Some CAD novices may wonder why the system sometimes asks them to enter a negative value for radius of an arc.
If users know two end points and radius when drawing arcs, and there are more than one arc that meets the condition, there are four actually, shown as below,
There are two arcs on both sides of the line segment defined by two end points: the arc that is smaller than 180° is called minor arc while the arc greater than 180° is called major arc.
The negative and positive values for radius are to solve this particular problem in CAD drafting, and we can control the drawing of major arc or minor arc by entering positive values or negative values for radius. But this is not to say that entering positive values is to get major arcs or entering negative values minor arcs, it all depends on the circumstances.
As we can see in the picture that there are four arcs meeting the condition, when we draw arc using start point, endpoint point and radius, we can switch direction by pressing CTRL. The clockwise or counterclockwise of arc segments is determined by the order of specifying start and end point, if we want to switch the direction, just CTRL, shown as below,
By default, the CAD system creates minor arc, if the direction is correct, entering a positive radius value creates minor arc segment and entering a negative radius value creates major arc segment.
If the direction is not correct, just press CTRL key to switch direction, then entering a positive radius value creates major arc segment and entering a negative raius value creates minor arc segment, shown as below,