Cnc machining nm09/2
Review questions — section 2
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- Navigate this page:
- Section 3: Job planning
- 4. Program verification
- 6. Transfer to machine memory
- 3.3 Tool radius compensation
- Advantages of each method
Review questions — section 2Basic operation of CNC machines
Answers are at the end of CNC Machining text Section 3: Job planningThis section covers the sequencing of operations for efficient job completion, the selection of tooling, the concepts of radius compensation and workpiece and machine checks before and after machining. ObjectivesAt the end of this section, you will be able to:
Safety reminders
The planning for NC machining operation is an essential part in the whole process of producing a component on NC machine. Planning may be defined as the step by step process for successful completion of the job. Good, thoughtful planning will result in efficient machine times, reduced waste, effective use of tooling and efficient operator time utilisation. All of these points are necessary to make the most effective use of expensive machine tools and tooling accessories. 3.1 The NC procedureTo utilise numerical control in manufacturing, the following steps must be accomplished. 1. Process planningThe engineering drawing of the workpiece must be interpreted in terms of the manufacturing processes to be used. This step is referred to as process planning and it is concerned with the preparation of a route sheet. The route sheet is a listing of the sequence of operations which must be performed on the workpiece. It is called a route sheet because it also lists the machines through which the part must be routed in order to accomplish the sequence of operations. We assume that most of the operations will be performed on one or more NC machines. 2. Part programmingA part programmer plans the process for the portions of the job to be accomplished by NC. Part programmers are knowledgeable about the machining process and they have been trained to program numerical control. They are responsible for planning the sequence of machining steps to be performed by NC and to document these in a special format There are two ways to program for NC:
In manual part programming, the machining instructions are prepared on a form called a part program manuscript. The manuscript is a listing of the relative cutter/workpiece positions which must be followed to machine the part. In computer-assisted part programming, much of the tedious computational work required in manual part programming is transferred to the computer. This is especially appropriate for complex workpiece geometry and jobs with many machining steps. Use of the computer in these situations results in significant savings in part programming time. 3. Program preparationA program is prepared from the part programmer's NC process plan. In manual part programming, the punched tape can be prepared directly from the part program manuscript on a typewriter like device equipped with tape punching capability. In computer-assisted part programming, the computer interprets the list of part programming instructions, performs the necessary calculations to convert this into a detailed set of machine tool motion commands, and then controls a tape punch device to prepare the tape for the specific NC machine. 4. Program verificationAfter the program has been prepared, a method is usually provided for checking the accuracy of the program. Sometimes the program is checked by running it through a computer program which plots the various tool movements (or table movements) on paper. In this way, major errors in the program can be discovered. The ‘acid test’ of the program involves trying it out on the machine tool to make the part. A foam machinable wax or plastic material is sometimes used for this tryout. Programming errors are not uncommon, and it may require several attempts before the program is correct and ready for use. 5. Direct linkWhen a program has been written on a CAM system, it can be fed to the CNC machine directly from the computer on which it was generated. In this case there is no facility for the program to be proved on the machine and the operator must rely on the proving of the program in the CAM system before it goes directly to the machine. Using the direct link is usually done when the program is too large for the machines memory. 6. Transfer to machine memoryThis can be done by sending the program via a direct link to the machines memory or by saving on a disk and then loading the machine from a computer next to the machine or from a disk drive built into the machine. Once the program has been loaded it can be proved and edited on the machine. 7. ProductionThe final step in the NC procedure is to use the NC program in production. This involves ordering the raw workpieces, specifying and preparing the tooling and any special fixtures that may be required, and setting up the NC machine tool for the job. The machine tool operator's function during production is to load the raw workpiece in the machine and establish the starting position of the cutting tool relative to the workpiece. The NC system then takes over and machines the part according to the instructions in the program. When the part is completed, the operator removes it from the machine and loads the next part. It is also part of the operators job to monitor the size of the parts coming off the machine and to make alterations to the wear offsets in the machine control to compensate for the small amounts of wear that occur to the cutting tools during machining. 
Figure 3.1 Process planning for NC machining 3.2 Planning stepsA list of points that should be considered when developing the job plan are as follows:
3.3 Operation sheetsAn operation sheet is a document that presents the order of machine operations and tooling required to produce the finished part. The operation sheet or sheets should contain information on:
Example: The sketch below is to have the holes drilled and tapped. The operators sheet shows the relevant details to allow setting the fixtures, clamps, part and tools correctly. 
Figure 3.2 Threaded plate 
Operators sheet 3.3 Tool radius compensationDefinitionTool radius compensation is the alteration of program co-ordinates to allow for the size or shape of the cutting tool. PurposeThe purpose of tool radius compensation is to achieve correctly sized and shaped parts. Methods
Advantages of each methodMethod I: At all times the exact position of the cutting tool will be known. Method 2: Less time is usually consumed in preparing a program. Different size cutters can be used easily. Disadvantages of each methodMethod I: Time consuming to determine calculations. Only one sized cutter can be used for those co-ordinates. Method 2: Thorough familiarity of the automatic system must be obtained by theoretical study if profile errors are not to be introduced to the part due to incorrect use of the system. When die sinking profiled surfaces, automatic compensation is not available with all controls, therefore method one must be used, for these circumstances. The coordinates in a NC program create machine movements following those coordinates. This is called the program path. It stands to reason that a cutting tool (eg. milling cutter) will reduce the sizes of the part by an amount related to the cutter radius. Two examples 1. A simple block would be reduced in size by an amount exactly equal to the size of the cutter radius.
2. A lathe tool facing to the work centre will leave a tit. Solution-Program to move the tool past the centre to a diameter equal to twice the tool nose radius.
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