November 3, 2009
No matter what drafting standards you adhereto, a CAD jockey should always strive todeliver information in the best and mostcomplete format to the manufacturing floor.
Our project is to create the drawing shown inFigure 1. For the purposes of publication I used an excessively large font for the bill of materials (BOM). Other than that, I believe this print would be presentable to a fabricator in order to have this product manufactured.
One of the advantages of using a 3-D CAD system to perform this work is illustrated by the isometric view shown in the top left corner of Figure 1. The exploded view was created in a few mouse clicks; using a drafting board and pencil to draw this would require more patience.
I'm guilty of not always adhering to ANSI drafting standards. I sometimes take shortcuts in an effort to shorten the delay between the development of the 3-D model and the arrival of the actual part for inspection. I don't recommend shortcutting as a good practice. I recommend that you always produce fully detailed drawings that any fabricator can interpret with certainty.
Nonetheless, I have some good habits in creating drawings for sheet metal parts. Including an isometric view on the drawing is one of them (see Figure 2). Note that the isometric view has no hidden lines visible and that the tangent edges are visible. This makes it easier to see where the bends in the sheet metal start.
The standard projection views do have hidden lines visible (see Figure 3). That makes it easier for the press brake operators to figure out the orientation and relationships of the bends. For that same reason, the tangent lines are hidden in the projected views. Otherwise, the brake operator might confuse a tangent line with a bend line.
Perhaps my most important "habit of best practice" is to make the projected views big enough so that the details are easy to read. I almost always plan for my drawings to be printed on B-size (11- by 17-in.) sheets of paper.
In the example shown in Figure 1 we have captive fasteners installed in a sheet metal part. Because multiple components are in the product, a bill of materials is needed. I do not put a BOM table on the drawing if the product has only one component. I see including a table with one row as drafting clutter.
Including a BOM has many benefits. It forces me to create the 3-D model in a more realistic way. It helps in estimating the cost and planning for the production process. It helps the purchasing and quality control departments because everything is listed and easily referenced.
This is where I get into the most trouble by violating ANSI standards: Dimensioning a drawing properly is critical to success, and every time I take a shortcut, I get into trouble.
In Figure 3 I'm trying to communicate to the fabricator about how to manufacture the product. I want the dimensions to be easy to interpret. I don't want a lot of clutter on the drawing that might hide the illustration of the part. I did an OK job of this in Figure 3.
However, I could have done better. No angles are dimensioned. No tick marks are included to indicate inside versus outside surfaces. I completely skipped geometric tolerance and dimensioning.
The 3-D CAD software I'm using has a work flow that starts with creating the 3-D model (see Figure 4). The next big event is to combine the 3-D model with what the 3-D CAD software terms a "drawing template" to create a 2-D drawing file. Once the views are set up, I adjust the position of the dimensions.
The information in the title blocks is pulled in automatically from the file properties of the 3-D model file, so I frequently use the same drawing template for all of my drawings. Note that "file properties" are a Microsoft thing— you can set file property values for nearly any file. Some CAD software uses values stored in file properties for useful tasks like filling in title blocks on drawings.
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 firstname.lastname@example.org.
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