AutoCAD

You Break Me Up!

15 Oct, 2004 By: Bill Fane Cadalyst

AutoCAD's Break command removes precise portions of lines, arcs, and circles.


It was a dark and stormless night. Captain LearnCurve and his gorgeous wife took the ski boat and ran 40 miles up the lake, 20 miles beyond any other humans. They cooked dinner on the beach and spread an air mattress under the stars. It was so warm out they didn't bother to pitch the tent. As they lay there watching the Perseid meteor shower, it was so quiet they could hear their own heartbeats.

Suddenly a shout rang out. "Hey, your column is late. Again."

Okay, so it wasn't a shout. It was an e-mail several days later. But you get the idea.

Back in the good old days, B.A. (before AutoCAD), we had to be methodical and careful in our drafting. The problem was, once the pencil touched the paper, it left a mark that could be difficult to edit. A heavier pencil line would actually indent the surface of the paper.

To minimize this effect, we would lay in very light construction lines while we worked out details. We would then erase and tidy up as desired. Finally, we would come back to trace over all the final lines to darken them up.

If we made a mistake or changed our mind, it was often difficult to completely erase the old line. Like the ghost of Christmas past, vestiges of that line would haunt our prints. In extreme cases of heavily edited drawings, it was possible to burn right through and erase a hole in the paper.

Fortunately, AutoCAD has changed all that.

Take a Break
The Break command is one of AutoCAD's primary editing commands. In its fundamental form, it lets us partially erase a section of line, arc, or circle, and hence lets us break a section out of one of these objects. We often find it faster to draw objects too long and then break them to length rather than draw them correctly the first time. Break does have a few tricks up its sleeve, though.

Let's begin with the basic command. Start by drawing several lines, arcs, and circles. Make sure the Osnap button at the bottom of your screen is off (in the up position).

figure
Figure 1. The Break button displays on the Modify toolbar.
As usual within AutoCAD commands, there are several ways to initiate Break. You can type it in at the Command line, pick Modify from the menu bar, then Break from the pull-down menu, or pick the Break button (figure 1) from the Modify toolbar. AutoCAD asks you to Select object. Pick a point on a line. Then it asks you to Enter second point (or F for first point). Pick a second point along the line, and watch as a section of the line disappears. You end up with two separate lines. The missing segment starts at the first point you picked on the line and ends at the second point (figure 2).

figure
Figure 2. Using the Break command on a line: a) select a point on a line, b) pick second point, then c) watch as the specified section breaks out.

And now, Campers, it's question and answer time! Can you stump Captain LearnCurve?

QUESTION: What if the second point is not on the line, but is off to one side?
ANSWER: AutoCAD picks the point on the line that is closest to the selected point by dropping an imaginary perpendicular from the point to the line, and then breaks the line (figure 3).

figure
Figure 3: The Break command reacts when you specify a second point that's not on the line.

QUESTION: What if the second point is beyond the end of the line?
ANSWER: AutoCAD simply breaks off the end off the line. Instead of ending up with two lines, we end up with one shorter one (figure 4).

figure
Figure 4: How Break works if your second point is beyond the end of the line.

QUESTION: What if I don't want the break to start at the first pick?
ANSWER: That's what the F is for in the second prompt. Instead of supplying the second point, type in F and press . AutoCAD asks for the first point over again, and then asks for the second point. It breaks the selected line between the last two points and ignores the original point used to select the line.

This is necessary because when you select an object, AutoCAD normally takes the youngest one that falls within the pickbox. You might be trying to break an older line where it crosses a younger one, but if you select it at the intersection point, AutoCAD keeps finding the wrong one. The F mechanism lets you select an object at an unambiguous location and then go back to specify the first break point.

In fact, this situation arises so often that I modified my menu file so Break automatically asks for the first point every time:

[Break ]^C^Cbreak \f

QUESTION: How small a section can I break out of an object?
ANSWER: Infinitesimally small. Remember relative coordinate inputs using the @ symbol? Whenever AutoCAD asks for the second point, just press @ (-2), then . AutoCAD assumes the second point is the same as the previous one and breaks out a section of zero length. It turns the single object into two that touch.

figure
Figure 5. The Break at Point button.
This combination arises so often, in fact, that the Modify toolbar includes a separate Break at Point button (figure 5) for this very purpose. When you click on it, it starts a menu macro consisting of the sequence ^C^C_break \_f \@. This macro cancels any command in progress and starts the Break command. It asks you to select an object, then it asks you to select the first point. Finally, it reuses the first point as the second point, simply cracking the object at that point.

QUESTION: Do running Object Snap modes and Object Snap overrides -- Int, End, Mid, and so forth -- work within the Break command and the Break at Point macro?
ANSWER: Of course, and sometimes this is bad news. For example, if you're trying to pick a point somewhere along a line, the running Osnap mode forces the point to jump to the end of the line.

QUESTION: Does Break work on other objects besides a line?
ANSWER: Yes. It also works as described on an arc, a polyline (pline), including a curve-fit polyline, a spline, and a 3D polyline. Polygons, rectangles, and donuts are actually polylines at heart, so Break works on those, too.

It works on a circle, turning it into an arc, but with one crunchy bit and one restriction.

First the crunchy bit: Two possible arcs could result from breaking a circle. AutoCAD always breaks out the section that results by travelling counter-clockwise from the first pick to the second one. If you end up with the wrong arc, just type in U, then to undo the last command and restore the circle. Now, break it again but pick the points in the opposite sequence. See the difference in the illustration below (figure 6). Both circles were broken from point A to point B.

figure
Figure 6: Sequence matters when you break a circle.

Now for the restriction: An arc can't be 360-degrees, so you can't simply enter @ for the second point on a circle to make it the same as the first one, nor can you use the Break at Point toolbar button. An ellipse has the same restriction. You can break it, provided it ends up with a finite gap in it.

QUESTION: What is half of infinity?
ANSWER: A construction line (xline) is infinitely long. If you break one, it turns into two rays, each of which starts from one of the break points and is infinitely long in one direction, away from the other ray.

QUESTION: What is infinity minus a little bit?
ANSWER: A ray is infinitely long in one direction from a start point. If you break one, the segment from the start point to the beginning of the ray becomes a regular line segment. The remaining piece is an infinitely long ray, starting from the end of the break.

QUESTION: What is the answer to life, the universe, and everything?
ANSWER: 42.

And Now For Something Completely Different
You should occasionally check the adjustment of the winglets at the tip of the tail fin of a high-performance water ski to make sure they have not come loose or been knocked out of adjustment. A typical setting should be 3- to 7-degrees downward, depending on the ski and the skier's preference. Under no circumstances should you ever set them even the slightest amount upward, or the ski will become dangerously unstable.


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Lynn Allen

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