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Parametrics 101: Don't Be Afraid!
28 Jan, 2010 By: Bill FaneLearning Curve tutorial: Parametrics in AutoCAD 2010 affect how we create and edit drawings.
It had been a dark and slightly stormy night, but by mid-morning it was turning into a very pleasant day. Captain LearnCurve and his gorgeous wife were on special assignment.
Okay, now what sort of strange car are you in, and in which exotic city?
Not a city, and not a car. He was at the helm as they sailed the Inside Passage between Alaska and British Columbia, Canada ...
Don't tell us you bought another boat!
No, even his MasterCard limit won't quite cover a 93,000-ton cruise ship.
Okay, with today's security restrictions how on earth did you get onto the bridge of a cruise ship while it was sailing?
It's not what you know that counts, nor is it who you know. The only thing that counts is what you know about who you know.
Sailing, sailing ...
Anyway, the image of the ship's wheel is a bit of a hoax. Most of the time the ship is controlled by using a joystick to load the appropriate parameters into the GPS navigation computer that controls the automatic pilot, not unlike a big video game ...
That's it! This month's topic! Parametrics!
By my reckoning, only been four events in history have significantly affected how we create engineering drawings.
The first was the introduction of orthographic plans and elevations, generally attributed to the Roman architect Marcus Vitruvius in the first century B.C. Contrary to popular opinion, I am not so old that I knew him personally.
Nothing much really changed until the introduction of CAD in the late twentieth century. I was there for this second event: my involvement goes back to AutoCAD 2.17g in September of 1986, as documented in more than 200 "Learning Curve" columns.
In the early twenty-first century, self-scaling annotations finally solved the dilemma of easily creating scale drawings. Okay, this third event only counts as half an event because the problem didn't exist until CAD came along.
Next comes parametrics — the fourth event. Parametrics count as an event and a half, because they not only affect how we create drawings but also how we edit them. Parametrics involves constraining drawing objects so they are related to each other. Changing or moving one object will affect all its siblings. Let's study 2D parametrics, particularly as introduced in AutoCAD 2010.
Two Kinds of Parametric Constraints
Geometric constraints are sticky object snaps. In standard AutoCAD, a Tangent object snap exists only for the split-second interval while the location is being calculated. In parametrics, however, the line remembers that it is tangent to the arc and vice versa. If you change or move one object, then the other will adjust itself to remain tangential.
As well as the usual object snaps, additional geometric constraints are available such as Equal (length or radius), Symmetric, Collinear, Coincident, and so on.
Objects don't need to touch for geometric constraints to work. A line in a front view can be equal in length to a line in the top view so that changing one line will change the other, and one end of the front-view line always can be vertical to the end top-view line. Now the views will stay in step if anything changes.
Let's start with a quick exercise to see this in operation. Draw a vertical and a horizontal line segment with Ortho and Polar turned off and with gaps between the ends of the lines, as shown in the figure below.
Start with a random pair of lines ...
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