Comprehensive Guide to CNC Canned Cycles for Milling Machines

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Understanding CNC Canned Cycles: An Essential Overview

When venturing into CNC machining, especially with mills, the sheer number of codes and cycles can initially seem daunting. Many professionals lean heavily on CAM software for programming, but gaining a fundamental understanding of manual codes remains invaluable. It empowers you to read, troubleshoot, and optimize your programs effectively, making you a more versatile machinist.

This guide aims to introduce you to the core canned cycles used in 3-axis CNC milling, specifically referencing Fanuc controllers. While it doesn’t cover every advanced detail, it provides a solid foundation on what each cycle does and how to implement them correctly. Remember, each machine may have its own quirks, so always consult your specific manual. Nonetheless, these cycles are generally consistent across Fanuc-based controllers, making this knowledge widely applicable.

Table of Contents

  • What is a Canned Cycle?
  • Common Canned Cycles for Fanuc Mills

What is a Canned Cycle?

In essence, a canned cycle is a pre-programmed set of instructions that automates repetitive machine operations. Instead of scripting each move individually, canned cycles encapsulate a pattern of motions—like drilling, tapping, or boring—into a single command. This simplifies programming and enhances process efficiency.

List of CNC Canned Cycles for Fanuc Milling Machines

G73 High-Speed Peck Drilling / Break Chip Cycle
G74 Left-Hand Tapping Cycle
G76 Precision Boring Cycle
G80 Cancel Current Canned Cycle
G81 Standard Drilling Cycle
G82 Counter Boring Cycle with Dwell
G83 Deep Hole Drilling Cycle
G84 Right-Hand Rigid Tapping Cycle
G85 Reaming and Boring Cycle
G86 Boring Cycle with Dwell
G87 Back Boring Cycle
G88 Boring Cycle with Manual Retract
G89 Advanced Boring Cycle with Dwell

Deep Dive into Canned Cycles

G73 – High-Speed Peck Drilling

The G73 cycle is often referred to as the break chip or high-speed peck drilling cycle. Its primary purpose is to prevent long, stringy chips that can cause damage or safety issues during drilling operations. Instead of continuous drilling, the tool retracts rapidly at specified intervals, breaking the chips into manageable pieces.

Key parameters for G73 include:

  • X: X-coordinate of the first hole (absolute with G90, incremental with G91)
  • Y: Y-coordinate of the first hole
  • Z: Final depth of the hole
  • R: Retract plane (default is the last Z position before G73 activation)
  • Q: Peck depth increment, e.g., 0.100″ between pecks
  • F: Feedrate for drilling
  • K: Number of repetitions (optional)

Example program snippet:

SAMPLE: G73 X1.0 Y0.0 Z-1.0 Q.15 R0.1 F10.0; 
G73 X2.0; 
G73 X3.0; 
G80;

Optimal for hole diameters between 1xD and 4xD, adjusting Q smaller for smaller tools enhances chip control. Typically, a Q of 0.1″ suffices for most applications.

G74 – Left-Hand Tapping Cycle

The G74 cycle performs left-hand (counterclockwise) tapping, suitable for threads requiring such orientation. Many CNC controls use G84 for right-hand tapping, but G74 remains relevant, especially with rigid tapping capabilities.

Essential parameters include:

  • X: Starting X position
  • Y: Starting Y position
  • Z: Depth of thread
  • R: Retract plane
  • F: Feedrate, critical for thread accuracy
  • K: Repetition count (optional)
  • M29: Enabling rigid tapping (check your manual)

Sample code:

SAMPLE: S400 M29; 
G74 X1.0 Y0.0 Z-1.0 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

Ensure the tap is engaged deeper than the thread’s depth to prevent cross-threading. Proper calculation of feed rates based on RPM and pitch ensures quality threads.

G76 – Precision Boring Cycle

The G76 cycle is ideal for high-precision boring operations, especially when unmarred internal surfaces are required. It includes a dwell at the bottom of the hole to allow for smooth finishing.

Important notes:

  • Works exclusively with single-fluted boring bars
  • Correct tool orientation is critical to avoid damaging the workpiece

Parameters include:

  • X, Y: Positioning coordinates
  • Z: Depth
  • R: Retract plane
  • Q: Shift at the bottom of the bore
  • P: Dwell time at the bottom (seconds)
  • F: Feedrate
  • K: Repeat count (optional)

Example:

SAMPLE: G76 X1.0 Y0.0 Z-1.0 R0.1 Q0.010 P0.5 F10.0; 
X2.0; 
X3.0; 
G80;

G80 – Cancel Canned Cycle

The G80 command terminates any active canned cycle, reverting the machine to a standard rapid or linear move mode. Since canned cycles are modal, G80 is essential after completing specific operations to prevent unintended repeats.

