Creating Circular Arc Segments

Close approximations to circular arcs can be created within planar regions of active curves using the arc tool - a circular tool analogous to a circle template or radius-aid in manual drafting. This is only possible with fitted curves.

The arc creation tool is associated with one particular curve at any time. It is activated by selecting an active curve, or a data point belonging to an active curve, then selecting CURVE \ ARC or . The active graphics window should be an orthogonal view that is parallel to the planar region of curve that the arc is to be created in.

The arc tool settings are accessed from a popup launched by clicking the Right Mouse button on the circle circumference.

There are 6 stages to creating an arc in a curve:

• Select the curve, or one of its data points, using the Left Mouse button in the appropriate orthogonal graphics view.

• Activate the arc tool

• Set the arc tool to be a Tangent or a Knuckle type

• Approximately size and position the arc tool

• Snap the arc tool precisely into place on the curve (this can be set to occur automatically when the tool come close enough to a satisfactory position)

• Apply the arc tool to modify the curve

The effect the arc creation tool has on the curve is determined by whether it is set to Knuckle or Tangent. The Knuckle setting (KNUCKLE TYPE on the arc tool popup menu) creates an arc that has knuckles (tangent discontinuities) where it joins with the rest of the curve.

Figure 3:65. Creating an arc with knuckle transitions: move to position, snap to curve, apply to curve

With the Tangent setting (TANGENT TYPE on the arc tool popup menu), the arc joins smoothly with the rest of the curve.

Figure 3:66. Creating an arc with smooth transitions: move to position, snap to curve, apply to curve

The size and position of the arc tool is controlled through the interactive positioning (dragging or nudging) of several controlling points:

• The centre point

• Three points on the circumference

• Points at the end of handles that control the tangency of the circle at the circumference points

Generally, moving one point causes some of the other to move. This behavior can be modified by locking the position of certain points (click the Right Mouse button on the point, and choose LOCK). The size and position of the arc tool can also be specified using the dialog launched by choosing OPTIONS… from the arc tool popup menu.

Figure 3:67. The dialog for controlling the arc tool

Prior to creating the arc in the curve, the arc tool must be snapped into a precise position, by selecting SNAP TO CURVE from the arc tool popup menu. This moves two of the controlling points onto the curve, defining the extents of the arc that will be created.

When using the Tangent setting, the way that the arc tool is snapped into position depends on whether the curve itself or one of its data points was selected when the arc tool was invoked:

• If the tool is associated with the whole curve, the radius remains fixed, and the tool is translated so that it is tangential to the curve at two distinct points.

• If the tool is associated with a data point, the tool remains tangential to the curve at this point, and the radius is changed such that it becomes tangential to another point on the curve.

The arc tool can only be snapped into place if it has a valid position relative to the curve to the curve:

• When using the Knuckle setting, the tool circumference must intersect the curve at two distinct points.

• When using the Tangent setting, the tool must be roughly tangential to two distinct points on the curve (it is best to allow the tool to slightly overlap the curve).

If the arc tool fails to snap to the curve, it becomes grey in colour.

If auto-snapping (AUTO-SNAPPING on the arc tool popup) is enabled, the arc tool will be snapped into position on the curve automatically upon releasing the Left Mouse button after dragging one of the controlling points, provided it is in a valid position.

Once the arc tool has been snapped into place, it is applied to the curve by selecting MODIFY CURVE from the arc tool popup menu. When using a Knuckle type tool, the side of the tool that is used is the one that has the third controlling point on.