The G74 cycle is commonly used in CNC (Computer Numerical Control) machining for performing left-hand tapping operations. Tapping is the process of cutting a thread inside a hole so that a cap screw or bolt can be threaded into the hole. Left-hand tapping is used when the thread needs to be tightened in the opposite direction of standard right-hand threads.

Usage Format

The typical format for a G74 command is as follows:

G74 X... Y... Z... R... P... Q... F...

Where:

• X and Y are the coordinates of the hole’s center.
• Z is the depth to tap to.
• R is the plane to retract to between pecks.
• P is the pause at the bottom of the hole, in milliseconds.
• Q is the depth of each peck.
• F is the feed rate, or how fast the tap drills into the material.

Explanation

The G74 cycle is initiated by positioning the tap above the hole. Once the cycle starts, the tap moves down to the specified depth at the feed rate. After reaching the depth, it retracts slightly to break the chip (pecking), then pauses if a dwell time is set, and repeats the process until the full depth is reached. Finally, the tap reverses out of the hole.

Example

Here’s an example of a G74 cycle in action:

% 
O1001 (Left hand tapping example)
T01 M06 (Tool change to tap)
G00 G90 G54 X0 Y0 (Rapid move to hole location)
S500 M03 (Spindle speed 500 RPM clockwise)
G43 H01 Z1 M08 (Tool length compensation and coolant on)
G74 X0 Y0 Z-0.5 R0.1 P200 Q0.05 F0.02 (G74 tapping cycle)
G80 (Cancel tapping cycle)
G00 Z1 M09 (Move to safe Z and coolant off)
M30 (End of program)
%

In this example, the tap will move to the hole located at X0 Y0, then perform a left-hand tapping operation to a depth of 0.5 inches, retracting 0.1 inches between pecks, pausing for 200 milliseconds at the bottom, with each peck being 0.05 inches deep, at a feed rate of 0.02 inches per revolution.