Self-centered

AutoCAD 2017 introduced a new associative functionality in the form of associative center lines. This involves two commands:

• CENTERLINES: Prompts you to select two lines. AutoCAD then draws a centerline midway between the two lines using a long-dash–short-dash line style.
• CENTERMARK: Prompts you to select a circle or an arc. AutoCAD then draws two centerlines that cross at the exact center of the circle or arc.

Now comes the magic part. Centerlines created using these two commands are fully associative to the lines, circles, or arcs that were used to define them. If you move an arc or a circle or change its radius, or if you move either line or either end of either line, the centerlines obediently follow and remain properly centered.

Associative centerlines are automatically placed on a Center lines layer with a Center2 linetype override. If this layer doesn’t exist, AutoCAD automatically creates it with a white/black color and the Continuous linetype. If this layer already exists in your template file, AutoCAD uses the color you specified, but ignores the linetype assigned to the layer and applies the Center2 linetype override.

Quick, dimension!

Three dimensioning commands can help you place multiple dimensions very quickly. They’re found near the middle of the Dimensions panel of the Annotate tab on the Ribbon.

• Continue — DimCOntinue (DCO): Having placed one linear dimension in the drawing, you can now carry on placing a series of dimensions end-to-end. AutoCAD automatically picks the end of the previous dimension as the start of the next one, so you need pick only the end of the next one.
• Base — DimBaseline (DBA): This one works much like DimCOntinue except that AutoCAD selects the start of the previous one as the start of the next one. You thus end up with a series of stacked dimensions, all measuring from a common starting point.
• Quick Dimensions — QDIM: Interestingly, this is one of the few dimensioning commands that doesn’t start with DIMxxx. When you invoke it, it invites you to select objects. The best bet usually is to use one or more window selections (pick from left to right), each of which completely surrounds several objects. Now whenever you press Enter, AutoCAD automatically applies DimLInear dimensions wherever it can, and you might be surprised at some of those places. It can place dimensions in several formats, including continue, baseline, staggered, ordinate, and several others. For convenience in repeat operations, it remembers the last mode you used.

If you want to be superefficient, memorize the three-letter command aliases for the dimension commands you use most often, of which DIM should top the list. Refer to Figure 14-1 to see the aliases of the most common commands.

And now for the easy way

AutoCAD 2016 added several enhancements that greatly simplify the application of dimensions. The first improvement is the DIM command, which has nothing to do with brightness. The DIM command can replace the following five dimension commands, resulting in the same dimensions but with a lot less effort:

• DImLinear: With DIM, after you select two points or an object, moving the cursor horizontally produces a vertical dimension and moving the cursor vertically produces a horizontal dimension.
• DimALigned: Using DIM, once you select two points or an object, moving the cursor perpendicular to the imaginary line between the defining points produces a dimension that is aligned parallel to the imaginary line between the defining points.
• DimANgle: If you pick two lines or an arc, DIM produces the angle dimension between them.
• DimDIameter and DimRAdius: If you select an arc or a circle, DIM knows the difference and applies a diameter or radius dimension as appropriate.

The DIM command has an additional set of characteristics to enhance your dimensioning experience:

• Unlike the older individual commands, you do not need to differentiate between picking exact start and end points as opposed to selecting an object. If you select an object such that your pick point is close to an object snap point, DIM will snap to it. But if you select an object in such a way that it doesn’t find a snap point, DIM automatically selects the picked object.
• Running object snaps won’t snap to the end of an extension line that is closest to the object being dimensioned. Instead, DIM snaps to the end of the dimension line itself and hence creates a continuing dimension.
• After you have placed a dimension, you don't need to restart the DIM command; it reverts to asking you to begin defining the next dimension. In this way, you can place the vast majority of dimensions in a typical drawing with just one run of the DIM command.
• While the DIM command is running, the command line shows several options that can be invoked, of which my favorite is Undo.
• To terminate the DIM command, press ESC or Enter.

