Common GCodes For Lathes

The biggest difference between NC Code for basic lathes and milling machines is that Z is a position along the turning axis and X can be either a radius or diameter value, depending upon the machine.

These GCodes Behave The Same As Milling Machines

G0    Rapid Positioning

    The machine moves at maximum speed to the specified locations. G0 is a modal code, rapid motion stays in effect until another motion code is encountered. 

G1    Linear Positioning

G2    Arc Clockwise

    The machine moves at the current feed rate in a circular or helical arc to the specified locations. The arc motion begins at  the current location and moves in a clockwise direction. G2 is a modal code, arc motion stays in effect until another motion code is encountered.

G3    Arc Counter-Clockwise

    The machine moves at the current feed rate in a circular or helical arc to the specified locations. The arc motion begins at  the current location and moves in a counter-clockwise direction. G3 is a modal code, arc motion stays in effect until another motion code is encountered. 

G4    Dwell

    The machine remains in the current position for the specified length of time, or number of spindle revolutions.

G20    Inches

    Specifies that the program units are in inches.

G21    Millimeters

    Specifies that the program units are in millimeters.

G90    Absolute Positioning

    The coordinates specified by position commands, such as X, Z, are absolute positions in the current work coordinate system.

G91    Incremental Positioning

    The coordinates specified by position commands, such as X, Z, are positions relative to the current position in the current work coordinate system.

These GCodes Are Specific To Lathes

G32/G33    Lathe Thread

    Specifies a lathe thread. The end of the thread coordinates are specified by X, and/or Z. The thread pitch maybe specified using either K or F, depending upon the machine.

G71    Lathe Roughing Cycle

    Specifies a lathe roughing cycle. Two formats.
    G71 U=depth of cut R=clearance height
    G71  P=first sequence number of path Q=last sequence number of path U=X stock to leave W=Z stock to leave F=roughing feed rate

P.S.    If I forgot anything, please let me know, I'll be glad to add it.
Thanks!