I’ve been working for a major A&E firm for more than a dozen years, and have worked with many CAD operators and designers with various skill levels, both within the office and outside consultants. As a multi-discipline A&E firm, we share files across disciplines within the firm and sometimes from outside consultants. I’ve seen some clean, proper CAD work out there, and some CAD work that should have never been allowed to happen. With a multi-discipline project and a multiple consultant design team environment, the primary design firm for a project should be setting the standards for the whole team, which we can only hope is logical and intuitive and accurate.
The purpose of this article is to highlight the importance of following good drafting practices and standards and discuss some of the problems that can occur when these practices and standards aren’t followed. Many of the problems I’ve seen are caused by people not following set standards, who are new to CAD design, or who have worked at a smaller shop doing their own thing. I would like to go “back to basics” and bring up some AutoCAD® topics that we veterans need to be reminded of, and make the new folks in the industry aware of some facts as well.
There are many things that can make a good design project go bad due to poor CAD drafting practices. I will point out just a few of the problems I’ve seen that have caused personal headaches. Most of my scenarios are geared toward the building design industry and CAD standards we use at the company I work for, but the basic concept of following good CAD drafting standards and practices holds true for all industries. Disclaimer: Of course, there are exceptions to any rule, including the ones brought up in this article. But remember—you need a very good reason for not following the rules (and laziness is not a good reason!).
One thing I’ve seen on projects is the dimensions on a drawing aren’t adding up or the geometry is inaccurate. This can cause problems for everyone using the drawing for information and expecting it to be accurate. This is usually due to sloppy drafting practices such as not using “ortho” when needed, not using “object snaps” when needed, or “fudging” the geometry (i.e., using loose tolerances so it “looks good,” but isn’t (see Figure below).
The resolution to these problems is found in simply adhering to good drafting practices. The CAD designer must draft everything accurately—full size in model space to an acceptable tolerance. If the item needs to be 10’-0” long, it should be drawn 10’-0” long; not 9’-11 7/8”—and the dimension precision rounded to 1/4” (see Figures below).
Designers should be using AutoCAD’s “ortho” when drafting truly horizontal and vertical lines. They also should be using proper “object snaps” when placing lines (where applicable) and dimensions. A seasoned designer will usually use a set of favorite “running object snaps” when drafting, or at least access the correct ones on the fly (see Figures below).
Another problem I see is a drawing created with entities not drawn “ByLayer.” This can cause issues in many situations. In a multi-discipline firm, we use a lot of xrefs (external references) and reference a lot of the same base files for each discipline’s drawings. It is necessary to manipulate the printed appearance of these xrefs through the layer manager for a particular purpose. Example: The mechanical designer uses the architectural floor plan as an xref background and draws mechanical equipment. They need to adjust all the architectural line weights to a light background to communicate their design on the plans. If there are entities created in the architectural plan that are not ByLayer, the ability to modify those entities’ line weight or color through layer manager is disabled and creates a problem for this designer and others who need to manipulate them (see Figure below).
Obviously, the resolution to this is to create AutoCAD entities ByLayer whenever possible, especially color and line weight. Line types are important, too, but I’ve seen drawings where the line type has been changed from continuous to hidden for a purpose and it didn’t cause too much of a problem. A great rule of thumb is: If it needs to appear different, make a layer for it and keep it ByLayer.
A similar problem I encounter is a drawing that contains entities on the wrong layer. I have worked with drawings created by others where even the most basic layer usage was ignored or abused. I recently saw a drawing that had doors on column layers, walls on equipment layers, and text on layer zero, and none of it was consistent throughout the drawing.
Layer zero (0) shouldn’t be used for drafted lines or annotation in a drawing unless, of course, it is within a block and placed on a proper layer. Another wrong layer issue is xrefs placed on any random layer and not on a proper layer. Some people forget that when attaching an xref, it comes in on whatever layer is current at the time, then they leave it that way. So if a designer happens to be working in the layer “A-ANNO-DIMS” when attaching an xref, and later on that layer needs to be frozen for some reason, the whole xref goes away and the designer is left scratching his head, wondering why. (See Figures below).
The solution to these issues is to place drawing entities on their proper layers. It’s also important to have a logical layer naming convention. The complexity of the project drives how detailed the layer naming should be. We use National CAD Standards (NCS) at our company, which utilizes the AIA layer naming convention.
Layer zero (0) is a built-in layer in AutoCAD and is reserved for (among other things) creating raw geometry for block creation. When placed on a layer, the defined block takes on that layer’s properties such as line weight and color. Another acceptable block-creating use for layer zero is a complex block made up of geometry on different layers, and the block definition is created on layer zero. It can get kind of tricky manipulating layers in this case, but at least these blocks are defined on a layer not being used for anything else.
Xrefs need to be placed on a proper layer, separate from layers for other purposes. Layer zero (0) comes to mind if you have nothing else designated for your CAD standards. In fact, you can consider an xref simply a block being referenced outside the drawing you’re in. An NCS suggested layer for xrefs is “G-ANNO-REFR” which stands for “General-Annotation-Reference.”
At my company, we place all references (drawings, images, etc.) on this layer. Of course, if you need to be discipline-specific with xref layers, you could use “S-ANNO-REFR” for structure, “M-ANNO-REFR” for mechanical, and so on… but I digress. The point is to have a layer designated for reference files that is separate from other “working” layers (see Figures below).
With proper drafting practices and procedures, design teams can work efficiently and accurately. They’ll save money that would have to be spent on rework and chasing problems that arise when creating the design drawings, and, ultimately, earn more money on their projects.
Ted Goulet is a senior structural CAD designer at a worldwide engineering firm, working in the A&E division in southern Maine, USA. His main function is lead drafter and designer for the structural engineering department. He is also experienced in architectural and civil design. He acts as technical support for the company’s CAD staff, helping with CAD-related issues and customization. Ted does a bit of programming, mostly writing AutoLISP routines to help with production, and script and batch files as well. He is also involved with creating and instituting company CAD standards and procedures. Ted has an Associates Degree in Technical Graphics and Design Technology. He works primarily in AutoCAD 2D and has experience in AutoCAD 3D, Revit Structure, Inventor, and MicroStation 2D.