.That leads us to model this as an assembly of individual parts, which makes it easier when it comes to the drafting of prints and generation of DXFs.

CAD Tricks

The 3-D CAD software will calculate the intersection curve between the tube and the plate, so we'll use that tool when it comes to making the cuts.We also know that a "normal" cut in a sheet metal part is perfect for 2-D cutting. With that in mind, our plates will be modeled as sheet metal parts in order to take advantage of the normal option.

Finally, the distance between our plates and the size of the plates may change in the future, so we want to set up our model for easy revision.

Putting Together the Parts

Modeling a square plate requires sketching a square and extruding it to the proper sheet metal thickness.In this example, the thickness is 0.5 in. The software doesn't really care even though we know that's pretty thick for sheet metal.

For our convenience in assembling the parts into a model, we will sketch the rectangle with the origin dead center (seeFigure 2).

We're also going to create a sketch that contains a single point, which will be used for locating the end of the tube in the assembly (see Figure 3).

We'll use an axis for creating circular patterns of holes and tube. This is a good time to add an axis to the model of the plate (see Figure 4).

The only difference between the models for the three plates is the overall size. Once I completed the first plate, I made copies of the file, renamed them, and then edited them to change the size from 9 in. by 9 in. to 18 in. by 18 in.

The model for the tube is based on a sketched circle with a diameter of 1.5 in. I used a thin extrude option to set the wall thickness to 0.2 in.—similar to Schedule 80 tubing—for a final outside diameter of 1.9 in.

I extruded the length to be well past the target of 24 in. because I intend to trim the tube to length in the assembly. You might prefer to model the tube differently—using Extrude to Surface, for example—but that's a topic for another column.

Call to Assembly

The plates are positioned in the assembly using mates that reference planes—top-to-top and front-to-front.The outer plates are mated at the 24-in. distance from each other.The middle plate is positioned with a width mate to center it between the other two plates.

The tube is then inserted into the assembly and positioned with mates to connect its center axis with the point sketches in the outer plate models (seeFigure 5).To fully constrain the tube, its origin is mated coincident with a surface on the middle plate.

Cutting to the Chase

Now we can answer Mike's question!With the assembly open, edit the model of the middle plate.Create a cut-extrude.Use the Intersection Curve tool—found under Tools>Sketch Tools >Intersection Curve—to calculate where the tube intersects the plate.The result is shown in Figure 6.

After the cut is complete, repeat using the other surface of the plate—otherwise the tube won't pass through the hole without interference.

Use the Circular Pattern tool to create the other three holes in the plate.For extra fun, create a circular component pattern to model the other three tubes.

Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to dand@the fabricator.com.



Gerald Davis Design and Consulting

Gerald Davis

Contributing Writer
Gerald Davis Design and Consulting

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