Thomas Industrial Library

Every CNC programmer and most of CNC machine operators have a simple chart of all common G-commands (G-codes) and M-functions (M-codes), usually tucked away somewhere under the lid of their tool box or they have them posted on any convenient machine side or cork board. This chapter covers most of those G-codes that are either uncommon, seldom used, special, or outright mysterious. Keep in mind that machine manufacturers often add G-codes and M-codes of their own. These special codes or functions cannot be covered in a general publication, such as this handbook.

Miscellaneous functions (M-functions) are not covered here at all, as they are often very much dependent on the machine tool manufacturer - for that reason, they are not part of this chapter. The situation is much different with various G-codes, some standard, some optional - they are covered here.

These special and less frequently used G-codes are as important as those used on a daily basis, even if only as accepting them for possible future use. Programmers often forget that there are many preparatory commands available that are not used very frequently. In this chapter, the focus will be on those G-codes that may sometimes become the key to solving a particular problem or achieving a particular programming goal. Some of these preparatory G-codes have a direct relationship with each other, in which case, all related commands will be considered together and explained together.

Divided into seven groups, seventeen preparatory commands covered in this chapter are:

Before studying this section, please take note:

Spindle speed is programmed in revolutions per minute (r/min or rpm). For example, the program may contain S1000 M03, selecting 1000 r/min at normal rotation. Ideally, the spindle should run at the constant 1000 r/min, however, that is not always the case. The actual spindle speed will fluctuate slightly, which is normal. Just watch the display of the spindle speed at the control. In most cases, the speed fluctuation (also called spindle speed variation) can be ignored, as it presents no problem. For those cases, when the fluctuation is high, the control system provides two preparatory commands:

Note that the G26 command does not actually remove the fluctuation, it only detects it . It only monitors it, and issues and alarm if the spindle is overheated - the main reason for spindle fluctuation is heat. The format for G26 uses three entries (Fanuc 16-18 format shown:

P = Time in milliseconds to start the check

Q = Percentage of allowed tolerance

R = Percentage of spindle speed fluctuation

The above descriptions are rather brief, and do not indicate what exactly happens. Using an example for spindle speed of 1000 r/min, the meaning of individual entries should becomes clearer.

The P value is the time in milliseconds (ms) that is counted from the spindle rotation function S to the start of check for possible overheating. The checking will start only if the programmed spindle speed is actually established during the time specified by P . If P is P2000 it is selected as 2000 ms; the system will check if the programmed spindle speed is reached within 2 seconds.

The Q value is specified as a percentage of allowed tolerance from the programmed spindle speed. For example, Q5 is 5% of S1000, which is 1000/100 _ 5 = 50. Subtracting 50 from the programmed speed results in 950 r/min for the spindle speed to reach. If the programmed speed is within 950 and 1000 r/min, the check of actual spindle speed begins.

The R value is the percentage of spindle speed fluctuation where the spindle speed is too fast and overheating is possible. R10 is 10% of the programmed spindle speed, and 1000/100 _ 10 = 100. Subtracting 100 from the programmed speed results in 900 r/min as the speed when alarm will be generated. Alarm will be issued if overheating is likely to occur.

Often the critical speeds are known, in which case the individual values of Q and R have to be calculated. There are two formulas to use for this purpose:

A = Actual spindle speed

H = High speed at which overheating is possible

S = Programmed spindle speed

For example, if A = 950, H = 900 and S = 1000 , then:

Q = (1 - 950 / 1000) _ 100 = 5

R = (1 - 900 / 1000) _ 100 = 10