CNC Lathe Programming Cycles - G-Code Tutor

Repetitive cycles are used in CNC Lathe Programming to minimise the amount of code that has to be written and give us more control over complex shapes and operations. The control system will understand the operation and perform the repetitive cycle following the commands that are given by the programmer until the feature is complete.

G70 - Finishing Cycle

G70 P Q;

P = start of the contour

Q = end of the contour

Example: G00 X70.0 Z2.0; G42; G70 P100 Q200; G00 X70.0 Z5.0; G53 X0.0 Z-210.0 M09; G40 M05;

This cycle is used after one of the roughing cycles and follows the contour programmed within that roughing cycle. The'P' and 'Q' values are used to locate the profile in that cycle and run a finishing cut that follows it.

G71 - Roughing Cycle

G71 U R; G71 P Q U W F;

U = Depth of cut

R = Retract value

P = Sequence number for the start of the program contour

Q = Sequence number for the end of the program contour

U = Finishing allowance in X

W = Finishing allowance in Z

F = Feed rate

Example: G71 U1.0 R1.0; G71 P100 Q200 U0.2 W.05 F0.2; N100 G00 X19.0; G01 G42 Z0.0 F0.2; Z-65.0 ,R5.0; X60.0; N200 G40 X70.0 Z5.0 F200;

The G71 roughing cycle follows the contour that is programmed between the N numbers. It is used to remove material by cutting along the Z-axis leaving some material on for a finishing cut using a G70 cycle.

Learn more about the G71 Roughing Cycle here

G72 - Facing Cycle

G72 W1 R; G72 P Q U W2 F;

W1 = Depth of cut

R = Retract value

P = Sequence number for the start of the program contour

Q = Sequence number for the end of the program contour

U = Finishing allowance in X

W2 = Finishing allowance in Z

F = Feed rate

Example: G72 W1000 R100; G72 P100 Q200 U0.03 W0.03 F0.01; N100 G00 Z-0.2 S500 P11; G01 X3.0 F0.01; X2.0 Z-0.5; Z-0.1; X0.5; N200 X0.0;

The G72 roughing cycle follows the contour that is programmed between the N numbers. It works the same as the G71 cycle but is used to remove material by cutting along the X-axis.

Learn more about the G72 Facing Cycle here

G73 - Pattern Repeating Cycle

G73 U(1) W(1) R; G73 P Q U(2) W(2) F;

U(1) = Amount of material to be machined on the X-axis

W(1) = Amount of material to be machined on the Z-axis

R = Amount of roughing passes

P = Sequence number for the beginning of the contour

Q = Sequence number for the end of the contour

U(2) = Finish allowance on the X-axis

W(2)= Finish allowance on Z-axis

F = Feed rate

Example: G73 U0.5 W0.2 R0.03; G73 P100 Q200 U0.4 W0.01 F0.01; N100 G00 X1.0 S1000 P11; G01 Z-0.4 F0.01; X2.0 Z-1.0; Z-1.5; N200 X4.0;

The G73 cycle is a pattern repeating roughing cycle used in boring or turning. It is followed by the G70 finishing cycle.

Learn more about the G73 Pattern Cycle here

G75 - Peck Grooving Cycle

G75 X Z P Q F;

X = Depth of groove (dia)

Z = Distance to groove from datum

P = Depth of peck

Q = Step over amount on the Z axis

F = Feed rate

Example: G00 X3.0 Z-6.0; G75 X2.0 Z-1.0 P0.1 Q0.15 F.002; G00 X10.0 Z12.0;

The G75 cycle is used to groove out an area larger than the grooving tool.

G76- Screw Thread Cycle

(Single Line)

G76 X Z I K D A F;

X = Core diameter of the thread

Z = End position of the thread

I = Taper

K = Depth of thread

D = Depth of first pass

A = Tool nose angle

F = Pitch of Thread

Example: G97 S400 M03; G00 X20.0 Z5.0 M08; G76 X18.2 Z-18.0 I-.01 K900 D100 A60 F1.5; G00 X25.0 Z10.0;

If the parameters of the machine are set up correctly, you can use one block to machine a screw thread. Note this might not work on every lathe or machining centre so the following method has been included

G76- Screw Thread Cycle

(Double Line)

G76 P (1) (2) (3) Q R; G76 X Z P Q R F;

