on these machines can be carried out automatically. Human involvement is limited to setting up, loading and unloading the workpiece and entering the amounts of dimensional offsets.
CNC programming is a method of defining machine tool movements through the application of numbers and corresponding coded letter symbols. As shown in the list below, all phases of production are considered in programming, beginning with the technical part drawing and ending with the final product:
Technical Part Drawing
Work Holding Considerations
Tool Selection
Preparation of the Part Program
Part Program Tool Path Verification
Measuring of Tool and Work Offsets
Program Test by Dry Run
Automatic Operation or CNC Machining
All programming begins by a close evaluation of the technical drawing and emphasizes assigned tolerances for particular operations, tool selection, and the choice of a machine. The next step is the selection of the machining process. The machining process refers to the selection of fixtures and determination of the operation sequence. Following that, is a selection of the appropriate tools and determination of the sequence of their application. Before writing a program, spindle speeds and feed rates must be calculated.
When program writing begins, special attention is given to the specific tool movements necessary to complete the finished part geometry, including non-cutting movements. Individual tools are identified and noted in the program manuscript. Miscellaneous functions are noted for each tool such as; flood coolant, spindle direction, r/min and feedrates (these items will be covered in greater detail in the following chapters). Then, once the program is written, it must be transferred to the machine through an input medium like one of the following: punched tape, floppy disk, USB, by RS-232 interface or by Ethernet.
Machining is initiated by preparing the machine for use, commonly called setup (for example, input of Workpiece Zero and Tool Length Offset into CNC memory registers). Many modern controllers have a function for graphical simulation of the programmed tool path on the Cathode Ray Tube (CRT). This enables the machinist or setup person to verify that the program has no errors, and to visually inspect the tool path movements. If all looks well, the first part can be machined with increased confidence. After completion, a thorough dimensional inspection will compare dimensions of the final product to those on the part drawing. Any differences between the actual dimensions and the dimensions on the drawing are corrected by values inserted into the offset register of the machine. In this manner, the correct dimensions of consecutively machined parts can be obtained.
INTRODUCTION TO THE COORDINATE SYSTEM
All machines are equipped with the basic traveling components, which move in relation to one another as well as in perpendicular directions. CNC Turning Centers are equipped with a tool carrier, which travels along two axes (see Figures 6 and 7).
Note that in the following drawings of lathes, the cutting tool and turret is located on the positive side of spindle centerline. This is a common design of modern CNC Turning Centers. For visualization purposes in this book the cutting tool will be shown upright. In reality it is mounted with the insert facing down and the spindle is rotated clockwise for cutting.
Note: the direction of spindle rotation, clockwise (CW) or Counterclockwise (CCW), in turning, is determined by looking from the headstock towards the tailstock and tool orientation.
Machining Centers are milling machines equipped with a traversing worktable, which travels along two axes, and a spindle with a driven tool that travels along a third axis.
All axes of machines are oriented in an orthogonal (each axis is perpendicular to the other) coordinate system, for example, the Cartesian coordinate system (right-hand rule system). (See Figure 9)
Figure 6 Turning Center Axes
Figure 7 Two Axis Turning Center Courtesy MAZAK Corporation
Figure 8 Three Axis Machining Center Courtesy MAZAK Corporation
Figure 9 Right Hand Rule
The Right-Hand Rule System
In discussing the X, Y, and Z axes, the right hand rule establishes the orientation and the description of tool motions along a positive or negative direction for each axis. This rule is recognized worldwide and is the standard for which axis identification was established.
Use Figure 9 above to help you visualize this concept. For the vertical representation, the palm of your right hand is laid out flat in front, face up the thumb will point in the positive X direction. The forefinger will be pointing the positive Y direction. Now fold over the little finger and the ring finger and allow the middle finger to point up. This forms the third axis, Z, and points in the Z positive direction. The point where all three of these axes intersect is called the origin or zero point. When looking at any vertical milling machine, you can apply this rule. For the Horizontal mill the same steps described above could be applied if you were lying on your back.
Visualize a grid on a sheet of paper (graph paper) with each segment of the grid having a specific value. Now place two solid lines through the exact center of the grid and perpendicular to each other. By doing this, you have constructed a simple, two-dimensional coordinate system. Carry the thought a little further and add a third imaginary line. This line passes through the same center point as the first two lines but as vertical; that is, it rises above and below the sheet on which the grid is placed. This additional line would represent the third axis in the three-dimensional coordinate system which is called the Z axis.
Two-Dimensional Coordinate System
A two-dimensional coordinate system, such as the one used on a lathe, uses the X and Z axes for measurement. The X runs perpendicular to the workpiece and the Z axis is parallel with the spindle centerline. When working on the lathe, we are working with a workpiece that has only two dimensions, the diameter and the length. On blueprints, the front view generally shows the features that define the finished shape of the part for turning. In order to see how to apply this type of coordinate system, study the following diagrams. (See Figures
