G71 Cycle | Stock Removal | Turning

[Hakan]

G71 CNC Code: The Ultimate Guide to Stock Removal in Turning

Introduction:

In CNC turning, efficient and precise material removal is crucial for productivity and part quality. The G71 Stock Removal Cycle is a powerful canned cycle used for rough turning operations on CNC lathes. It automates the process of removing large amounts of material from a workpiece, leaving a defined allowance for a subsequent finishing operation (typically performed with G70). This guide provides a detailed explanation of G71, covering its function, syntax, different types (Type I and Type II), applications, variations across control systems (Fanuc, Siemens, Haas, Mazatrol, Mitsubishi, Heidenhain, and others), best practices, troubleshooting, and programming examples. This article is created for all levels of CNC users.

1. What is the G71 Stock Removal Cycle?

G71 is a G-code command that initiates a canned cycle for rough turning on a CNC lathe. A canned cycle is a pre-programmed sequence of operations that simplifies the programming of repetitive machining tasks. Instead of programming each individual cut, the programmer defines the overall shape of the part and a few key parameters, and the control automatically generates the necessary toolpath.

Key Concepts:

• Rough Turning: Removing the bulk of the material from the workpiece, leaving a small amount for finishing.
• Canned Cycle: A pre-programmed sequence of operations.
• Stock Removal: The process of removing material from the workpiece.
• Longitudinal Turning: Turning operations where the tool moves primarily along the Z-axis (parallel to the axis of rotation). G71 is primarily used for longitudinal turning.
• Facing: Turning operations where the tool moves primarily along the X-axis (perpendicular to the axis of rotation). G72 is the facing equivalent of G71.
• Profiling: Creating the shape.
• P and Q: These parameters define the toolpath for G71.

2. Type I and Type II G71 Cycles

There are two main types of G71 cycles: Type I and Type II. The type determines the direction of the roughing cuts and the shape of the material that can be removed.

• G71 Type I:

  • The roughing cuts are made parallel to the Z-axis (longitudinal turning).
  • The defined profile must be monotonic in the X-axis. This means that the X-diameter must either continuously increase or continuously decrease along the profile. No “undercuts” or “pockets” are allowed in the X-direction.
  • This is the most common type of G71 cycle.

• G71 Type II:

  • The roughing cuts are made parallel to the X-axis (facing cuts).
  • The defined profile must be monotonic in the Z-axis. This means that the Z-dimension must either continuously increase or continuously decrease along the profile.
  • Type II allows for the machining of profiles with “undercuts” or “pockets” in the X-direction.
  • Not all CNC controls support Type II G71. Check your machine’s manual.

Visualizing Type I and Type II:

Imagine a simple stepped shaft:

• Type I: The roughing cuts would be made along the length of the shaft (Z-axis), stepping in towards the center (X-axis) with each pass.
• Type II: The roughing cuts would be made across the face of the shaft (X-axis), stepping in along the length (Z-axis) with each pass.

The choice between Type I and Type II depends on the shape of the part and the capabilities of your CNC control.

3. G71 Syntax and Parameters

The syntax for the G71 command typically involves two G71 blocks:

First G71 Block:

G71 U[Depth of Cut] R[Retract Amount]

• G71: The G-code for the stock removal cycle.
• U[Depth of Cut]: Specifies the depth of cut for each roughing pass. This is a radial value (radius) on most controls, not a diametral value. Be careful with this!. A common mistake is to enter a diametral value, resulting in a much larger cut than intended.
• R[Retract Amount]: Specifies the amount the tool retracts (in all axes) at the end of each cutting pass before returning to the start point for the next pass. This provides clearance for chip evacuation and prevents the tool from rubbing against the workpiece.

Second G71 Block:

G71 P[Start Block] Q[End Block] U[X Allowance] W[Z Allowance] F[Feed Rate] S[Spindle Speed] T[Tool Number]

• G71: Again, the G-code for the stock removal cycle.
• P[Start Block]: The starting block number (N-number) of the sequence of blocks that defines the finished profile of the part.
• Q[End Block]: The ending block number (N-number) of the profile definition sequence.
• U[X Allowance]: The amount of material to leave for finishing in the X-axis (diameter). This is a signed value. A positive value leaves material on the outside diameter (for OD turning). A negative value leaves material on the inside diameter (for ID turning).
• W[Z Allowance]: The amount of material to leave for finishing in the Z-axis. This is also a signed value. A positive value leaves material on the right face (in the +Z direction). A negative value leaves material on the left face.
• F[Feed Rate]: The feed rate to be used during the roughing cuts.
• S[Spindle Speed]: The spindle speed (RPM) to be used during the roughing cuts (if using G97 constant RPM mode). If using G96 (Constant Surface Speed), the S value is specified in the G96 command.
• T[Tool Number]: The tool number to be used (often redundant if the tool has already been selected).

