Machining Fundamentals 400 Copyright Goodheart-Willcox Co., Inc. stored separately from the machine. These programs can then be uploaded using portable storage devices, such as USB memory devices. Sometimes machines are networked into a company’s computer systems. In these cases, programs can be uploaded into the control through the computer network. Most programs are written using the Cartesian coordi- nate system because defining the location of a given point relative to another point is easier in Cartesian coordinates. This is because distances between important points are usu- ally provided on prints using dimensions that are based on the Cartesian coordinate system. 21.6.2 Polar Coordinate System The polar coordinate system is used in CNC programs only when a straight-line distance and travel angle from a tool’s current point location is known. An example of a perfect application for polar coordinate programming is a circular pattern of holes around a given point, Figure 21-20. The locations for the holes are defined using a distance from the center point of the circular pattern and an angle relative to an axis that is parallel to either the X or Y axis of the machine. Cartesian coordinates are not used in these cases because doing so would require the programmer to use trigonometry to convert the polar coordinate information into Cartesian coordinates. This would involve rounding multiple decimal values to the third or fourth place, which would cause errors in programming. The polar coordinate system is not often used. How- ever, when a part is defined with dimensions using the polar coordinate system, having the capability to create programs using this system reduces time and errors associated with the mathematical conversions that would otherwise be required. 21.7 CNC Movement Systems CNC axes are positioned using electric motors. The two basic types of motors used to drive CNC axes are stepper motors and servomotors. Vertical spindle Z Y X +X X Z Y +Z Horizontal spindle Horizontal Milling Machine Lathe Vertical Milling Machine Goodheart-Willcox Publisher Figure 21-18. Axes of machine tool movements. Spindle motion is assigned to the Z axis and therefore differs between vertical spindle and horizontal spindle machines. + Z + Y + X IBM Figure 21-19. Machine tool axes are specifi ed according to the right-hand system of Cartesian coordinates. When the right hand is held as shown, the thumb, forefi nger, and third fi nger point in the positive (+) directions of X, Y, and Z axes.