Statements for influencing the NC program flow sequence

The syntax for control block statements is:

$<statement>

<statement> Control block string acc. to the following chapters. Note that between $ and <statement> no blank characters are permissible.

Statements for influencing the program flow (control-block) enable the realization of:

Conditional jumps, to trigger for instance, an optional machining step depending upon a measured value,
Incremental digital loops, to simplify programming of several repetitive machining steps, for instance, during line milling or during boring of hole-circles,
Loops with run-condition, to allow to repetition of several machining steps till the abort condition is fulfilled. For instance, if the infeed of the tool and a machining operation should be carried out till a definite coordinate-value is reached. Loops could be programmed as endless-loops in case of missing or unfulfilled run-condition.

In the use of control-block-statements, the following rules are valid:

In one NC-block, only one control-statement may be present!
Nested control-block-statements are allowed. The nesting depth is firmly given.
Only the block number and "/" may be programmed in front of the control-block-statement.
In the invalid branch of a control-block-statement, a syntax-check is made for block-numbers and further (nested) control-block-statements (see example of IF-ELSE-branching).

Programming example

Syntax check in an invalid branch:

N10 $IF 0 
N20 XY (Here no syntax check takes place) 
N30 $ENDIF 
N10 $IF 0 
N20 ... 
N30 $IF XY (Syntax-check because of nested control-block-)
(statement; Error message because of the unknown term) 
N40 ... 
N50 $ENDIF 
N60 $ENDIF 
N10 $IF 0 
NXY (Syntax check of block number;) 
(Error message because of unknown term) 
N30 $ENDIF

Because of inaccuracy in the calculation and internal representation of parameters, comparative operations in control-block-statements, could lead to an erroneous result (see Chapter 2.6.1.3: "Arithmetical expressions"). Therefore, in doubtful cases, checking should be made not for precise value but for a tolerance range.

Programming example

Wrong:

N10 $FOR P1 = 0, 10, 1 
N20 P2 = P2 + 0.01 
N30 $ENDFOR 
N40 $IF P2 == 0.1 (Due to inaccuracies of calculation P2) 
N50 ... (can be unequal to 0.1, so that the $ELSE-) 
N60 $ELSE (branch is executed) 
N70 G04 X20 
N80 $ENDIF

Correct:

N10 $FOR P1 = 0, 10, 1 
N20 P2 = P2 + 0.01 
N30 $ENDFOR 
N40 $IF [P1 - 0.1] <= .000001 (Verification of a tolerance range non-) 
(problematical for NC manufacture;) 
N50 G04 X5 ($IF-branch will be executed) 
N60 $ELSE 
N70 ... 
N80 $ENDIF

Further Information