One of the main reasons to strictly format CNC programs has to do with making it easy to write your first few programs. When writing your first program, the related commands will by no means be memorized. However, if you have good example formats to go by, those first few programs will be much easier.
It's like driving a car. It is unlikely that any driver can recite from memory all road signs used to direct traffic. However, when a driver sees a road sign, it is quite likely he will recognize its meaning. In the same way, it is unlikely that even an experienced CNC programmer can recite every word used with CNC programming. But when even a relative newcomer to CNC sees a command, it is likely its meaning will be remembered. One of our intentions with program formatting is to keep you from having to memorize all commands needed for programming. Instead, you will be looking at an example and simply recollecting the function of each command.
A second reason for strict program formatting is consistency. Once you have a format that works, use it. If you use the same format (or structure) for all programs you write, you will be able to repeat past successes. If all programmers in your company use the same format for a given CNC machine, each programmer will be easily able to work on another's program.
The third (and most important) reason for strictly formatting programs is related to multiple tool jobs. Almost all CNC machining center and turning center programs require more than one tool in the program. For this kind of program, there will be many times when it will be necessary to rerun only one tool in the program a second, third, or fourth time.
Say, for example, you have a machining center program that uses ten tools. After running a workpiece, you determine that the fifth tool in the program did not go quite deep enough. After fixing the problem (changing tool offset or Z position in the program), you will need to run the fifth tool again. However, you would not want to run the entire program just to get to tool number five. Doing so would be a waste of time and may actually cause unwanted problems with workpiece ac-curacy and finish. Instead, you will want to be able to run only tool five a second time.
To do so will require that information necessary to get the machine running (just like at the beginning of the program) is included at the beginning of tool five. If the programmer makes certain assumptions related to modal information from a previous tool, it may not be possible to run tool number five by itself (without editing the program).
Here is an example of a time when the programmer must include some redundant information at the beginning of a tool in order to provide the operator the ability to rerun the tool. As in our previous ten-tool example, we still wish to run tool number five a second time. Say that tools four and five both happen to run at 500 rpm. Say the last tool in the program (tool number ten) runs at 1500 rpm. Spindle speed is modal. While writing the program, say the programmer decides to leave out the S500 word at the beginning of tool five, expecting it to carry over from tool four.
If the program is run in the normal (sequential) manner, everything will work just fine. But when the operator attempts to rerun tool number five it will start at the same spindle speed as the last tool in the program (1500 rpm), not 500 rpm!
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