G71 Rough Turning Cycle – Complete Guide with Programming Examples

G71 Rough Turning Cycle – Complete Guide with Programming Examples

Introduction

The G71 Rough Turning Cycle is one of the most commonly used canned cycles in Fanuc CNC Turning programming. Instead of writing multiple G01 cutting commands for every roughing pass, G71 automatically removes material in multiple passes until the finished profile is reached.

This cycle is widely used in production because it reduces programming time, minimizes errors, and ensures consistent machining. Whether you're a beginner learning CNC programming or an experienced machinist looking to optimize cycle time, understanding G71 is an essential skill.

In this guide, you'll learn how the G71 cycle works, its syntax, programming examples, parameters, common mistakes, and best practices.

What is G71 Rough Turning Cycle?

G71 is a rough turning canned cycle used in Fanuc CNC lathes to remove excess material from the outside diameter (OD) or inside diameter (ID) of a workpiece.

Instead of programming each roughing pass manually, the programmer only defines the final part profile. The CNC control automatically calculates and performs multiple roughing passes based on the specified depth of cut.

This makes G71 one of the fastest and most efficient methods for rough machining.

Why Use G71?

Using G71 offers several advantages:

Reduces program length

Automatically generates roughing passes

Saves programming time

Improves machining efficiency

Reduces operator errors

Produces consistent material removal

Easy to modify when part dimensions change

For production environments, G71 is considered a standard programming practice.

G71 Two-Line Format

Most modern Fanuc controls use the two-line G71 format.

G71 U2.0 R0.5; G71 P100 Q200 U0.2 W0.1 F0.25;

Parameter Explanation

U (First Line)

Depth of cut for each roughing pass.

Example:

U2.0

The machine removes 2 mm material during each roughing pass.

R

Retract amount after every pass.

Example:

R0.5

The tool retracts 0.5 mm before returning to the starting position.

P

Starting block number of the profile.

Example:

P100

The profile begins at block N100.

Q

Ending block number.

Example:

Q200

The profile ends at block N200.

U (Second Line)

Finish allowance in the X-axis.

Example:

U0.2

Leaves 0.20 mm material for the finishing cycle.

W

Finish allowance in the Z-axis.

Example:

W0.1

Leaves 0.10 mm material in the Z direction.

F

Feed rate used during rough turning.

Example:

F0.25

Feed = 0.25 mm/rev

Complete Programming Example

Suppose we need to machine a stepped shaft.

Raw Material

Diameter = Ø60 mm

Final Part

Ø50 × 30 mm

Ø40 × 50 mm

Program

G50 S2500;

G96 S180 M03;

T0101;

G00 X62 Z2;

G71 U2.0 R0.5;

G71 P100 Q150 U0.2 W0.1 F0.25;

N100 G00 X50;

G01 Z0;

X40 Z-30;

Z-80;

N150 X60;

G70 P100 Q150 F0.1;  

M30;

In this program:

G71 performs all roughing passes automatically.

G70 finishes the profile using the same geometry.

The programmer defines the profile only once.

This approach significantly reduces program size.

How G71 Works

The machining sequence is simple:

Tool moves to the start position.

Machine removes material by the specified depth of cut.

Tool retracts by the R value.

Tool returns safely.

Another roughing pass begins.

Process repeats until the finish allowance remains.

G70 performs the finishing pass.

The operator only needs to define the final contour.

G71 Type I vs Type II

Type I

Suitable for simple profiles.

Characteristics:

Straight turning

Steps

Tapers

Basic contours

Type II

Used for complex profiles.

Supports:

Pockets

Undercuts

Complex shapes

Reverse profiles

Not all CNC controls support Type II programming.

Best Practices

To obtain the best machining performance:

Use appropriate depth of cut.

Select correct insert geometry.

Choose the correct cutting speed.

Leave sufficient finishing allowance.

Verify the profile before machining.

Simulate the program whenever possible.

Use coolant for difficult materials.

Following these practices improves both tool life and surface finish.

Common Programming Mistakes

Wrong P and Q Numbers

If the block numbers are incorrect, G71 cannot locate the profile.

Incorrect Tool Position

Always position the tool safely before calling G71.

Finish Allowance Too Small

Leaving no finishing allowance may produce poor surface finish.

Large Depth of Cut

Excessive depth of cut can overload the insert and cause tool breakage.

Incorrect Profile Direction

The profile should be programmed continuously without unnecessary rapid moves inside the contour.

Example Machining Parameters

These values are starting recommendations. Always adjust according to machine rigidity, insert grade, workholding, and coolant conditions.

Advantages of G71

Shorter CNC programs

Faster programming

Automatic roughing passes

Reduced machining time

Better consistency

Easier program editing

Improved productivity

Standard Fanuc programming method

Frequently Asked Questions (FAQ)

What is G71 used for?

G71 is used for automatic rough turning of OD and ID profiles.

What is the difference between G70 and G71?

G71 removes bulk material during roughing, while G70 performs the final finishing pass using the same profile.

Can G71 machine tapers?

Yes. G71 supports straight, stepped, and tapered profiles. Advanced controls also support more complex contours.

Can G71 be used for internal boring?

Yes. By programming the appropriate internal profile and using the correct boring tool, G71 can be applied to ID roughing.

Should I always use G70 after G71?

In most production programs, yes. G71 intentionally leaves finishing allowance, and G70 removes that remaining material to achieve the final dimensions and surface finish.

Conclusion

The G71 Rough Turning Cycle is one of the most powerful productivity features available in Fanuc CNC turning controls. Instead of manually programming every roughing pass, G71 automatically calculates and executes the rough machining process based on the final profile.

By understanding the syntax, selecting proper cutting parameters, and following programming best practices, you can create shorter, cleaner, and more reliable CNC programs while reducing machining time and improving overall efficiency.

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