Dimensioning a Drawing - AutoCAD 2011 and AutoCAD LT 2011: No Experience Required - page 631

using ordinate Dimensions
Ordinate dimensions are widely used by the mechanical engineering profession
and related trades. They differ from the kind of dimensioning you have been doing
so far in this chapter in that ordinate dimensioning specifies x- and y-coordinate
values for specific points in a drawing based on an absolute or relative Cartesian
Coordinate System, rather than on a distance between two points. This method is
used to dimension centers of holes in sheet metal or machine parts.
You don't need ordinate dimensions in the cabin project, so you'll now go through
a quick exercise in setting them up to dimension the holes in a steel plate. Doing so
will give you a glimpse of the tools that AutoCAD provides for this type of work. ( If
you aren't interested in ordinate dimensioning, move on to the next section, “Using
Leader Lines,” to modify the dimensions you've already created for the cabin.)
1. Open a new drawing, and leave the units at the default of Decimal
with a precision of four decimal places. Turn Polar Tracking on.
2. Set up a new text style, and set 0.125 as the height. Click Apply and
then Close to make it the current text style.
3. Draw a rectangle using 0,0 as the first point and 6,-4 as the second.
4. Use Zoom To Extents, and then zoom out to see the area around the
object. Use the UCSICON command to move the icon to the Noorigin
position.
5. Somewhere in the upper-left quadrant of the rectangle, draw a circle
with a radius of 0.35 units.
6. Using Polar Tracking or Ortho mode, copy that circle once directly to
the right, once directly below the original, and to two other locations
that are not aligned with any other circle, so the configuration looks
something like the top of Figure 12.41.
7. Set the Endpoint and Center osnaps to be running, and turn on
Ortho mode.
What you are concerned with in ordinate dimensioning isn't how
far the holes are from each other but how far the x- and y-coordinates
of the centers of the holes are from a reference point on the plate.
You'll use the upper-left corner of the plate as a reference point, or
datum point , because it's positioned at the origin of the drawing, or at
the 0,0 point.