Cnc machining nm09/2
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- Advantages of computer aided drafting
- Use of automatic programming routines
- Ease of data storage and management
- 4.14 Writing simple programs
- 4.15.1 Word address format
- 4.15.2 Tab sequential format
- 4.15.3 The APT programming system
- 4.15.4 APT Program example
- Description of letter codes
- ‘G’ preparatory functions
4.12 Computer softwareNo matter how sophisticated input and output devices may be and, no matter how powerful the CPU may be, a computer is useless without software. The software used depends on what you are trying to achieve, for example word processing software for written communication and CAD (Computer Aided Drafting) for graphical communication. 4.13 CAD/CAM softwareThe computers you will be using for CNC programming are loaded with a CAD/CAM (Computer Aided Drafting /Computer Aided Manufacturing) program for which the operator instructions are included in the following section. Advantages of computer aided draftingSaving in geometry definitionSince the part geometry data has already been created during the design phase using the CAD/CAM graphics system, the part programmer is not required to redefine the geometry of the part. Immediate visual verificationThe graphics terminal provides a display of the tool path for immediate verification by the part programmer. Most programming errors can be detected by the user and corrected at the time the error is made. with conventional APT or other NC languages, there is a delay between writing the program and the verification/correction process. Use of automatic programming routinesFor common part programming situation such as profiling and pocketing, the use of automatic macro type routines (sub routines) yields a significant reduction in part programming time. One of a kind jobsBecause the part programming time is significantly reduced when using a CAD/CAM system, numerical control becomes an economically attractive method for producing one of a kind jobs. without CAD/CAM, the time required to prepare the part program represents a significant obstacle which often precludes the use of NC for one/off production. Ease of data storage and managementComputer hard drives and floppy disks provide a means of storing the equivalent of thousands of drawings and tool path geometry. Apart from the security and conveniences of electronic storage changes, edits and revisions of data is far easier through computing than any other method. 4.14 Writing simple programsIn manual CNC part programming, the process of writing any program starts with the programmer interpreting the drawing and producing a job plan which details such things as dimensions, types of operations, tools required, feed and speeds. Once this data has been established the process of writing a program manuscript can begin. 4.15 Program formatsOver the years a number of programming formats have been developed. However, the one which has dominated is ISO word address 4.15.1 Word address formatThe term word address means that each word in a block is pre-fixed by a letter. The letter acts as an address and directs the particular word to are addressed to their respective storage registers. The N word would be directed to the N word register, the word X register, and so on. If there is no requirement for a word in a particular block, the word need not be shown on the tape. For example: If there were no z motion required, there would be no need to show a z word on the tape. 
Figure 4.20 Word address format Technically the words may be in any order. However, a standard has been developed which specifies that they be arranged with the sequence number word being first, and the other words in the following order. N Sequence number word G Preparatory word X } Y} Dimension words (straight line) Z} A}
B } Dimension words (rotary) C} Other programming formats are: 4.15.2 Tab sequential formatThis format is very helpful when programming for point-to-point equipment since the majority of tapes for point-to-point machines are prepared on a device that resembles a typewriter. As far as the tape format is concerned, it may be noted that the tab character is used to separate the words in a block. See below. Also, if the words are kept in a fixed order, such as the sequence number word coming first, the G word next, etc., then it is not necessary to note the letter addresses, since the words will be directed automatically to their proper registers in the control unit. In some point-to-point machines the trailing zeros as well as the plus (+) sign before the X and Y words need not be written. While there is no harm in showing the plus (+) sign, it is not necessary to do so with most NC systems since the system interprets the absence of a sign as a plus (+) sign. The minus (-) sign, however, must be shown. 
Figure 4.21 4.15.3 The APT programming systemThe APT or Automatically Programmed Tool system, was one of the first computer part programming systems developed. Although APT is not the simplest computer programming system, it has been chosen for description in this text because it is the system having the widest use and the greatest range of capabilities. It can be used with more types of numerical control machines than any other programming system. A complete part program, which would be suitable for preparing a tape via the computer, consists of four types of statements. These are:
Usually the first statement of a part program are auxiliary and post processor statements, then follow geometry statements, and last motion statements. There is no fixed rule covering this, and auxiliary and post processor statements may also be scattered throughout the program. Where necessary, geometry statements may be mixed in with motion statements providing any symbol used in a geometry or motion statement has been described prior to its use. Shown below is a typical APT language program for the part out line shown below. 
Figure 4.22 4.15.4 APT Program example
Unlike the WORD ADDRESS and TAB Sequential formats, APT language is not a manual programming format, it requires a computer with the appropriate post processing software to encode such statements as ‘GOTO’ (GOTO) ‘GORIGHT’ (GORGT) and ‘GOLEFT’ (GOLFT) as signals which the MCU will accept. ISO Word addressBy far the most accepted program format used through out the world is the ‘ISO WORD ADDRESS —variable length’ format. This format is in accordance with the following international standards. EIA RS 224 ISO IS0840 The addresses and their numerical values are normally grouped into a block in the following order: N G X Y Z I J K F T S M Each block does not contain all addresses as their value may not always change from block to block. Normally only those values that are changing need to be entered, exceptions to this will be noted in the following descriptions. Description of letter codes‘N’ sequence numberThe sequence number is employed to allow the operator or programmer to identify that portion of the program. The information is the four digits after N. The sequence number is not a counter, therefore the sequence count (NI, N2, N3 ...... N9999), is the responsibility of the programmer. It must be changed for a given block, in order to be able to use the sequence number search facility to find that block. ‘G’ preparatory functionsThe two digit code preparatory word designates to the control the mode of operations and the means by which tools should move from one point to another. G codes for example, can change the control to an inch mode or specify rapid traverse. These will be dealt with in more detail shortly. ‘G Codes’G codes can be split into groups on their function. Any number of G codes may appear in the same block, but the first ‘G’ code which requires a second letter address (GOI XIOO) will terminate the scanning of the block. Example: N12G91G17G01X100 Directory: file -> view view -> Western Europe after wwii view -> Author study: paul fleischman view -> Qpa1 7ela study Guide view -> Arctic Oil/Gas Neg view -> November 1970 Approximately 150,000-500,000 deaths Bhola Cyclone Bangladesh view -> Grade Level: 6 Unit of Study: Caribbean glce view -> Citation: Duffield, Katy view -> Table of Contents Lesson # view -> Chinese Wind Energy Disad Download 0.53 Mb. Share with your friends:
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