The AutoCAD dimensioning commands prompt you with useful information at the command line or at dynamic input prompts. Read the prompts during every step of the command, especially when you’re trying a dimensioning command for the first time. When all else fails …

Where, oh where, do my dimensions go?

In this section, I explain where to put dimensions, not where to place them. Huh? Right. (By now you should know to place dimensions outside the part outline, with smaller ones closer to the part and longer ones farther away, and so on.) In this section, I show you the location in the drawing where your dimensions will live. You can choose one of three different methods for dimensioning your drawings:

• You can place dimensions in model space using non-annotative dimensions. This method, the oldest, for many years was the only possible way. I mention it here because you’re bound to encounter many of the millions and millions of existing drawings that were created this way.
  • Advantage: Dimensions are available in model space as you edit the drawing.
  • Disadvantage: As with text and hatching, dimension objects have to be scaled to suit the drawing scale. Placing dimensions on details at other scales can get truly ugly. I cover scale factors in Chapters 13 and 15 as well as later in this chapter.
• You can place dimensions in paper space. This was introduced as the second-generation method in an attempt to overcome the scaling issue. You can reach from paper space through a layout viewport and snap onto the model space objects. Dimensions remain associative and usually change when the model space objects change. Paper space layouts are discussed in Chapter 12.
  • Advantage: You don’t need to set a dimension scale factor to suit the drawing scale. The dimension scale factor is always 1:1 because paper space layouts are usually plotted at full size, and the viewport scale sets the model space scale factor.
  • Disadvantages: The dimensions aren’t available when editing in model space, so you’re flying blind. In addition, the associative connections between the paper space dimensions and the model space objects are fragile and easily broken. There’s a repair command, but it’s a nuisance to have to use it.
• You can place dimensions in model space using annotative dimensions. This is the newest, latest, greatest, and best way to place dimensions. Annotative dimensions automatically size themselves to suit the drawing scale that you choose. I discuss annotative text in Chapter 13, annotative hatching in Chapter 15, and annotative dimensions later in this chapter.
  • Advantages: Dimensions are available in model space while you’re drawing and editing, you don’t need to mess with dimension scale factors, and applying dimensions to drawing details that are at a different scale from the main drawing becomes trivial.
  • Disadvantage: Annotative dimensions have been slow to catch on because people are stuck in the older ways (“That’s how we’ve always done it!”) and because they don’t understand a couple of subtleties about them that I reveal in Chapter 13. If you’re the first in your office to adopt annotative dimensions, everyone else will hail you as the Great AutoCAD Guru and will pester you to help them learn how to use them.

Creating dimension styles

A dimension style (or dimstyle, for short) is a collection of drawing settings, called dimension variables (or dimvars, for short), which are a special class of the system variables that I describe in Chapter 23.

If you do need to create your own dimension styles or you want to tweak existing ones, use the Dimstyle command. You can invoke it by clicking the small, diagonal arrow in the lower-right corner of the Dimensions panel of the Annotate tab on the Ribbon. The Dimension Style Manager dialog box opens, as shown in Figure 14-4.

Every drawing comes with a default dimension style named Standard for Imperial (feet-and-inches) drawings, or ISO-25 for metric drawings, and matching annotative styles. (I cover annotative dimensions later in this chapter.) Although you can use and modify the Standard or ISO-25 style, I suggest that you leave them as they are and create your own dimension style(s) for the settings appropriate to your work. This approach ensures that you can always use the default style as a reference. More important, it avoids a potential conflict that can change the way your dimensions look if the current drawing is inserted into another drawing: AutoCAD refers to styles by name, so if you have two styles of the same name, one will override the other. Chapter 18 describes this potential conflict.

When you install AutoCAD, it checks with the operating system (OS) to see what country you selected when the OS was installed, and then sets its measurement system accordingly. If you’re in the United States or one of the few other countries that still use Imperial units, the default dimension style is Standard; most of the rest of the world defaults to ISO-25. Canada, on the other hand, is nominally metric, but most people still use Imperial. The system variable MEASUREINIT controls the default action. If set to 0 (zero), AutoCAD uses Imperial units; if set to 1, metric units are used. Among other things, this setting also affects text styles (see Chapter 13), hatching (see Chapter 15), and noncontinuous line types (see Chapter 9).

