CNC Canned Cycles: Simplifying Machining Operations
Canned cycles are pre-programmed subroutines within a CNC machine's control system. They allow you to perform complex machining operations with a single block of code, significantly simplifying programming and reducing the risk of errors. Here's a breakdown of common canned cycles:
1. Stock Removal Cycles (Turning/Facing)
These cycles are designed for efficient removal of material along the length (turning) or across the face (facing) of a workpiece.
External Turning Cycle (G71)
The G71 cycle is a powerful tool for rough turning external diameters. It involves multiple passes parallel to the Z-axis (longitudinal feed).
Format:
G71 U(Δd) R(r);
G71 P(ns) Q(nf) U(Δu) W(Δw) F(f) S(s) T(t);
Where:
- U(Δd): Depth of cut per pass (radius value).
- R(r): Retract distance in the X-axis after each pass.
- P(ns): Sequence number of the first block defining the contour.
- Q(nf): Sequence number of the last block defining the contour.
- U(Δu): Finish allowance in the X-axis (diameter value).
- W(Δw): Finish allowance in the Z-axis.
- F(f): Feed rate.
- S(s): Spindle speed.
- T(t): Tool number.
Operation:
- The tool moves to the starting point defined before the G71 block.
- It takes a depth of cut (U) and moves along the Z-axis.
- It retracts in the X-axis by the amount (R).
- It moves back to the starting X-coordinate of the current pass.
- Steps 2-4 are repeated until the roughing contour defined by blocks Ns to Nf is achieved, considering the finish allowances.
- Finally, a finish pass is made along the defined contour, offset by the finish allowances (Δu and Δw).
Internal Turning Cycle (G71 - Similar Logic)
The G71 cycle can also be used for internal turning (boring) with a similar logic. The main difference lies in the direction of cutting and the tool path. The U value will still represent the radial depth of cut.
External Facing Cycle (G72)
The G72 cycle is used for rough facing operations, removing material perpendicular to the Z-axis (radial feed).
Format:
G72 W(Δd) R(r);
G72 P(ns) Q(nf) U(Δu) W(Δw) F(f) S(s) T(t);
Where:
- W(Δd): Depth of cut per pass (Z-axis value).
- R(r): Retract distance in the Z-axis after each pass.
- P(ns): Sequence number of the first block defining the contour.
- Q(nf): Sequence number of the last block defining the contour.
- U(Δu): Finish allowance in the X-axis (diameter value).
- W(Δw): Finish allowance in the Z-axis.
- F(f): Feed rate.
- S(s): Spindle speed.
- T(t): Tool number.
Operation:
- The tool moves to the starting point.
- It takes a depth of cut (W) along the X-axis.
- It retracts in the Z-axis by the amount (R).
- It moves back to the starting Z-coordinate of the current pass.
- Steps 2-4 are repeated until the roughing contour is achieved.
- A finish pass is made along the defined contour, offset by the finish allowances.
Internal Facing Cycle (G72 - Similar Logic)
Similar to internal turning, G72 can be adapted for internal facing operations.
Stock Removal in Z-axis with Finish Profile (G70)
The G70 cycle is always used after a G71 or G72 cycle. It performs the finish pass along the contour defined by the blocks between the P and Q addresses in the preceding roughing cycle.
Format:
G70 P(ns) Q(nf);
Where:
- P(ns): Sequence number of the first block defining the finish contour (same as in G71/G72).
- Q(nf): Sequence number of the last block defining the finish contour (same as in G71/G72).
2. Grooving Cycles
Grooving cycles simplify the creation of grooves, which are narrow recesses in the workpiece diameter or face.
External Grooving Cycle (G75)
The G75 cycle is used for creating external grooves.
Format:
G75 R(e);
G75 X(u) Z(w) P(Δi) Q(Δk) R(d) F(f);
Where:
- R(e): Retract amount in the X-axis for each pass.
- X(u): Diameter at the bottom of the groove (absolute or incremental).
- Z(w): Z-coordinate of the groove (absolute or incremental).
- P(Δi): Depth of each cutting pass (radius value, no sign).
- Q(Δk): Shift amount in the Z-axis after each pass (no sign).
- R(d): Chamfer amount at the groove edges (radius value, no sign).
- F(f): Feed rate.
Operation:
- The tool moves to the starting position.
- It makes a series of plunge cuts in the X-axis to the specified depth (controlled by P), retracting by R after each cut.
- After each plunge cut, it shifts along the Z-axis by the amount Q.
- If R(d) is specified, chamfers are created at the groove edges.