Example:

G71 U2.0 R1.0 ; Depth of cut 2mm (radius), retract 1mm
G71 P100 Q200 U0.4 W0.2 F0.3 S1000 T0101; Profile from N100-N200

4. Control System Variations

While the general concept of G71 is consistent, the specific implementation and available options vary between CNC control systems:

• Fanuc:

  • Type I and Type II: Fanuc controls typically support both Type I and Type II G71 cycles. The type is determined by the shape of the profile defined by the P and Q blocks.
  • Two-Line Format: The two-line G71 format (described above) is standard.
  • Widely Used: Fanuc is a very common control system.

• Siemens (SINUMERIK):

  • CYCLE95 (Stock Removal Cycle): Siemens primarily uses CYCLE95 for stock removal. This cycle combines roughing and finishing into a single, more flexible cycle. It’s not a direct equivalent to G71, but it achieves the same purpose.
  • Parameters: CYCLE95 uses a variety of parameters to define the cutting strategy, depth of cut, finishing allowance, etc. These parameters are entered in a dedicated input screen, not directly in the G-code line.
  • ShopTurn/ShopMill: Siemens’ conversational programming interfaces further simplify the process.

• Haas:

  • Syntax: Very similar to Fanuc (two-line G71).
  • Type I and Type II: Supports both types.
  • Intuitive Interface: Haas controls are known for their user-friendliness.

• Mazatrol (Mazak):

  • Conversational Programming: Stock removal is typically defined within the conversational programming interface, as part of the overall machining process. You wouldn’t usually write a G71 command directly. The system automatically generates the roughing passes based on the part geometry and the specified parameters.
  • EIA/ISO (G-code): Mazak machines can run standard G-code. In this case, G71 would be used similarly to Fanuc.

• Mitsubishi:

  • Syntax: Similar to Fanuc (two-line G71).
  • Type I and Type II: Supports both types.

• Heidenhain:

  • CYCLE: Heidenhain uses different cycles for that.

• Other Controls (Okuma, Fagor, etc.): Consult the specific programming manual for your control.

Key Takeaways:

• Fanuc is the most common reference: The two-line G71 format is widely used.
• Siemens uses CYCLE95: This is a more comprehensive cycle that handles both roughing and finishing.
• Mazatrol is conversational: Stock removal is defined within the conversational programming environment.
• Always check your machine’s manual! This is essential for understanding the specific implementation on your control.

5. The Profile Definition (P and Q Blocks)

The P and Q parameters in the second G71 line are crucial. They define the start and end block numbers of the sequence of G-code commands that describe the finished shape of the part.

Example:

N10 G71 U2.0 R1.0
N20 G71 P100 Q200 U0.4 W0.2 F0.3 S1000 T0101 ;
N100 G00 X40.0 Z2.0  ; Start of profile (P100)
N110 G01 Z0.0 F0.15  ; Move to Z0
N120 G01 X20.0 Z-20.0 ; Turn a taper
N130 G03 X30.0 Z-30.0 R10.0 ; Circular interpolation (CCW)
N140 G01 Z-50.0       ; Turn a straight section
N150 G02 X50.0 Z-60.0 R10.0 ; Circular interpolation (CW)
N160 G01 X60.0       ; Turn to final diameter
; ... (More profile blocks, if needed) ...
N200 G01 Z-80.0      ; End of profile (Q200)
;

Important Points:

• Only G00, G01, G02, and G03 are allowed within the P-Q block sequence. You cannot include tool changes (M06), spindle speed changes (S), feed rate changes (F), or other canned cycles within this sequence. These commands should be placed before the G71 block.
• The first move in the P-Q sequence should be a rapid move (G00) or feed move (G01) only in X axis. The control uses this move to determine the starting point for the roughing cuts. Do not program Z axis movement.
• The profile definition must be monotonic (either continuously increasing or decreasing in diameter) for G71 Type I. For Type II, it must be monotonic in Z.
• The Shape: Machine will cut the material according to P and Q blocks.
• No other G-codes: You can not use different G-codes between P and Q blocks.

6. The G70 Finishing Cycle (and its Relationship to G71)

After the G71 roughing cycle is complete, a G70 Finishing Cycle is typically used to machine the part to its final dimensions and achieve the desired surface finish.

