Learning Curve: The Slant on Isometrics

14 May, 2005 By: Bill Fane Cadalyst

Isometric dimensioning in AutoCAD is easier than it looks

It was a hot, sunny winter's day. Captain LearnCurve and his gorgeous wife were relaxing on the beach of an all-inclusive Mexican resort. As he watched the iguanas (figure 1) scurrying amongst the vegetation at the edge of the beach, he idly fished one of the ice cubes out of his drink. As he twisted and turned it in his hand, he suddenly got the inspiration for this month's article.

Figure 1. A Mexican iguana.

The Meaning of Isometric
The word "isometric" comes from two Greek words: "iso" meaning "equal" and "metros" meaning "measure." Isometric, therefore, means "equal measure."

Now let's go back to the Mexican beach. Pretend you're there with the Captain. Hold the ice cube in front of you and look straight at it. All you will see is a square and a few water drops as the ice cube melts. Ignore the water drops.

Now twist the top of the ice cube towards you and rotate it on the vertical axis. As you do so, the side and rear edges come into view. The side edges seem to get longer as the front and rear edges appear to get shorter. At some point, all visible edges of the cube will seem to be the same length. Now we see an isometric view of the ice cube (figure 2).

Figure 2. Rotating a cube from front to isometric view.

Actually, I lied. This is not an isometric "view;" it is an isometric "projection." At the isometric angle, the edges only appear to be approximately 82.03108568% of their actual length, not including the ice melting. For convenience, an isometric "view" is usually drawn about 121.90500562189% larger than the projected view, so that the apparent lengths in the view measure the same as the actual lengths on the object.

Engineering drawings date back at least to Roman times, but pictorial drawings, and isometric in particular, came on strong during World War II. As the manufacturing of war materials ramped up, hundreds of thousands of new workers flooded into the factories. With no time to teach them the subtleties of orthographic drawings, they made extensive use of isometric ones.

The hard part comes when it's time to apply dimensions on the object. For ease of understanding, dimensions are created at the same isometric angle as the faces and edges, so they appear to be lying in the same plane as the feature that is dimensioned.

Okay, so I lied again. Applying isometric dimensions is not really difficult; it just seems that way until you learn the tricks.

How Many Dimensions?
Once upon a time there was a TV program called Name That Tune. The host would briefly describe a song, and the competitors would bid on how quickly they thought they could identify the song when it was played.

"I can name that tune in 12 notes."

"I can name that tune in 11 notes."

And so on. The bidding ended at say four notes, and then the band would play the first four notes of the tune. If the low bidder could identify the tune, they would win; but if they could not, the process would step up to the next-highest bidder and their number of notes, and so on until someone won.

Figure 3 shows a dimensioned isometric drawing of a part. How many different text and dimension styles do you think it needs to dimension it?

Figure 3. A dimensioned isometric drawing.

Well, I can dimension that part with just two text styles and two dimension styles.
We covered text styles in my previous article.

In that article, I noted that you should normally create a separate text style for use in dimensioning. The reason is that the default text height in the standard text style is zero, which means that AutoCAD asks you for the height of every text item as you enter it. To overcome this nuisance, I suggested modifying the text style to use a specific height.

The problem is that if the text style has a height greater than zero, dimensioning uses that height instead of the one specified in the dimension style. This becomes a nuisance when you apply a DimScale factor to scale the size of dimensions.

Having said that, we will now see why we need two styles for dimensions. Let's begin by creating the styles and applying them with blind obedience to my instructions. The examples and explanations will come later.

Start the Style command (Format / Text Style) and click the New button in the Text Style dialog box. Enter Left for the style name. You can choose the font you want, but make sure the height is 0.00 (zero). Set the obliquing angle to -30, and then click Apply.

Click New again, and enter Right for the text style name. It starts from a copy of Left. Change the obliquing angle to 30 and click on Apply. Click Close.

Now start the DimStyle command (Format > Dimension Style or Dimension > Style).

When the Dimension Style Manager dialog box appears, click New. In the Create Dimension Style dialog, enter Left as the New Style Name, and then click Continue.

Click the Text tab of the New Dimension Style dialog box, and then click the down arrow at the right end of the Text Style window. Select Left from the drop-down list of text styles. Set the text alignment to be aligned with dimension line.

Set any other dimension style variables that you want, including the text height, and then click OK.

Repeat the preceding steps using Right as the dimension style name, Left as the Start With choice, and Right as the text style.

Okay, now we are ready to apply our isometric dimensions. Let's start with a simple 2D isometric drawing of a cube.

Set the Right dimension style to be current. Apply an Aligned dimension to the upper right edge of the cube (figure 4).

Figure 4. An Aligned dimension applied to a cube.

It doesn't look quite right, does it? No problem. We now come to a cunning trick that fixes it.

Start the DimEdit command and invoke the Oblique option (Dimension / Oblique). Select the dimension, and then press . Now type in 30 as the obliquing angle, and press .

Bingo! The dimension now looks like it is applied on the top plane of the cube (figure 5).

Figure 5. The aligned dimension with a 30-degree obliquing angle applied.

Let's try it again. Set the Left dimension style to be current. Apply an Aligned dimension to the upper left edge of the cube, and then edit it to have an obliquing angle of 150 degrees (figure 6).

Figure 6. A Left dimension set to an obliquing angle of 150

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