CNC Lathe Programming with Fanuc G71 and G74 cycles

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CNC lathes are widely used in manufacturing for their ability to create detailed parts with precision. Programming these machines is a critical skill, and understanding the G-code commands is essential. Two such commands are the G71 Turning Cycle and the G74 Peck Drilling Cycle, which are used for efficient material removal and drilling operations, respectively.

Usage Format

G71 Turning Cycle:

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);

G74 Peck Drilling Cycle:

G74 R(Initial Plane) Z(Drilling Depth) Q(Depth Increment) F(Feed Rate);

Explanation

G71 Turning Cycle is used for rough turning operations. It allows for the removal of large amounts of material by specifying a roughing depth and a finishing allowance. The cycle follows a defined profile between the P and Q points.

G74 Peck Drilling Cycle is designed for deep hole drilling. It breaks the drilling operation into smaller increments or ‘pecks’, allowing for chip breaking and heat dissipation, which prevents tool breakage and workpiece damage.

Example

Consider a scenario where we need to rough turn a part and then perform a peck drilling operation.

G71 Example:

G71 U2.0 R0.5;
G71 P100 Q200 U0.5 W0.2 F0.15;
N100 G00 X50.0 Z5.0;
N101 G01 Z-50.0 F0.2;
N102 X45.0;
N103 Z-5.0;
N104 G02 X40.0 Z-55.0 R5.0;
N105 G01 Z-60.0;
N106 X50.0;
N200 G00 X100.0 Z100.0;

G74 Example:

G74 R1.0 Z-30.0 Q2.0 F0.1;

In the G71 example, the tool will rough turn the profile from N100 to N200, leaving a finish allowance of 0.5mm in X and 0.2mm in Z. The G74 cycle will drill a hole to a depth of 30mm, retracting 1mm after each 2mm peck.

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