• G70 Syntax: G70 P[Start Block] Q[End Block]
• Relationship to G71: The G70 command uses the same P and Q block numbers as the preceding G71 command. This tells the control to follow the same profile definition, but this time, it will remove the finishing allowance left by the G71 cycle.
• Separate Tool: It’s best practice to use a separate finishing tool for the G70 cycle.

Example (Continuing from previous example):

; ... (G71 roughing cycle and profile definition as above) ...

N210 T0202 ; Select finishing tool
N220 G96 S250 M03 ; Constant Surface Speed for finishing
N230 G70 P100 Q200 ; Finishing cycle (using the same P and Q as G71)
N240 G00 X100.0 Z100.0 ; Rapid retract
N250 M30 ; Program end

7. Best Practices for Using G71

• Choose the Correct Type (I or II): Select Type I or Type II based on the part geometry and your control’s capabilities.
• Appropriate Depth of Cut (U): Choose a depth of cut that is appropriate for the material, tool, and machine. Too large a depth of cut can lead to tool breakage or poor surface finish.
• Appropriate Retract Amount (R): Ensure the retract amount is sufficient to clear chips and prevent the tool from rubbing against the workpiece.
• Correct Finishing Allowances (U and W): Leave enough material for the G70 finishing cycle to achieve the desired dimensions and surface finish, but not so much that the finishing cycle takes an excessively long time.
• Optimize Feed and Speed: Use appropriate feed rates and spindle speeds for roughing.
• Use Coolant: Use appropriate coolant to keep the tool and workpiece cool and to aid in chip evacuation.
• Verify the Toolpath: Always simulate your program (including the G71 and G70 cycles) to visually check the toolpath and ensure there are no errors or unexpected movements.
• Consider Tool Nose Radius Compensation: If you need a very precise finishing, you can use G41/G42 in P and Q blocks.

8. Troubleshooting Common G71 Problems

• “Sequence Number Not Found” Alarm:
  • Cause: The P or Q block number specified in the G71 command does not exist in the program, or the block numbers are out of order.
  • Solution: Double-check the P and Q values and ensure they correctly match the start and end block numbers of the profile definition.
• “Illegal Argument” or “Invalid G-Code” Alarm:
  • Cause: Syntax errors in the G71 command, or unsupported parameters for your control system.
  • Solution: Consult your machine’s programming manual and carefully review the G71 syntax.
• “Non-Monotonic Motion” Alarm (Type I):
  • Cause: The profile definition (between the P and Q blocks) does not have a continuously increasing or decreasing X-diameter (for Type I). There may be an “undercut” or a change in direction in the X-axis.
  • Solution: Redesign the profile to be monotonic in X, or use G71 Type II (if supported by your control) or consider manual programming for that section.
• “Non-Monotonic Motion” Alarm (Type II):
  • Cause: The profile definition (between the P and Q blocks) does not have a continuously increasing or decreasing Z-dimension. There may be an “undercut” or a change in direction in the Z-axis.
  • Solution: Redesign the profile to be monotonic in Z, or use G71 Type I (if supported by your control) or consider manual programming for that section.
• Incorrect Dimensions:
  • Cause: Incorrect U and W values (finishing allowances), incorrect tool offsets, or errors in the profile definition.
  • Solution: Double-check all parameters and offsets, and carefully review the profile definition.
• Excessive Tool Wear or Breakage:
  • Cause: Too large a depth of cut (U in the first G71 block), excessive feed rate, or inappropriate tool selection.
  • Solution: Reduce the depth of cut, reduce the feed rate, use a more robust tool, or consider using multiple roughing passes with different tools.
• Poor Surface Finish:
  • Cause: Excessive depth of cut, incorrect feed rate or spindle speed, tool vibration, or inappropriate tool selection for roughing.
  • Solution: Reduce the depth of cut, adjust the feed rate and spindle speed (refer to tool manufacturer recommendations), check for machine stability, and ensure you’re using a tool designed for roughing.

9. Conclusion: The Workhorse of Rough Turning

The G71 Stock Removal Cycle is a fundamental and indispensable canned cycle in CNC turning. It provides a highly efficient and reliable method for removing large amounts of material, preparing the workpiece for subsequent finishing operations. By understanding the two types of G71 (Type I and Type II), the key parameters (U, R, P, Q, U, W, F), the variations between control systems, and best practices for programming and troubleshooting, CNC machinists and programmers can significantly improve their productivity and the quality of their turned parts. G71, combined with G70 (Finishing Cycle), forms the backbone of many turning operations.