The arc command button allows the user to create arcs on the current layer. The sides button, or the Sides entry in the shapes menu in electrical mode, can be used to reset the number of segments used to represent the circle containing the arc. Press button 1 first to define the center. Subsequent presses, (or drag releases) define the inner and outer radii, the arc start angle, and the arc terminal angle. In physical mode, if the arc path width is set to zero, a round disk is created, as with the round button. If the angle given is 360 degrees, then the created figure is identical to that produced by the donut button. In electrical mode, the arc function is entered through the arc entry in the menu brought up with the shapes button. In this case, the arc path has no width, so that the inner and outer radii are equal and not separately definable. Arcs have no electrical significance, but can be used for illustrative purposes.
While the command is active in physical mode, the cursor will snap to horizontal or vertical edges of existing objects in the layout if the edge is on-grid, when within a small distance. When snapped, a small dotted highlight box is displayed. This makes it much easier to create abutting objects when the grid snap spacing is very fine compared with the display scaling. This feature can be controlled from the Edge Snapping group in the Snapping page of the Grid Setup panel.
In electrical mode, an arc is actually a wire, and as such should not be used on the SCED layer. If the current layer is the SCED layer, the arc will be created using the ETC2 layer, otherwise the arc will be created on the current layer. Although there is no error, arc vertices on the SCED layer are considered in the connectivity establishment, leading to inefficiency. If the user insists on the arc being on the SCED layer, the Change Layer command in the Modify Menu can be used to move it to that layer.
If the user presses and holds the Shift key after the center location is defined, and before the perimeter is defined by either lifting button 1 or pressing a second time, the current radius is held for x or y. The pointer location of the Shift press defines whether x is held (pointer closer to the center y) or y is held (pointer closer to the center x). This allows elliptical arcs to be generated. This similarly applies when defining the outer radii, so that the inner and outer surfaces can have different elliptical aspect ratios, though the outer radius must be larger than the inner radius at all angles.
The Ctrl key also provides useful constraints. Pressing and holding the Ctrl key when defining the radii produces a radius defined by the pointer position projected on to the x or y axis (whichever is closer) defined from the center. Otherwise, off-axis snap points are allowed, which may lead to an unexpected radius on a fine grid. When defining the angles of arcs with the Ctrl key pressed, the angle is constrained to multiples of 45 degrees. Ordinarily, the arc angle snaps to the nearest snap point.
When the command is expecting a mouse button press to define a radius, the value as defined by the mouse pointer (in microns) is printed in the lower left corner of the drawing window, or the X and Y values are printed if different. Pressing Enter will cause prompting for the value(s), in microns. If one number is given, a circular radius is accepted, however one can enter two numbers separated by space to set the X and Y radii separately.
Similarly, the angles are displayed, and can be entered in this manner. Prompts can be obtained for the start and end angles separately. The angle should be entered in degrees. Zero degress points along the X axis, and positive angles advance clockwise.