Starting a new drawing from an ISO template, such as acadiso.dwt, forces everything to metric, and starting a new drawing from a non-ISO template (for example, acad.dwt) forces everything to Imperial, regardless of the setting of MEASUREINIT.

The system variable MEASUREMENT overrides the default for a specific drawing, but it affects only text, hatching, and noncontinuous line types.

Follow these steps to create your own dimension style(s):

1. On the Ribbon’s Home tab, click the label of the Annotation panel to open the panel slideout, and then click the Dimension Style button.

   Alternatively, if that just sounds like too much work, you can type D and press Enter. The Dimension Style Manager dialog box appears.

2. In the Styles list, select the existing dimension style whose settings you want to use as the starting point for the settings of the new style.

   For example, select the default dimension style named Standard or ISO-25.

3. Click the New button to create a new dimension style that’s a copy of the existing style.

   The Create New Dimension Style dialog box appears.

4. Enter a New Style Name and then select or deselect the Annotative check box. Click Continue.

   Select the Annotative check box to create an annotative dimension style, or deselect it for a non-annotative style.

   The New Dimension Style dialog box appears. This dialog box is virtually identical to the Modify Dimension Style dialog box, which is displayed when you edit a dimension style.

5. Modify dimension settings on any of the seven tabs in the New Dimension Style dialog box.

   See the descriptions of these settings in the next section of this chapter.

6. Click OK to close the New Dimension Style dialog box.

   The Dimension Style Manager dialog box reappears.

7. Click Close.

   The Dimension Style Manager dialog box closes, and the new dimension style becomes the current dimension style that AutoCAD uses for future dimensions in this drawing.

8. Draw dimensions to test the new dimension style.

Avoid changing existing dimension styles that you didn’t create, unless you know for sure what they’re used for. When you change a dimension style setting, all existing dimensions using that style change to reflect the revised setting. Thus, one small dimension variable change can affect a large number of existing dimensions! To play it safe, rather than modify an existing dimension style, create a new style by copying the existing one and then modifying the copy.

A further variation on the already convoluted dimension styles picture is that you can create dimension substyles (also called style families), which are variations of a main style that affect only a particular type of dimension, such as radial or angular. If you open the Dimension Style Manager dialog box and see names of dimension types indented beneath the main dimension style names, be aware that you’re dealing with substyles.

Adjusting style settings

After you click New or Modify in the Dimension Style Manager dialog box, AutoCAD displays a tabbed New Dimension Style dialog box or Modify Dimension Style dialog box (the two dialog boxes are identical except for their title bars) with a mind-boggling — and potentially drawing-boggling, if you’re not careful — array of settings.

Fortunately, the dimension preview that appears on all tabs — as well as on the main Dimension Style Manager dialog box — immediately shows the results of most setting changes. With the dimension preview and some trial-and-error setting changes, you can usually home in on an acceptable group of settings. For more information, use the Help feature in the dialog box: Just hover the mouse pointer over the setting that you want to know more about.

If you find the preview image hard to read in the Dimension Style Manager, New Dimension Style, or Modify Dimension Style dialog boxes, click and drag the right edge of the dialog box to increase the size of the preview image.

Before you start messing with dimension style settings, know what you want your dimensions to look like when they’re plotted. If you’re not sure how it’s done in your industry, ask others in your office or profession, or look at a plotted drawing that someone in the know represents as being a good example. A general rule that I’ve found to be helpful in virtually all aspects of life is to stick with the defaults unless you know specifically what you want to change and why.

The following few sections introduce you to the more important tabs in the New/Modify Dimension Style dialog boxes and highlight useful settings. Note that whenever you specify a distance or length setting, you should enter size you want it to be when the drawing is plotted. I discuss plotting size and scale factors in Chapters 13 (text) and 15 (hatching) and later in this chapter.

