Preparatory functions are the G-codes that identify the type of activities the machine will execute. A program block may contain one or more G-codes.

The letter address G and specific numerical codes allow communication between the controller and the machine tool. This combination of letters and numerical values is commonly called G-Code. In order to perform a specific machining operation, a G-Code must be used. There are two types of G-Codes, modal and non-modal. Modal commands remain in effect, in multiple blocks until they are changed by another command from the same group; whereas, non-modal commands are only in effect for the block in which they are stated.

For example:

• Group 00, are non-modal "One-Shot" commands.

• Group 01, are modal commands.

There are several different groups of G-codes as indicated in column 2 of the following chart, "Preparatory Functions (G-Codes) specific to Machining Centers". One code from each group may be specified in an individual block. If two codes from the same group are used in the same block, the first will be ignored by the control and the second will be executed. There are G-Codes that are active upon startup of the machine indicated by an asterisk (*) in the chart. A Safety Block is commonly placed in the first line of the program where cancellation codes are used to cancel all G-Codes that have been in effect in prior programs (described later in this section, "Explanation of the Safety Block"). Typically, they are: (G40) cutter...