AutoCAD ordinate dimensions are a wonderful method of reducing drawing clutter when dimensioning numerous features in a small area. Ordinate Dimensions greatly simplifies the machining process because the dimensions on your drawing directly match the digital readouts on the milling machines.
Ordinate dimensioning is used when the X and the Y coordinates, from one location, are the only dimensions necessary. Usually the part has a uniform thickness, such as a flat plate with holes drilled into it. The dimensions to each feature, such as a hole, originate from one datum location (typically the lower left corner of the part).
Ordinate dimensions have only one datum. The datum location is usually the lower left corner of the object. Ordinate dimensions appearance is also different. Each dimension has only one leader line and a numerical value. Ordinate dimensions do not have extension lines or arrows.
The below drawing of a small plate with 8 holes, this shows the typical baseline linear dimensions:
As you can see, this can end up being very cluttered, especially if each hole was located in a different position.
Now, if we use Ordinate Dimensions, notice how much cleaner the drawing looks:
This blog will discuss the issues arising from using Ordinate Dimensions when the lower left corner of the part is not at 0,0 or if you need to move the origin to get the correct Ordinate Dimension Values.
I have a part drawn in AutoCAD and I start my Ordinate Dimensions (Command line: DIMORD)
I select my first point, and notice how the Ordinate Dimension is not 0.0:
This is because the origin is at 0,0, and the part’s lower left corner is not on that point. Now, here is how we correct that.
We need to re-define the User Coordinate System (UCS) of the drawing. When an AutoCAD drawing is created, the UCS is defined automatically in model space and you probably will see the UCS icon indicating its location.
The default setting for AutoCAD is to display the UCS icon at the origin of the drawing (0,0). There are two methods to verify / set this functionality. One of these is to select the diagonal arrow on the Coordinates panel under the Visualize tab. If you do not see the Visualize tab, right click on the ribbon and select Show Tabs to make it visible.
In the dialog box, verify that the check mark is beside "Display at UCS origin point" and “on”, then click OK to close
I highly recommend that you set the Origin to the part, before you begin any Ordinate Dimensions. To do that, you need to establish where the drawing UCS origin is located. There are a number of ways to accomplish this but the easiest is to use the "UCS" command or select the icon shown below on the Coordinates panel.
Using your Object Snaps, place the origin on the lower left-hand corner of the plate. This moved the UCS origin, but my two placed ordinate dimensions do not update.
Before:
After:
This is because each set of ordinate dimensions are related to the active UCS origin at the time of their placement. Since the two ordinate dimensions were placed before I moved the UCS origin, they are measuring from the UCS’s previous location. I would generally recommend erasing these and start with new Ordinate Dimensions
The second method to move the UCS, you can click on the UCS icon and drag the UCS origin to any point on your model depending on your design.
Click on the grip point in the highlighted UCS icon and move the UCS to the desired location.
The most important part to remember and practice is to set the UCS origin correctly before you begin Ordinate Dimensions.
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