P is a six digit character, two digits each for (1), (2) and (3)

(1) = number of finishing passes

(2) = chamfering amount

(3) = inclusive angle of the tool tip

Q = Minimum cutting depth

R = Finishing allowance

X = Minor Diameter of the thread

Z = End position of the thread

P = Depth of thread

Q = Depth of first pass

R = Taper

F = Pitch

Example: G00 X20.0 Z5.0 M08; G76 P040060 Q100 R.02; G76 X18.2 Z-18.0 P180 Q160 F1.5; G00 X25.0 Z10.0;

The more common two-line format of the G76 screw cutting cycle when CNC lathe programming

Learn more about the G76 Thread Cycles here

G83 - Z-axis Peck Drilling Cycle

G83 Z Q R P F ;

Z = Depth of hole

Q = Peck distance

R = Distance from initial point

P = Dwell time at bottom of hole (milliseconds)

F = Feed rate

Example: Z1.0; G83 Z-50.0 P500 Q2000 P1000 F0.08; G80;

The G83 peck drilling cycle is used with live tooling to drill offset from the centreline on the Z-Axis.

G84 - Z-axis Tapping Cycle

G84 Z Q R F;

Z = Depth of hole

Q = Peck distance

R = Distance from initial point

F = Feed rate

Example: X0; G84 Z-5.0 Q2000 R1000 F0.0625; G80 G0 X5.0;

The G84 cycle is used for tapping along the Z-axis when not tapping in the centre line of the part.

G87 - X-axis Peck Drilling Cycle

G87 X R Q P F;

X = Depth of hole

R = Retract Value

Q = Peck distance

P = Dwell time at bottom of hole (milliseconds)

F = Pitch

Example: X42.0; G87 X-19.5 R-5.0 P1000 Q2000 F30; G80;

The G87 cycle is used for peck drilling on the X-axis with live tooling.

G88 - X-axis Tapping Cycle

G88 X R Q P F;

X = Depth of hole

R = Retract Value

Q = Peck distance

P = Dwell time at bottom of hole (milliseconds)

F = Pitch

Example: X42.0; G88 Z-48.0 C90.0 X30.0 R42.0 P200 S100 F1.0; G80;

The G88 cycle is used for tapping on the X-axis with live tooling.

How Lathe Programs Are Structured for Predictable Machining

Programming a CNC lathe is like giving the machine a clear set of instructions that tells it what to cut, where to cut, and in what order. A well-written program groups moves into logical steps, starting with safety and setup, then material removal, and finishing with cleanup and part release.

On most controllers, a lathe program begins with commands that prepare the machine for work: selecting the right coordinate system, setting units, choosing the spindle speed and direction, and picking a tool. From there, each line of code guides the tool either along the part profile or into specific operations such as facing, turning, boring, or threading.

Why Sequence Matters in Lathe Programming

The order you put your moves in has a big effect on surface finish and tool life. Common practice is to:

put roughing passes first to remove bulk material with conservative feed rates,

follow with finishing passes that lightly touch the part for accuracy,

and then handle threading or hole work last so you don't stress thin walls created earlier in the program.

This flow helps reduce vibration and tool wear, and makes mistake recovery easier if there's a problem during cutting.

Axis Motion and Cut Definitions

On a lathe, most motion happens in two axes:

Z axis controls movement along the length of the part,

X axis controls radial movement toward and away from the workpiece surface.

Turning moves generally push the tool inward on the X axis to remove stock, while feed moves along the Z axis carry the tool toward the part's end. Since most lathe tools cut on just one side, it's important to orient your motions so the tool engages the material correctly and safely.

Coordinate Zero and Work Offsets

Lathe programs reference a zero point from which all positions are measured. Typical setups place this at the part's face or a shoulder feature so that coordinates are easy to reason about. Controls also let you define work offsets, so the same program can run on different machines or fixtures without rewriting every coordinate.

Mixing Cycles with Manual Moves

While many controls support cycles for roughing, finishing, threading, and peck drilling, not every situation needs them. Sometimes a simple G00 rapid move or G01 feed move gives you finer control, especially when part geometry is irregular or close to a tolerance limit.

Using cycles where they make sense cuts down on code length and reduces transcription errors, but thoughtful placement and review of each move still matters.

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