G81 – Basic Drilling Cycle

The simplest and most frequently used drilling cycle, G81 positions the tool at the hole location, then feeds down to the specified Z depth, and retracts rapidly. It simplifies repetitive drilling tasks, saving programming time.

Parameters:

  • X, Y: Coordinates
  • Z: Depth
  • R: Retract plane
  • F: Feedrate
  • K: Repetition count (optional)

Sample:

SAMPLE: G81 X1.0 Y0.0 Z-1.0 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

G82 – Counterbore with Dwell

G82 enhances basic drilling with a dwell at the bottom, allowing for cleaner counterbore or spot drilling. It ensures the tool remains stationary for a specified time, resulting in superior surface finish.

Parameters:

  • X, Y: Positioning
  • Z: Depth
  • R: Retract plane
  • P: Dwell time (seconds)
  • F: Feedrate
  • K: Repeat count

Example:

SAMPLE: G82 X1.0 Y0.0 Z-1.0 R0.1 P0.1 F10.0; 
X2.0; 
X3.0; 
G80;

Choosing appropriate dwell time (e.g., one full spindle revolution) is key to achieving optimal surface quality.

G83 – Deep Hole Drilling Cycle

Designed for deep holes, typically exceeding four times the diameter, G83 pulls chips upward efficiently. It cycles through feeding down to the depth, retracting, and then repeating until reaching the final Z depth.

Parameters:

  • X, Y: Coordinates
  • Z: Final depth
  • R: Retract plane
  • Q: Peck depth increment
  • F: Feedrate
  • K: Repetition count

Sample code:

SAMPLE: G83 X1.0 Y0.0 Z-1.0 Q.15 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

G84 – Right-Hand Rigid Tapping

G84 is the standard for tapping threads in CNC mills. It combines spindle rotation, synchronized feed, and precise motion control to produce accurate threads. It’s nearly identical to G74 but with the spindle rotating clockwise by default.

Critical points include:

  • Spindle rotation (CW or CCW)
  • Synchronized feed rate based on RPM and thread pitch
  • Automatic reversal at the bottom of the thread

Parameters:

  • X, Y: Positioning
  • Z: Thread depth
  • R: Retract plane
  • F: Feedrate
  • K: Repeats
  • M29: Enable rigid tapping (check your machine manual)

Sample:

SAMPLE: S400 M29; 
G84 X1.0 Y0.0 Z-1.0 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

To determine the correct feed rate, multiply RPM by the thread pitch, considering the number of starts. For example, for a 1/4-20 thread (20 threads per inch), the pitch is 0.050″. The formula is:

Feed Rate = RPM x Pitch x Number of Starts

G85 – Reaming or Boring

This cycle is similar to G81 but uses the feed rate on retraction, providing a smoother finish. It’s suitable for reaming and finishing bores with minimal oversize risk.

Parameters:

  • X, Y: Positioning
  • Z: Depth
  • R: Retract plane
  • F: Feedrate
  • K: Repetition count
SAMPLE: G85 X1.0 Y0.0 Z-1.0 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

G86 – Boring Cycle with Dwell

The G86 cycle extends G85 by adding a dwell at the bottom, ideal for smooth finishing or reaming. After reaching the depth, the spindle stops, dwells, then retracts.

Parameters:

  • X, Y: Positioning
  • Z: Final depth
  • R: Retract plane
  • P: Dwell time at bottom (seconds)
  • F: Feedrate
  • K: Repetition count
SAMPLE: G86 X1.0 Y0.0 Z-1.0 R0.1 P0.1 F10.0; 
X2.0; 
X3.0; 
G80;

G87 – Back Boring Cycle

G87 is used for back boring, involving a complex sequence to cut a larger diameter hole from the inside. It requires precise offsets and careful setup, making it suitable for specialized operations like counterboring or creating steps.

Key steps include:

  • Spindle stops and reorients
  • Tool offsets to fit within the existing hole
  • Rapid entry into the hole
  • Tool feeds in to the bore
  • Retraction and repositioning

Sample code snippet:

SAMPLE: G87 X1.0 Y0.0 Z-1.1 I0.050 J0.0 K0.60 R0.1 F10.0; 
X2.0; 
X3.0; 
G80;

G88 – Manual Boring Cycle

Unlike G86 or G85, G88 requires operator intervention to manually retract the tool, making it suitable for one-off or special operations where precise control is needed. The cycle pauses at the bottom, awaiting operator action.

G89 – Boring Cycle with Dwell

This cycle combines features of G85 and G86, allowing a dwell at the bottom for finishing. It’s useful for achieving smooth, accurate holes, especially when the material requires extra finishing time.

Overall, mastering these canned cycles will significantly improve your efficiency and quality in CNC milling. Always test cycles with scrap material to fine-tune your parameters before running production parts, ensuring safety and precision.

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