Programming of CNC Machines. Ken Evans
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Reference information is similar to the other planning documents. Included are:
The date the document is prepared or revised
The name of the person checking the part
The part name and part number (from the blueprint).
On the check sheet, 100 % of the blueprint dimensions and their tolerances are written down in list form. Using this method, sequentially go through each of the print dimensions and log the results. This assures that the machined part meets the specifications given on the blueprint. As the part is checked and verified, some dimensions may not meet specifications. It is important to make sure that these incorrect values are noticed. A good method for relating this is to write those dimensions that are out of print in red ink or use a highlighter pen to emphasize them. You could also include details in the comments section of the QC Check Sheet. If dimensions are found that do not meet specifications, corrective action must be taken.
TYPES OF NUMERICALLY CONTROLLED MACHINES
There are two basic groups of numerically controlled machines, Numerical Control (NC) and Computer Numerical Control (CNC).
In an NC system, the program is run from a punched tape, it is impossible to store such a program in memory. For a punched tape to be used again to machine another part, it must be rewound and read from the beginning. This routine is repeated every time the program is executed. If there are errors in the program and changes are necessary, the tape will need to be discarded and a new one punched. The process is costly and error prone and while this type is still in use, it is becoming obsolete.
Machines with a CNC system are equipped with a computer, consisting of one or more microprocessors and memory storage facilities. Some CNC Machines have hard drives and are network configurable. Program data is entered through Manual Data Input (MDI) at the control panel keyboard, via an RS232 communications interface port or via Ethernet from a remote source like a Personal Computer (PC) network. The control panel enables the operator to make corrections (edits) to the program stored in memory, thereby eliminating the need for new punched tape.
Types of CNC Machines have expanded vastly over the last decade. Turning and Machining Centers are the focus of this book but there are many other types of machines using Computerized Numerical Control. For example there are: Electrical Discharge Machines (EDM), Grinders, Lasers, Turret Punches, and many more. Also, there are many different designs of Machining and Turning Centers. Some of the Machining Centers have rotary axes and some Turning Centers have live tooling and secondary spindles. For this text, the focus will be limited to Vertical Machining Centers with three axes and Turning Centers with two axes. These types of machines are considered the foundation of all CNC learning. All operations 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