When you have everything set the way you want it, click OK to close the Modify Dimension Style dialog box, and then click Close to exit the Dimension Style Manager. The new style is now current.

Changing styles

To switch to a different style, click in the dimension style window in the Dimensions panel of the Annotate tab on the Ribbon, and select a style from the drop-down list. Beginning with AutoCAD 2014, if you know the name of the style that you want to use, just begin typing it at the command line. The AutoCAD command line searches for dimension styles; when you see the one you want, just click it.

Editing dimension geometry

The easiest way to change the location of dimension parts is to use grip editing. I describe editing with grips in Chapter 11. Just click a dimension, click one of its grips, and maneuver away. You’ll discover that certain grips control certain directions of movement. Selecting a dimension displays grips generally at the text, the ends of the dimension lines, and the defpoints.

In AutoCAD 2012, dimensions joined the group of objects that feature multifunction grips. Click the text grip on a linear dimension and use the grip menu to adjust the text location. Click an arrow grip, and you can create a continuous or baseline dimension from that end of the dimension, or you can flip the arrow. You can do these things by selecting a dimension and changing items in the Properties palette, but the multifunction grips are more efficient.

If you want to change the look of a component of a specific, individual dimension (for example, substitute a different arrowhead or suppress an extension line), use the Properties palette. See Chapter 11 for more on the Properties palette. All dimension settings in the New/Modify Dimension Style dialog boxes (see “Adjusting style settings,” earlier in this chapter) are available from the Properties palette when you select one or more dimensions.

Follow these tips for editing dimensions:

• Use smart breaks. In manual drafting, it’s considered bad form to cross object lines (that is, real geometry) with dimension lines or extension lines, or to have anything cross a dimension line. Dimension Break (DIMBREAK) prompts you to select a dimension and then an object to break it. The wild aspect is that it’s a smart break. If you do anything to the dimension or to the object being broken to change where they cross, the break follows accordingly — and if you change either item so that they no longer cross, the break heals itself. Better yet, if you change things again so that they cross again, the break reappears!
• Use the DIMSPACE command. The Dimension Space (DIMSPACE) command applies a specified separation between existing linear or angular dimensions. If you don’t use the DimBAseline command as the dimensions are created, spacing dimensions equally afterwards will require tedious manipulation with Snap Mode and the Move command.

• You don’t always have to draw everything at full size. A fundamental mantra in AutoCAD is that you should always draw everything at full size. On occasion, however, it isn’t always practical. For example, you may design a power-transmission shaft for a large machine. The shaft is 4 inches in diameter and 12 feet long. It has a variety of splines, keyways, and bearing shoulders on each end, but the 11-foot section in the middle is simply a straight cylinder. If you draw it at full size and scale the plot to fit a suitable paper size, you’ll never see the details at each end.

  Common practice would be to draw the interesting end details, break out and remove the boring center section, and then bring the ends closer together. Now you can create a reasonable plot. The problem is that any dimension that crosses over the break, such as the overall length, doesn’t show the correct value. The solution is to override the value, and then to use the DIMJOGLINE command to insert a jog in the dimension line to indicate that the dimension line isn’t the true length. I explain how a little later in the section “Editing dimension text.”

  In spite of the name similarities, don’t confuse DIMJOGLINE with the DimJOgged command, covered earlier in this chapter. DIMJOGLINE is for linear dimensions, and DimJOgged is for radial dimensions.

• Right-click for useful options. If you select one or more dimensions and right-click, the menu displays a number of useful options for overriding dimension settings or assigning a different style.

• Sometimes, it’s better to create a new style. When you change a setting in the Properties palette, you’re overriding the default style setting for that dimension. If you need to make the same change to a bunch of dimensions, it’s usually better to create a new dimension style and assign that style to them. You can use the Properties palette or the right-click menu to change the dimension style that’s assigned to one or more dimensions.

  If you manually change a dimvar setting, the setting is applied to the current dimension style as an override, and all subsequent dimensions that are placed by using this style have the overridden appearance. This can cause much the same problem as overriding object properties instead of using different layers; if you edit a dimension style, all existing dimensions that use it update, including the ones that you hadn’t expected because they seem to be different from the ones you want.

