Centerlines are typically used in drafting as a dimensioning reference to axes of symmetry. You can create a centerline by selecting two line segments, including segments of polylines. The centerline is drawn between the apparent midpoints of the start and end points of the two lines you select. If you select nonparallel lines, the centerline is drawn between the imaginary intersection point and the endpoints of the selected lines. Centerlines bisect the angles of intersecting lines.
A centerline includes its calculated length—the initial length defined by the geometry the centerline is associated to—and the overshoot—the distance each extension line extends beyond the defined object.
You can use system variables to control the layer on which the centerline is created, the linetype used, its linetype scale, and how far the centerline extends beyond the associated geometry. For example, at the command line, type CENTERLAYER and press ENTER. The program prompts you to enter a new value for CENTERLAYER. Type “MyCenter” and then press ENTER. You have just specified the layer on which centerlines will be created. If the layer does not yet exist, it will be created when you place your first centerline.
To change the distance that the centerline extends beyond the associated geometry, type CENTEREXE and press ENTER. The program prompts you to enter a new value for CENTEREXE. Type “1” and press ENTER.
On the Home ribbon, in the Layers panel, when you expand the Layer drop-down you can see that the drawing already includes just two layers and Layer 0 is the current layer. And in the Properties panel, when you expand the Linetype drop-down, you can see that the drawing already includes the Continuous and Hidden linetypes.
The exercise file includes two views of a part. To add a centerline to the view on the left, on the Annotate ribbon, in the Centerlines panel, click the Centerline tool. The program prompts you to select the first line. Move the cursor over one of the angled lines. Even though the object was drawn using a polyline, the individual line segment highlights. Click to select that line. The program then prompts you to select a second line. Move the cursor over the other angled line until it highlights, and then click. As soon as you do, the centerline is created and the command ends.
When you move the cursor over the centerline, you can see that it is a centerline object. It was created on the MyCenter layer—the layer you specified—and it was created using the CENTER2 linetype, which was automatically loaded when you used the CENTERLINE command.
When you select the centerline, it highlights and you can see several grips. You can use those grips or tools in the Properties palette to modify the centerline. For example, you can click the triangular grip at the left end of the centerline to make it the hot grip and then drag it to the left to lengthen the centerline.
Since the centerline is associative, if you modify the geometry of the part, the centerline automatically updates. For example, press ESC to deselect the centerline. Then, on the Home ribbon, in the Modify panel, click the Move tool. Select the upper portion of the part on the left, complete the object selection, and then move it up approximately 2 units in the positive Y-direction. The centerline immediately changes so that it remains centered between the two portions of the cross section. Note that the rest of the geometry, including the view on the right, change as well because the drawing has been geometrically constrained.
Select the centerline. Then, click the middle square grip to make it the hot grip and then move the centerline. Press ESC to deselect the centerline.
By moving the centerline in this way, you have disassociated it from the geometry. Although you could always use the UNDO command to fix this, you would need to do so before making other changes. But you can easily reassociate the centerline to the geometry at any time. To do so, at the command line, type CENTERREASSOCIATE and press ENTER. The program prompts you to select a center mark or centerline to reassociate. Click to select the centerline. The program then prompts you to select the first line. Click to select one of the angled lines. The program then prompts you to select the second line. Click to select the other angled line. As soon as you do, the centerline is once again associated to those two lines and is located back at the axis of symmetry.