Internal Grooving Cycle (G75 - Similar Logic)
The G75 cycle can also be used for internal grooving operations. The X-axis values will correspond to the internal diameter.
External Face Grooving Cycle (G74)
The G74 cycle is used for creating grooves on the face of the workpiece.
Format:
G74 R(e);
G74 X(u) Z(w) P(Δi) Q(Δk) R(d) F(f);
Where:
- R(e): Retract amount in the Z-axis for each pass.
- X(u): Diameter of the groove (absolute or incremental).
- Z(w): Z-coordinate at the bottom of the groove (absolute or incremental).
- P(Δi): Shift amount in the X-axis after each pass (radius value, no sign).
- Q(Δk): Depth of each cutting pass (Z-axis value, no sign).
- R(d): Chamfer amount at the groove edges (Z-axis value, no sign).
- F(f): Feed rate.
Operation:
- The tool moves to the starting position.
- It makes a series of plunge cuts in the Z-axis to the specified depth (controlled by Q), retracting by R after each cut.
- After each plunge cut, it shifts along the X-axis by the amount P.
- If R(d) is specified, chamfers are created at the groove edges.
Internal Face Grooving Cycle (G74 - Similar Logic)
The G74 cycle can be adapted for internal face grooving.
3. Threading Cycles
Threading cycles simplify the creation of screw threads, both external and internal.
External Single-Point Threading Cycle (G76)
The G76 cycle is a versatile cycle for cutting external threads. It allows for multiple passes with infeed at a decreasing angle to improve thread quality.
Format:
G76 P(m)(r)(a) Q(Δdmin) R(d);
G76 X(u) Z(w) R(i) P(k) Q(Δd) F(f);
Where:
First Block:
- P(m): Number of finish passes (01-99).
- P(r): Number of spring passes (00-99).
- P(a): Thread angle (60, 55, 30, 29, etc., without the decimal point).
- Q(Δdmin): Minimum cutting depth in the radial direction.
- R(d): Finish cutting allowance in the radial direction.
Second Block:
- X(u): Minor diameter of the thread (absolute or incremental).
- Z(w): Z-coordinate of the thread end point (absolute or incremental).
- R(i): Radius of the thread run-out (taper).
- P(k): Thread height (radius value, no sign).
- Q(Δd): Depth of the first cutting pass (radius value, no sign).
- F(f): Lead of the thread (distance advanced per spindle revolution).
Operation:
- The tool moves to the starting position.
- It makes multiple roughing passes, gradually increasing the depth of cut. The infeed angle is determined by the thread angle (a).
- A minimum cutting depth (Δdmin) is maintained.
- Spring passes (r) can be included to improve surface finish.
- Finally, the specified number of finish passes (m) are executed with the finish allowance (d).
Internal Single-Point Threading Cycle (G76 - Similar Logic)
The G76 cycle can be used for internal threading operations. The X-axis values will correspond to the major diameter of the internal thread.
CNC Program Execution Modes
CNC machines offer different execution modes to control how the program is run, which is crucial for setup, testing, and production.
1. Memory (AUTO) Mode
- Description: This is the standard mode for automatic program execution during production. The entire program is loaded into the machine's memory and runs sequentially without interruption until the end is reached or a stop command is encountered.
- Usage: Used for running proven programs for producing parts.
- Advantages: Continuous and efficient for mass production.
- Disadvantages: Not suitable for single-step testing or manual intervention during execution.
2. Single Block Mode
- Description: In this mode, the machine executes the program one block (line of code) at a time. After each block is executed, the machine stops and waits for the operator to press the cycle start button to execute the next block.
- Usage: Primarily used for program testing, debugging, and verifying tool paths, especially for new or modified programs. It allows the operator to closely monitor each step of the machining process.
- Advantages: Enhances safety and allows for detailed verification of each command.
- Disadvantages: Very slow and not practical for production runs.
3. Manual Data Input (MDI) Mode
- Description: This mode allows the operator to enter and execute single blocks of G-code or M-code directly through the machine's control panel. The entered commands are not stored in the program memory and are executed immediately.
- Usage: Used for performing quick, single operations like tool changes, spindle starts/stops, manual axis movements, or testing specific functions.
- Advantages: Useful for setup, troubleshooting, and performing isolated tasks.
- Disadvantages: Not suitable for running entire programs or repetitive operations.
Understanding and utilizing these canned cycles and execution modes effectively is fundamental to efficient and accurate CNC machining. Always refer to your machine's specific programming manual for detailed information and any variations in syntax or functionality.
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