Fanuc’s lathe programming offers a suite of commands that make internal diameter (ID) machining accurate and time-efficient. Among these, the G70, G71, and G74 commands are pivotal for roughing and finishing operations.

Usage Format

• G70 Finishing Cycle: Used after a G71 roughing cycle to finish the part to its final dimensions.
• G71 Roughing Cycle: Removes large amounts of material in a controlled manner.
• G74 Peck Drilling Cycle: Used for drilling or grooving operations with a pecking motion to break chips.

Explanation

The G71 cycle is used for roughing material in a pattern defined by a subroutine. The cycle follows this format:

G71 U(Roughing depth) R(Retract amount);
G71 P(Start of profile) Q(End of profile) U(Finish allowance in X) W(Finish allowance in Z) F(Feed rate);

After roughing, the G70 cycle is used to finish the part:

G70 P(Start of profile) Q(End of profile);

For grooving or drilling, the G74 cycle is used:

G74 X(Drilling diameter) Z(Drilling depth) R(Retract position) P(Peck increment) F(Feed rate);

Example

Let’s consider an example where we need to machine an internal bore with a roughing and finishing cycle.

Text Diagram:

|----------------|
|                |
|     Bore       |
|                |
|----------------|

Roughing Cycle (G71):

G71 U2.0 R0.5;
G71 P100 Q150 U0.5 W0.2 F0.15;
N100 G00 X50.0 Z5.0; (Move to start position)
N110 G01 Z0.0; (Start of bore)
N120 X55.0 Z-50.0; (End of bore)
N150 G00 X100.0 Z100.0; (Safety position)

Finishing Cycle (G70):

G70 P100 Q150;

In this example, the G71 cycle roughs out the bore with a 2.0mm depth of cut, leaving a 0.5mm allowance for finishing in both X and Z axes. The G70 cycle then finishes the bore to the exact dimensions defined in the subroutine between N100 and N150.