by Mark Middlebrook, www.markcad.com.
|
Many engineers misconstrue AutoCAD as an "electronic pencil": a computer-assisted way to draw lines, arcs, and text and to copy them quickly. This debilitating myth saps the efficiency of CAD and bears much of the responsibility for mediocre drafting productivity in many structural firms.
AutoCAD is a database management system (DBMS) for making drawings. This assertion might seem surprising (or even pretentious) at first glance, but it's the foundation for effective CAD. Databases and managing data are the heart of almost all computer software, so it's really no surprise that these concepts are fundamental to CAD as well. |
|
Scaling is handled in CAD exactly the opposite from the way it is in manual drafting. In manual drafting, you squeeze real-world objects (the building perimeter, footings, 2x4s, and so on) down by a specific scale factor, like 96 or 16, so that they fit nicely on a sheet of paper. Naturally, you always draw text and other annotations the size you want them to appear on the paper (e.g., 1/8" high), regardless of the scale of the drawing.
In AutoCAD, on the other hand, you draw real-world objects at their actual size and stretch annotations up by the scale factor. That means that 1/8" text in a 1/8"=1'-0" plan will be 12" high in the AutoCAD drawing, while the same text in a 3/4"=1'-0" detail drawing will be 2" high. When you plot, everything gets scaled back down to fit on the paper. This approach seems peculiar at first to someone who was schooled in manual drafting, but it's actually more sensible and offers many advantages in CAD drafting. As a result, even so-called "not to scale" drawings should be set up and drawn to a specific scale.
The database approach to CAD that I mentioned in the beginning really comes into play when one starts thinking about how to organize a project's drawings with CAD. Many firms are still stuck in a "sheet-centric" approach to creating drawings, in which the drawing sheet provides the only organizational structure. This approach doesn't exploit CAD's strengths, and in many cases accentuates its weaknesses.
A superior approach is to view the project drawings as reports on a graphical database. The database structure should be determined first and foremost by the structure of the graphical subcomponents: the grid system, column layouts that read up through the structure, repeated framing subassemblies, typical and project-specific details, and so forth. The requirements of building size, scale, and structural drafting convention will dictate how the graphical components are assembled and carved up on sheets.
The fundamental idea underlying drawings as databases is that, as in all database management, data should be organized so that repetition is kept to a minimum. AutoCAD offers several relational database-like features, including blocks, external reference files ("xrefs"), and paper space, that can help a well-versed CAD drafter achieve this goal.
Blocks and xrefs are two ways to group objects together into a single subassembly whose definition resides in one place. The advantage of centralizing common subassemblies is that the drafter can change all instances of the subassembly by changing a single definition, rather than having to edit every instance of the subassembly. Paper space is a plot layout tool for showing the same AutoCAD drawing in different ways (e.g., with different areas masked out or different layers visible). It can be useful in structural drafting for plans that require match lines (although xref clipping provides a more straightforward approach). On the other hand, paper space is not appropriate in all situations. In particular, it's an inefficient way to assemble detail sheets or other collections of discrete drawings.
For a more in-depth and technical look at the subject of drawings as databases, see my "Manager's Corner" column in the January 1995 issue of CADalyst magazine.
AutoCAD's customization interfaces include scripts, menu macros, AutoLISP, and ObjectARX. Scripts and menu macros are within reach of a knowledgeable CAD manager, while AutoLISP and ObjectARX are full-blown programming languages that usually require the savvy of a software developer or consultant.
Structural firms have several options for customizing AutoCAD, none of which are mutually exclusive: commercial third party software, freeware and shareware programs, custom-developed applications, and "roll your own" applications. For all but the largest firms, starting with a commercial application makes the most sense. Even with a third party application, most firms will want to customize their systems further.
The first step is to agree on what the office's standards should be. Items for standardization include layers (names, colors, and linetypes), color-to-plotted-lineweight correspondence, text (fonts and heights), dimension styles, and symbology (section marks, connection symbols, etc.). Don't forget about drawing organization standards. These are more difficult to standardize than layer names, but there should be some office guidelines about when and how to use blocks, paper space, and xrefs.
