Today’s full-blown machining centres allow the programmer to control just about any function required through programmed commands. This section lists the things that the programmer can usually control within a program. Also explained is how each function is controlled. The MCD format discussed here conforms to ISO 6983-1 (1982).
The CNC machine program is structured in blocks of data. A fixed set of alphabetic, numeric and special characters is used. Any characters that are not to be processed are included within parenthesis. Characters “:” or “%” can be used for display purposes. To identify a machine program, an identifier may be placed immediately after the program start character and before the first “end of block” character.
A block of data consists of a sequence number word and one or more than one data words (also known as NC words). Tab characters, which are optional for the tabulation of a printed copy of the data, may be inserted between words but are usually ignored by the control system. A data word always starts with an address character. The characters are usually presented in the following sequence and are not repeated within one block,
Preparatory words “G”;
“Dimension” words. These words are arranged in the following sequence: X, Y, Z, U, V, W, P, Q, R, A, B, C;
“Interpolation or thread cutting lead words” I, J and K;
“Feed function (F)” word;
“Spindle speed function (S)” word;
“Tool function (T)” word; and
“Miscellaneous function (M)” words.
Some words may be omitted in a specific block of data, indicating that there is no change in the condition of the machine with respect to the function denoted by the omitted word. This word is named as a “modal” word. Table 1 summarises the address characters commonly used in a modal word.Table 1.
Commonly used address characters
|A||Angular dimension about X axis|
|B||Angular dimension about Y axis|
|C||Angular dimension about Z axis|
|D||Second tool function|
|E||Second feed function|
|F||First feed function|
|I||Interpolation parameter of thread lead parallel to X|
|J||Interpolation parameter of thread lead parallel to Y|
|K||Interpolation parameter of thread lead parallel to Z|
|P||Tertiary dimension parallel to X or parameter|
|Q||Tertiary dimension parallel to Y or parameter|
|R||Tertiary dimension parallel to Z or parameter|
|S||Spindle speed function|
|T||First tool function|
|U||Secondary dimension parallel to X|
|V||Secondary dimension parallel to Y|
|W||Secondary dimension parallel to Z|
|X||Primary dimension X|
|Y||Primary dimension Y|
|Z||Primary dimension Z|