Machinery's Handbook, 31st Edition
CNC PROGRAMMING 1351 As a key person in the production environment, the CNC programmer has the responsi bility for developing safe, efficient, and error free programs for CNC machines installed in the machine shop. Many CNC programmers have machining skills and hands-on experi ence as machine operators working on manual or CNC machines. Typical skills of a CNC programmer include the ability to interpret drawings, mathematical aptitude, knowledge of setup and tooling, and the very important understanding of “ how to machine a part ”. The programmer has to be able to visualize all tool motions and recognize any restricting factors that may be involved. The programmer collects, analyzes, processes, and logically integrates all data into a single, production ready, part program. Equally important skills include knowledge of mathematics, particularly the ability to solve equations and trigo nometry. The knowledge of manual programming is absolutely essential even in the age of computers and computer based programming software. Another important quality of a professional CNC programmer is his or her ability to listen to and work with other people such as engineers, operators, customers, and managers. CNC Coordinate Geometry A basic step in understanding CNC principles is to understand the geometry of machine tools. This term covers the relationship between the machine data, part data, and tool data, which includes setup. As with all CAD/CAM systems (see CAD/CAM on page 1390), CNC is based on the same principles of a system of coordinates that define the location of a point in three dimensional space (3-D). A system of coordinates is founded on the concept of two perpendicular lines (named axes “X” and “Y”), intersecting at a point called origin , where both coordinates have a value of zero (X0, Y0). Both lines are divided into equal units of measurement. A point can be defined in a plane (any two axes = 2D) or in space (three axes = 3D). For CNC work, the rectangular coordinate system —also known as the Cartesian coordinate system —is the most commonly used system. It is based on three standard axes: X,Y,Z. Points are defined as locations with a distance from the origin defined by projecting a line at 90 degrees to each axis, forming a visual rectangle. For example, in Fig. 2 , P1 is defined as X3.0, Y2.0, P2 is defined as X-4.0, Y1.0, P3 as X-2.0,Y-3.0, and P4 as X2.0, Y-2.0.
Y+
Y axis
1 2 3 4
Quadrant II X-Y+
Quadrant I X+Y+
P1
P2
X-
X+
-4 -3 -2 -1
1 2 3 4
X axis
-4 -3 -2 -1
P4
Quadrant III X-Y-
Quadrant IV X+Y-
P3
Y- Fig. 2. Absolute Coordinates.
Fig. 1. Rectangular Coordinate System.
Once the point locations are established, the programmer can use them as points repre senting the center of individual holes to be machined or as endpoints for a continuous contour. The points can also be connected by providing specific motion instructions, as a toolpath between given points. Fig. 3 shows a linear motion from P1 to P2 to P3 to P4 and back to P1.
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