The second step is to build standards into your office's AutoCAD system. CAD drafting manuals are fine as a way of codifying office standards, but they're a lousy way to implement and enforce standards, particularly when computers are so good at automating such drudgery. All of the following should be automatic or nearly so in your firm's AutoCAD system: drawing setup; layer creation and management; scaling of text, annotations, and hatching; creation of standard symbology; and dimension configuration. Third party applications and additional customization are the tools for making this happen.
Objects (also called entities) are the indivisible units of an AutoCAD drawing. Objects come in several flavors, the most popular of which are line, arc, polyline (a series of connected lines and arcs), circle, and text string. Every AutoCAD objectpossesses, in addition to its geometric definition, a group of properties. The most important of these for our purposes are layer, linetype, and color.
The fundamental purpose of layers is to group related objects: columns go on one layer, beams on another layer, grid lines on still another layer, and so on. By grouping objects logically on layers, the drafter can choose what to show on the screen or on the plot, since turning off a layer makes all objects on that layer invisible.
Linetypes are the dash-dot patterns that a drafter uses to show a grid line, match line, footing, or other hidden line. Line color determines not only the color of objects on the screen (which helps the drafter identify what layer an object is on), but also their thicknesses on plots. That's because in AutoCAD you control plotted lineweight by mapping each color to a pen or line thickness.
AutoCAD provides two ways to control the color and linetype of each object. In the absence of instructions to the contrary, each object inherits the color and linetype of the layer on which it resides (in AutoCAD jargon, this is called color and linetype by layer). The drafter can, however, give objects explicit colors or linetypes that override the settings of their layers. In general, controlling the color and linetype of each object by layer is good. Assigning explicit color and linetype to objects usually is bad.
Drawing precision is much more important in CAD than it is in manual drafting, because a lack of precision makes later editing, hatching, and dimensioning tasks more time-consuming. AutoCAD offers many methods for ensuring precision, including object snaps (grabbing the endpoints, midpoints, intersections, etc. of existing objects) and snap (constraining points to an imaginary Cartesian grid). A good CAD drafter will be familiar with these and other methods and will almost never specify a point without using one of them.
AutoCAD provides several ways of grouping objects into a single "meta-object", and accomplished CAD drafters use these features extensively. Polylines join a series of connected lines and arcs that belong together (e.g., the lines representing a spread footing, the edge of a slab, or the outline of a wide flange). Associative dimensions group all the pieces of a dimension (witness lines, dimension line, ticks or arrows, and text) together. Blocks are groups of objects that get reused in one or more drawings (e.g., a north arrow or a repeated framing subassembly). Blocks can include attributes, which are variable text fields that the drafter fills in each time he uses (or inserts) the block (for example, the detail and sheet number in a section mark or sheet number in a title block). An xref is a special kind of block that resides in a separate file and updates automatically when the drafter edits the external file.
Drafters spend much more time editing existing objects than they do creating new ones, and as a result, efficient object selection is critical to CAD productivity. AutoCAD has a mind-boggling array of options for selecting objects by location in the drawing or by properties. Good CAD drafters are familiar with the full range of selection options and know how to select objects with a minimum of picks. Mediocre drafters tend to select objects one by one.
Most computer displays are smaller than E-size sheets, so all CAD programs include a way to zoom in or out on the drawing and pan around it. AutoCAD has enough zoom options to keep a George Lucas happy, and once again, the accomplished CAD drafter masters most of them so that navigating around in a drawing is as efficient as possible. When working in large drawings, it's important to understand the difference between a screen redraw (a relatively fast repainting of the graphics vectors) and a regeneration (a relatively slow conversion from AutoCAD's double-precision storage format to a less precise but much faster integer format used for the screen representation). Good CAD drafters know how to keep the number and time of regenerations to a minimum.
As I mentioned earlier, layers also provide control over what appears on the screen, and all drafters should be completely familiar with the differences among the layer options turning off (makes objects invisible), freezing (makes objects invisible and excludes them from consideration during regenerations), and locking (leaves objects visible but makes them immune to editing).
Text styles and dimension styles are the keys to efficient, consistent creation of text and dimensions in AutoCAD. A text style definition includes the font, width factor, and perhaps height (although most people prefer to define variable height styles, which means that the drafter can specify the height separately for each string). A dimension style definition captures a group of dimension variable settings and stores them under a name. Dimension styles make creating and controlling dimensions much easier, but many CAD drafters haven't yet mastered this important feature because of lack of training and/or lack of office-specific customization.
|
Return to www.markcad.com.