• Use a mask for the dimension text. You can use the Properties palette to turn on AutoCAD’s background mask feature (described in Chapter 13) for the text of individual dimensions: Select the dimensions, display the Text area in the Properties palette, and find the Fill Color item. Click in the list box, scroll down, and select Background to use the drawing background color, which usually gives the best results. To ensure that dimension text lies on top of other objects, use the TEXTTOFRONT (Chapter 13) or DRAWORDER (Chapter 18) commands.
• Don’t explode dimensions. The AutoCAD eXplode command on the Home tab’s Modify panel blows a dimension to smithereens — or at least into a bunch of line and multiline text objects. Resist the temptation. Exploding a dimension makes it much harder to edit cleanly and eliminates AutoCAD’s ability to update the dimension text measurement automatically.

Editing dimension text

In most cases, you shouldn’t have to edit dimension text. Assuming that you draw the geometry accurately and pick the dimension points precisely, AutoCAD always displays the right measurement. If you change the size of the associated object, AutoCAD updates the dimension and its measurement. However, you may occasionally want to override the dimension text (that is, replace it with a different measurement) or add a prefix or a suffix to the true measurement.

AutoCAD creates dimension text as multiline text, so dimension text has the same editing options as ordinary text. You have several ways to launch the dimension text editor, but the easiest is to simply double-click the dimension text.

AutoCAD displays the true dimension length as text in the actual dimension and updates the text if you change the size or location of the object. You can override the true length by typing a specific length or another text string. You can preserve the true length by adding a prefix or suffix by inserting <> (that is, the left and right angle-bracket characters) as placeholders for the dimension value. For example, if you enter About <> and the actual distance is 12.000, AutoCAD displays About 12.000 for the dimension text. If you stretch the object later on so that the actual distance changes to 14.528, AutoCAD changes the dimension text automatically to read About 14.528. Now you can appreciate the importance of drawing and editing geometry precisely!

Avoid the temptation to override the default dimension text by replacing the angled brackets with a numeric value unless you plan to use the DIMBREAK command on the dimension. Doing so eliminates AutoCAD’s ability to keep dimension measurements current; even worse, you get no visual cue that the default distance has been overridden unless you edit the dimension text. If you’re overriding dimension text a lot, it’s probably a sign that the creator of the drawing didn’t pay enough attention to using precision techniques when drawing and editing. I don’t point fingers, but you probably know whom to talk to.

Controlling and editing dimension associativity

When you add dimensions by selecting objects or by using Object Snap modes to pick points on the objects, AutoCAD normally creates associative dimensions, which are connected to the objects and move with them. This is the case in new drawings that were originally created in any release of AutoCAD starting with 2002.

If you have to work on drawings created or last edited in versions older than AutoCAD 2002, you must set the DIMASSOC system variable to 2 before later releases will create associative dimensions. An easy way to make this change for the current drawing is to open the Options dialog box (click the Application button and choose Options from the bottom of the Application menu), click the User Preferences tab, and turn on the Make New Dimensions Associative setting. Be aware that this setting affects only new dimensions that you draw from now on. To make existing non-associative dimensions associative, use the DIMREASSOCiate command described in the following list. Search for the term DIMASSOC system variable in the AutoCAD Help system for more information.

You aren’t likely to need any of these three commands often, but if you do, look up the command name in the online Help system:

• DIMREASSOCiate: If you have dimensions that aren’t currently associative (probably because they were created in older versions of AutoCAD) or they’re associated with the wrong objects, you can use the DIMREASSOCiate command to associate them with points on the objects of your choice.
• DimDisAssociate: You can use the DimDisAssociate (DDA) command to sever the connection between a dimension and its associated object.
• DIMREGEN: In a few special circumstances, AutoCAD doesn’t automatically update geometry-driven associative dimensions. In those cases, the DIMREGEN (no command alias) command fixes things.